Table of contents for issue 1, volume 867, The Astrophysical Journal

We present the rest-frame optical sizes of massive quiescent galaxies (QGs) at z ∼ 4 measured at the K′-band with the Infrared Camera and Spectrograph and adaptive optics (AO) facility, AO188, on the Subaru telescope. Based on a deep multiwavelength catalog in the Subaru XMM-Newton Deep Survey Field, covering a wide wavelength range from the u-band to the IRAC 8.0 μm over 0.7 deg², we evaluate the photometric redshift to identify massive (M_⋆ ∼ 10¹¹M_⊙) galaxies with suppressed star formation. These galaxies show a prominent Balmer break feature at z ∼ 4, suggestive of an evolved stellar population. We then conduct follow-up K′-band imaging with AO for the five brightest galaxies (K_AB,total = 22.5 ∼ 23.4). Compared to lower redshift ones, QGs at z ∼ 4 have smaller physical sizes of effective radii r_eff = 0.2–1.7 kpc. The mean size measured by stacking the four brightest objects, a more robust measurement, is r_eff = 0.5 kpc. This is the first measurement of the rest-frame optical sizes of QGs at z ∼ 4. We evaluate the robustness of our size measurements using simulations and find that our size estimates are reasonably accurate with an expected systematic bias of ∼0.2 kpc. If we account for the stellar mass evolution, massive QGs at z ∼ 4 are likely to evolve into the most massive galaxies today. We find their size evolution with cosmic time in the form of . Their size growth is proportional to the square of stellar mass, indicating that size–stellar mass growth is driven by minor dry mergers.

Low-redshift, spatially resolved Lyman continuum (LyC) emitters allow us to clarify the processes for LyC escape from these starburst galaxies. We use Hubble Space Telescope (HST) WFC3 and ACS imaging of the confirmed low-redshift LyC emitter Tol 1247−232 to study the ionization structure of the gas and its relation to the ionizing star clusters. We perform ionization parameter mapping (IPM) using [O iii] λλ4959, 5007 and [O ii] λ3727 imaging as the high- and low-ionization tracers, revealing broad, large-scale, optically thin regions originating from the center and reaching the outskirts of the galaxy, consistent with LyC escape. We carry out stellar population synthesis modeling of the 26 brightest clusters using our HST photometry. Combining these data with the nebular photometry, we find a global LyC escape fraction of f_esc = 0.12, with uncertainties also consistent with zero escape and all measured f_esc values for this galaxy. Our analysis suggests that, similar to other candidate LyC emitters, a two-stage starburst has taken place in this galaxy, with a 12 Myr old, massive central cluster likely having precleared regions in and around the center and the second generation of 2–4 Myr old clusters dominating the current ionization, including some escape from the galaxy.

Several nearby protoplanetary disks have been observed to display large-scale crescents in the (sub)millimeter dust continuum emission. One interpretation is that these structures correspond to anticyclonic vortices generated by the Rossby wave instability within the gaseous disk. Such vortices have local gas overdensities and are expected to concentrate dust particles with a Stokes number around unity. This process might catalyze the formation of planetesimals. Whereas recent observations showed that dust crescents are indeed regions where millimeter-size particles have abnormally high concentration relative to the gas and smaller grains, no observations have yet shown that the gas within the crescent region counterrotates with respect to the protoplanetary disk. Here we investigate the detectability of anticyclonic features through measurement of the line-of-sight component of the gas velocity obtained with ALMA. We carry out 2D hydrodynamic simulations and 3D radiative transfer calculations of a protoplanetary disk characterized by a vortex created by the tidal interaction with a massive planet. As a case study, the disk parameters are chosen to mimic the IRS 48 system, which has the most prominent crescent observed to date. We generate synthetic ALMA observations of both the dust continuum and ¹²CO emission around the frequency of 345 GHz. We find that the anticyclonic features of the vortex are weak but can be detected if both the source and the observational setup are properly chosen. We provide a recipe for maximizing the probability of detecting such vortex features and present an analysis procedure to infer their kinematic properties.

The X-ray signal from hydrogen-rich supernovae (SNe II) in the first tens to hundreds of days after the shock breakout encodes important information about the circumstellar material (CSM) surrounding their progenitors before explosion. In this study, we describe a way to generate SN II X-ray light curves from hydrodynamical simulations performed with the code Athena++, using the X-ray package XSPEC. In addition, we employ a radiation diffusion hydrodynamic code SNEC to generate the optical light curves in different bands. In this numerical setup, we model the X-ray and optical emission from a set of progenitor models, consisting of either two (red supergiant + low-density, steady wind) or three (red supergiant + dense CSM + low-density, steady wind) components. We vary the density in the wind and the slope in the CSM to see how these parameters influence the resulting X-ray and optical light curves. Among our models, we identify one that is able to roughly reproduce both optical and X-ray data of the well-observed SN 2013ej. In order to achieve this, the slope of the dense CSM in this model should be steeper than the one of a steady wind (ρ ∝ r⁻²) and closer to ρ ∝ r⁻⁵. On the other hand, we show that too-steep and extended CSM profiles may produce excessive X-ray emission in the first few tens of days, up to a few orders of magnitude larger than observed. We conclude that the ability to reproduce the observed X-ray signal from SNe II together with their optical light curves is crucial in establishing the validity of different CSM models.

Short-period binary star systems dissipate orbital energy through tidal interactions that lead to tighter, more circular orbits. Using a sample of binaries with subgiant, giant, and red clump star members that is nearly an order of magnitude larger than that of Verbunt & Phinney, we reexamine predictions for tidal circularization of binary stars with evolved members. We confirm that binary star systems in our sample predicted to have circular orbits (using equilibrium tide theory) generally have negligible measured eccentricities. At a fixed stellar mass, the transition period is correlated with the surface gravity (i.e., size) of the evolved member, indicating that the circularization timescale must be shorter than the evolutionary timescale along the giant branch. A few exceptions to the conclusions above are mentioned in the discussion. Some of these exceptions are likely systems in which the spectrum of the secondary biases the radial velocity measurements, but four appear to be genuine, short-period, moderate-eccentricity systems.

The gravitational wave event GW170817 associated with the short gamma-ray burst (GRB) 170817A confirms that binary neutron star (BNS) mergers act as one of the origins of short GRBs. The associated kilonova emission, radioactively powered by nucleosynthesized heavy elements, was also detected. Black hole–neutron star (BH–NS) mergers have also been argued to be a promising candidate for the origin of short GRBs and kilonovae. Numerical simulations show that the ejecta in BH–NS mergers is geometrically much more anisotropic than the BNS merger case. In this paper, we investigate observational signatures of kilonova emission from the anisotropic ejecta in BH–NS mergers. We find that a bump appears on the bolometric luminosity light curve due to the inhomogeneous mass distribution in the latitudinal direction. The decay slope of the single-band light curve becomes flatter and the spectrum also deviates from a single-temperature blackbody radiation spectrum due to the gradient in the velocity distribution of the ejecta. Future detection or nondetection of such signatures would be useful to test the mass ejection geometry in BH–NS mergers.

N103B is a Type Ia supernova remnant (SNR) in the Large Magellanic Cloud (LMC). We carried out new ¹²CO(J = 3–2) and ¹²CO(J = 1–0) observations using ASTE and ALMA. We have confirmed the existence of a giant molecular cloud at V_LSR ∼ 245 km s⁻¹ toward the southeast of the SNR using ASTE ¹²CO(J = 3–2) data at an angular resolution of ∼25″ (∼6 pc in the LMC). Using the ALMA ¹²CO(J = 1–0) data, we have spatially resolved CO clouds along the southeastern edge of the SNR with an angular resolution of ∼18 (∼0.4 pc in the LMC). The molecular clouds show an expanding gas motion in the position–velocity diagram with an expansion velocity of ∼5 km s⁻¹. The spatial extent of the expanding shell is roughly similar to that of the SNR. We also find tiny molecular clumps in the directions of optical nebula knots. We present a possible scenario that N103B exploded in the wind-bubble formed by the accretion winds from the progenitor system, and is now interacting with the dense gas wall. This is consistent with a single-degenerate scenario.

We report for the first time on the magnitude of the intrinsic [Fe/H] spread among 10 old globular clusters (GCs) of the Large Magellanic Cloud (LMC). Such spreads are merely observed in approximately 5% of the Milky Way GCs and recently gained more attention in theoretical models of GC evolution. We derived metallicities with a typical precision of 0.05 dex ≤ σ[Fe/H] ≤ 0.20 dex for an average of 14 red giant branch stars per GC from Strömgren photometry. The respective, metallicity-sensitive indices have been calibrated to precise and accurate high-dispersion spectroscopy. For all clusters, we found null [Fe/H] spreads with a typical uncertainty of 0.04 dex, with the possible exception of NGC 1786 that shows an intrinsic dispersion of 0.07 ± 0.04 dex. The mean, observed standard deviation of the derived metallicities for nearly 40% of our GC sample amounted to smaller than 0.05 dex. At present, we cannot exclude the fact that the remaining GCs also have intrinsic Fe-abundance variations in excess of 0.05 dex, but in order to significantly detect those, the measurement errors on individual [Fe/H]-values would need to be lowered to the 0.03–0.07 dex level. These findings suggest, along with those from ages and light element abundances, that the LMC GCs studied here are similar to the majority of Galactic GCs.

Both interplanetary space and Earth’s magnetosphere are populated by low-energy (≤300 keV) protons that are potentially able to scatter on the reflecting surface of the Wolter-I optics of X-ray focusing telescopes and reach the focal plane. This phenomenon, depending on the X-ray instrumentation, can dramatically increase the background level, reducing the sensitivity or, in the most extreme cases, compromising the observation itself. The use of a magnetic diverter, deflecting protons away from the field of view, requires a detailed characterization of their angular and energy distribution when exiting the mirror. We present the first end-to-end Geant4 simulation of proton scattering by X-ray optics and the consequent interaction with the diverter field and the X-ray detector assembly, selecting the ATHENA Wide Field Imager as a case study for the evaluation of the residual, soft-proton-induced background. We find that in the absence of a magnetic diverter, protons are indeed funneled toward the focal plane, with a focused non-X-ray background well above the level required by ATHENA science objectives (5 × 10⁻⁴ counts cm⁻² s⁻¹ keV⁻¹), for all the plasma regimes encountered in both L1 and L2 orbits. These results set the proton diverter as a mandatory shielding system on board the ATHENA mission and all high throughput X-ray telescopes operating in the interplanetary space. For a magnetic field computed to deflect 99% of the protons that would otherwise reach the WFI, Geant4 simulations show that this configuration, in the assumption of a uniform field, would efficiently shield the focal plane, yielding a residual background level of the order or below the requirement.

The collision of magnetic reconnection jets is studied by means of a three-dimensional numerical simulation at the kinetic scale, in the presence of a strong guide field. We show that turbulence develops due to the collision of jets, producing several current sheets in reconnection outflows, aligned with the guide field direction. The turbulence is mainly two-dimensional, with stronger gradients in the plane perpendicular to the guide field and low wave-like activity in the parallel direction. First, we provide a numerical method to isolate the central turbulent region. Second, we analyze the spatial second-order structure function and prove that turbulence is confined in this region. Finally, we compute local magnetic and electric frequency spectra, finding a trend in the subion range that differs from typical cases for which the Taylor hypothesis is valid, as well as wave activity in the range between ion and electron cyclotron frequencies. Our results are relevant to understand observed collisions of reconnection jets in space plasmas.

A possible surface type that may form in the environments of M-dwarf planets is sodium chloride dihydrate, or “hydrohalite” (NaCl · 2H₂O), which can precipitate in bare sea ice at low temperatures. Unlike salt-free water ice, hydrohalite is highly reflective in the near-infrared, where M-dwarf stars emit strongly, making the effect of the interaction between hydrohalite and the M-dwarf spectral energy distribution necessary to quantify. We carried out the first exploration of the climatic effect of hydrohalite-induced salt-albedo feedback on extrasolar planets, using a three-dimensional global climate model. Under fixed CO₂ conditions, rapidly rotating habitable-zone M-dwarf planets receiving 65% or less of the modern solar constant from their host stars exhibit cooler temperatures when an albedo parameterization for hydrohalite is included in climate simulations, compared to simulations without such a parameterization. Differences in global mean surface temperature with and without this parameterization increase as the instellation is lowered, which may increase CO₂ build-up requirements for habitable conditions on planets with active carbon cycles. Synchronously rotating habitable-zone M-dwarf planets appear susceptible to salt-albedo feedback at higher levels of instellation (90% or less of the modern solar constant) than planets with Earth-like rotation periods, due to their cooler minimum dayside temperatures. These instellation levels where hydrohalite seems most relevant correspond to several recently discovered potentially habitable M-dwarf planets, including Proxima Centauri b, TRAPPIST-1e, and LHS 1140b, making an albedo parameterization for hydrohalite of immediate importance in future climate simulations.

We study the mass–richness relation of 116 spectroscopically confirmed massive clusters at 0.4 < z < 2 by mining the Spitzer archive. We homogeneously measure the richness at 4.5 μm for our cluster sample within a fixed aperture of 2′ radius and above a fixed brightness threshold, making appropriate corrections for both background galaxies and foreground stars. We have two subsamples, those which have (a) literature X-ray luminosities and (b) literature Sunyaev–Zel’dovich effect masses. For the X-ray subsample we re-derive masses adopting the most recent calibrations. We then calibrate an empirical mass–richness relation for the combined sample spanning more than one decade in cluster mass and find the associated uncertainties in mass at fixed richness to be ±0.25 dex. We study the dependence of the scatter of this relation with galaxy concentration, defined as the ratio between richness measured within an aperture radius of 1 and 2 arcmin. We find that at fixed aperture radius the scatter increases for clusters with higher concentrations. We study the dependence of our richness estimates with depth of the 4.5 μm imaging data and find that reaching a depth of at least [4.5] = 21 AB mag is sufficient to derive reasonable mass estimates. We discuss the possible extension of our method to the mid-infrared WISE All Sky Survey data and the application of our results to the Euclid mission. This technique makes richness-based cluster mass estimates available for large samples of clusters at very low observational cost.

The majority of hydrogen in the interstellar medium (ISM) is in atomic form. The transition from atoms to molecules and, in particular, the formation of the H₂ molecule, is a key step in cosmic structure formation en route to stars. Quantifying H₂ formation in space is difficult, due to the confusion in the emission of atomic hydrogen (H i) and the lack of a H₂ signal from the cold ISM. Here we present the discovery of a rare, isolated dark cloud currently undergoing H₂ formation, as evidenced by a prominent “ring” of H i self-absorption. Through a combined analysis of H i narrow self-absorption, CO emission, dust emission, and extinction, we directly measured, for the first time, the [H i]/[H₂] abundance varying from 2% to 0.2%, within one region. These measured H i abundances are orders of magnitude higher than usually assumed initial conditions for protoplanetary disk models. None of the fast cloud formation model could produce such low atomic hydrogen abundance. We derived a cloud formation timescale of ∼6 × 10⁶ years, consistent with the global Galactic star formation rate, and favoring the classical star formation picture over fast star formation models. Our measurements also help constrain the H₂ formation rate, under various ISM conditions.

The nearby cluster Abell 1795 is used as a testbed to examine whether hot gas in cluster galaxies is stripped by the ram pressure of the intracluster medium (ICM). The expected X-ray emission in and around Abell 1795 galaxies is likely dominated by the ICM, low-mass X-ray binaries, active galactic nuclei, and hot gas halos. In order to constrain these components, we use archival Chandra X-ray Observatory and Sloan Digital Sky Survey observations of Abell 1795 and identify 58 massive (M_⋆ > 10¹⁰M_⊙) spectroscopic cluster members within 5′ of the Chandra optical axis. X-ray images at 0.5–1.5 and 4–8 keV were created for each cluster member and then stacked into two clustercentric radius bins: inner (0.25 < R_clust/R₅₀₀ < 1) and outer (1 < R_clust/R₅₀₀ < 2.5). Surface brightness profiles of inner and outer cluster members are fit using Markov chain Monte Carlo sampling in order to generate model parameters and measure the 0.5–1.5 keV luminosities of each model component. Leveraging effective total Chandra exposure times of 3.4 and 1.7 Ms for inner and outer cluster members, respectively, we report the detection of hot gas halos, in a statistical sense, around outer cluster members. Outer members have 0.5–1.5 keV hot halo luminosities () that are six times larger than the upper limit for inner cluster members (L_X < 1.3 × 10³⁹ erg s⁻¹). This result suggests that the ICM is removing hot gas from the halos of Abell 1795 members as they fall into the cluster.

Structure imprinted in foreground extragalactic point sources by ionospheric refraction has the potential to contaminate Epoch of Reionization (EoR) power spectra of the 21 cm emission line of neutral hydrogen. The alteration of the spatial and spectral structure of foreground measurements due to total electron content gradients in the ionosphere creates a departure from the expected sky signal. We present a general framework for understanding the signatures of ionospheric behavior in the 2D neutral hydrogen power spectrum measured by a low-frequency radio interferometer. Two primary classes of ionospheric behavior are considered, corresponding to dominant modes observed in Murchison Widefield Array (MWA) EoR data, namely, anisotropic structured wave behavior and isotropic turbulence. Analytic predictions for power spectrum bias due to this contamination are computed and compared with simulations. We then apply the ionospheric metric described in Jordan et al. to study the impact of ionospheric structure on MWA data, by dividing MWA EoR data sets into classes with good and poor ionospheric conditions, using sets of matched 30-minute observations from 2014 September. The results are compared with the analytic and simulated predictions, demonstrating the observed bias in the power spectrum when the ionosphere is active (displays coherent structures or isotropic turbulence). The analysis demonstrates that unless ionospheric activity can be quantified and corrected, active data should not be included in EoR analysis in order to avoid systematic biases in cosmological power spectra. When data are corrected with a model formed from the calibration information, bias reduces below the expected 21 cm signal level. Data are considered “quiet” when the median measured source position offsets are less than 10–15.

Magnetic flux ropes are commonly observed throughout the heliosphere, and recent studies suggest that interacting flux ropes are associated with some energetic particle events. In this work, we carry out 2D particle-in-cell (PIC) simulations to study the coalescence of two magnetic flux ropes (or magnetic islands), and the subsequent plasma energization processes. The simulations are initialized with two magnetic islands embedded in a reconnecting current sheet. The two islands collide and eventually merge into a single island. Particles are accelerated during this process as the magnetic energy is released and converted to the plasma energy, including bulk kinetic energy increase by the ideal electric field, and thermal energy increase by the fluid compression and the nonideal electric field. We find that contributions from these different energization mechanisms are all important and comparable with each other. Fluid shear and a nongyrotropic pressure tensor also contribute to the energy conversion process. For simulations with different box sizes ranging from L_x ∼ 25–100d_i and ion-to-electron mass ratios m_i/m_e = 25, 100, and 400, we find that the general evolution is qualitatively the same for all runs, and the energization depends only weakly on either the system size or the mass ratio. The results may help us understand plasma energization in solar and heliospheric environments.

Atmospheric chemistry models have shown that molecular oxygen can build up in CO₂-dominated atmospheres on potentially habitable exoplanets without input of life. Existing models typically assume a surface pressure of 1 bar. Here we present model scenarios of CO₂-dominated atmospheres with the surface pressure ranging from 0.1 to 10 bars, while keeping the surface temperature at 288 K. We use a one-dimensional photochemistry model to calculate the abundance of O₂ and other key species, for outgassing rates ranging from a Venus-like volcanic activity up to 20 times Earth-like activity. The model maintains the redox balance of the atmosphere and the ocean, and includes the pressure dependency of outgassing on the surface pressure. Our calculations show that the surface pressure is a controlling parameter in the photochemical stability and oxygen buildup of CO₂-dominated atmospheres. The mixing ratio of O₂ monotonically decreases as the surface pressure increases at very high outgassing rates, whereas it increases as the surface pressure increases at lower-than-Earth outgassing rates. Abiotic O₂ can only build up to the detectable level, defined as 10⁻³ in volume mixing ratio, in 10-bar atmospheres with the Venus-like volcanic activity rate and the reduced outgassing rate of H₂ due to the high surface pressure. Our results support the search for biological activities and habitability via atmospheric O₂ on terrestrial planets in the habitable zone of Sun-like stars.

The gravitational waves from the first binary neutron star merger, GW170817, were accompanied by a multiwavelength electromagnetic counterpart, from γ-rays to radio. The accompanying γ-rays seem at first to confirm the association of mergers with short gamma-ray bursts (sGRBs). The common interpretation was that we see an emission from an sGRB jet seen off-axis. However, a closer examination of the subluminous γ-rays and the peculiar radio afterglow was inconsistent with this simple interpretation. Here we present results of 3D and 2D numerical simulations that follow the hydrodynamics and emission of the outflow from a neutron star merger, form its ejection and up to its deceleration by the circum-merger medium. Our results show that the current set of γ-rays, X-rays, and radio observations can be explained by the emission from a mildly relativistic cocoon material (Lorentz factor ∼2–5) that was formed while a jet propagated through the material ejected during the merger. The γ-rays are generated when the cocoon breaks out from the engulfing ejecta, while the afterglow is produced by interaction of the cocoon matter with the interstellar medium. The strong early UV/optical signal may be a Lorentz-boosted macronova/kilonova. The fate of the jet itself is currently unknown, but our full-electromagnetic (EM) models define a path to resolving between successful and choked jet scenarios, outputting coupled predictions for the image size, morphology, observed time-dependent polarization, and light-curve behavior from radio to X-ray. The predictive power of these models will prove key in interpreting the ongoing multifaceted observations of this unprecedented event.

According to LCDM theory, hierarchical evolution occurs on all mass scales, implying that satellites of the Milky Way should also have companions. The recent discovery of ultra-faint dwarf galaxy candidates in close proximity to the Magellanic Clouds provides an opportunity to test this theory. We present proper motion (PM) measurements for 13 of the 32 new dwarf galaxy candidates using Gaia data release 2. All 13 also have radial velocity measurements. We compare the measured 3D velocities of these dwarfs to those expected at the corresponding distance and location for the debris of a Large Magellanic Cloud (LMC) analog in a cosmological numerical simulation. We conclude that four of these galaxies (Hor1, Car2, Car3, and Hyi1) have come in with the Magellanic Clouds, constituting the first confirmation of the type of satellite infall predicted by LCDM. Ret2, Tuc2, and Gru1 have velocity components that are not consistent within 3σ of our predictions and are therefore less favorable. Hya2 and Dra2 could be associated with the LMC and merit further attention. We rule out Tuc3, Cra2, Tri2, and Aqu2 as potential members. Of the dwarfs without measured PMs, five of them are deemed unlikely on the basis of their positions and distances alone being too far from the orbital plane expected for LMC debris (Eri2, Ind2, Cet2, Cet3, and Vir1). For the remaining sample, we use the simulation to predict PMs and radial velocities, finding that Phx2 has an overdensity of stars in DR2 consistent with this PM prediction.

There has been suggestive evidence of intermediate-mass black holes (IMBHs; 10³⁻⁵M_⊙) existing in some globular clusters (GCs) and dwarf galaxies, but IMBHs as a population remain elusive. As a main-sequence star passes too close by an IMBH it might be tidally captured and disrupted. We study the long-term accretion and observational consequence of such tidal disruption events. The disruption radius is hundreds to thousands of the BH’s Schwarzschild radius, so the circularization of the falling-back debris stream is very inefficient due to weak general relativity effects. Due to this and a high mass fallback rate, the bound debris initially goes through a ∼10 yr long super-Eddington accretion phase. The photospheric emission of the outflow ejected during this phase dominates the observable radiation and peaks in the UV/optical bands with a luminosity of . After the accretion rate drops below the Eddington rate, the bolometric luminosity follows the conventional t^−5/3 power-law decay, and X-rays from the inner accretion disk start to be seen. Modeling the newly reported IMBH tidal disruption event candidate 3XMM J2150-0551, we find a general consistency between the data and predictions. The search for these luminous, long-term events in GCs and nearby dwarf galaxies could unveil the IMBH population.

Luminous and ultra-luminous infrared galaxies ((U)LIRGs) are rare today but are increasingly abundant at high redshifts. They are believed to be dusty starbursts, and hence should have high rates of supernovae (multiple events per year). Due to their extremely dusty environment, however, such supernovae could only be detected in rest-frame infrared and longer wavelengths, where our current facilities lack the capability of finding them individually beyond the local universe. We propose a new technique for higher redshifts, which is to search for the presence of supernovae through the variability of the integrated rest-frame infrared light of the entire hosts. We present a pilot study to assess the feasibility of this technique. We exploit a unique region, the “IRAC Dark Field” (IDF), that the Spitzer Space Telescope has observed for more than 14 years in 3–5 μm. The IDF also has deep far-infrared data (200–550 μm) from the Herschel Space Observatory that allow us to select high-redshift (U)LIRGs. We obtain a sample of (U)LIRGs that have secure optical counterparts, and examine their light curves in 3–5 μm. While the variabilities could also be caused by AGNs, we show that such contaminations can be identified. We present two cases where the distinct features in their light curves are consistent with multiple supernovae overlapping in time. Searching for supernovae this way will be relevant to the James Webb Space Telescope (JWST) to probe high-redshift (U)LIRGs into their nuclear regions where JWST will be limited by its resolution.

Observations with the Parkes radio telescope of the eclipsing millisecond binary pulsar PSR J1748−2446A, which is in the globular cluster Terzan 5, are presented. These include the first observations of this pulsar in the 3 GHz frequency band, along with simultaneous observations in the 700 MHz band, and new observations around 1400 MHz. We show that the pulsar signal is not eclipsed in the 3 GHz band and observe the known eclipses in the lower-frequency bands. We find that the observed pulse signal becomes depolarized during particular orbital phases and postulate that this depolarization occurs because of rotation-measure fluctuations resulting from turbulence in the stellar wind responsible for the eclipses.

The Wide Field InfraRed Survey Telescope (WFIRST) was the highest-ranked large space-based mission of the 2010 New Worlds, New Horizons decadal survey. It is now a NASA mission in formulation with a planned launch in the mid 2020s. A primary mission objective is to precisely constrain the nature of dark energy through multiple probes, including Type Ia supernovae (SN Ia). Here, we present the first realistic simulations of the WFIRST SN survey based on current hardware specifications and using open-source tools. We simulate SN light curves and spectra as viewed by the WFIRST wide-field channel (WFC) imager and integral field channel (IFC) spectrometer, respectively. We examine 11 survey strategies with different time allocations between the WFC and IFC, two of which are based upon the strategy described by the WFIRST Science Definition Team, which measures SN distances exclusively from IFC data. We propagate statistical and, crucially, systematic uncertainties to predict the Dark Energy Task Force figure of merit (FoM) for each strategy. Of the strategies investigated, we find the most successful to be WFC focused. However, further work in constraining systematics is required to fully optimize the use of the IFC. Even without improvements to other cosmological probes, the WFIRST SN survey has the potential to increase the FoM by more than an order of magnitude from the current values. Although the survey strategies presented here have not been fully optimized, these initial investigations are an important step in the development of the final hardware design and implementation of the WFIRST mission.

Luminous stars in background galaxies straddling the lensing caustic of a foreground galaxy cluster can be individually detected due to extreme magnification factors of ∼10²–10³, as recently observed in deep HST images. We propose a direct method to probe the presence of dark matter subhalos in galaxy clusters by measuring the astrometric perturbation they induce on the image positions of magnified stars or bright clumps: lensing by subhalos breaks the symmetry of a smooth critical curve, traced by the midpoints of close image pairs. For the giant arc at z = 0.725 behind the lensing cluster Abell 370 at z = 0.375, a promising target for detecting image pairs of stars, we find that subhalos of masses in the range of 10⁶–10⁸M_⊙ with the abundance predicted in the cold dark matter theory should typically imprint astrometric distortions at the level of 20–80 mas. We estimate that ∼10 hr integrations with JWST at ∼1–3 μm may uncover several magnified stars whose image doublets will reveal the subhalo-induced structures of the critical curve. This method can probe a dynamic range in the subhalo-to-cluster halo mass ratio of m/M ∼ 10⁻⁷–10⁻⁹, thereby placing new constraints on the nature of dark matter.

Filaments of the cosmic web have long been associated with the threadlike structures seen in galaxy redshift surveys. However, despite their baryon content being dominated by hot gas, these filaments have been an elusive target for X-ray observations. Recently, detections of filaments in very deep (2.4 Ms) observations with Chandra were reported around Abell 133 (z = 0.0559). To verify these claims, we conducted a multiobject spectrographic campaign on the Baade 6.5 m telescope around Abell 133; this resulted in a catalog of ∼3000 new redshift measurements, of which 254 are of galaxies near the cluster. We investigate the kinematic state of Abell 133 and identify the physical locations of filamentary structure in the galaxy distribution. Contrary to previous studies, we see no evidence that Abell 133 is dynamically disturbed; we reject the hypothesis that there is a kinematically distinct subgroup (p = 0.28) and find no velocity offset between the central galaxy and the cluster (). The spatial distribution of galaxies traces the X-ray filaments, as confirmed by angular cross-correlation with a significance of ∼5σ. A similar agreement is found in the angular density distribution, where two X-ray structures have corresponding galaxy enhancements. We also identify filaments in the large-scale structure of galaxies; these filaments approach the cluster from the direction the X-ray structures are seen. While more members between R₂₀₀ and 2 × R₂₀₀ are required to clarify which large-scale filaments connect to the X-ray gas, we argue that this is compelling evidence that the X-ray emission is indeed associated with cosmic filaments.

The amplitude of redshifted 21 cm fluctuations during the Epoch of Reionization (EoR) is expected to show a distinctive “rise and fall” behavior with decreasing redshift as reionization proceeds. On large scales (k ≲ 0.1 Mpc⁻¹) this can mostly be characterized by evolution in the product of the mean 21 cm brightness temperature and a bias factor, . This quantity evolves in a distinctive way that can help in determining the average ionization history of the intergalactic medium from upcoming 21 cm fluctuation data sets. Here we consider extracting using a combination of future redshifted 21 cm and [C ii] line-intensity mapping data sets. Our method exploits the dependence of the 21 cm–[C ii]–[C ii] cross-bispectrum on the shape of triangle configurations in Fourier space. This allows one to determine yet, importantly, is less sensitive to foreground contamination than the 21 cm auto-spectrum and so can provide a valuable cross-check. We compare the results of simulated bispectra with second-order perturbation theory: on the largest scales well probed by our simulations (k ∼ 0.05 Mpc⁻¹), the perturbative estimate of matches the true value to within 10% for . The perturbative formula is most accurate early in the EoR. We consider the 21 cm auto-bispectrum and show that this statistic may also be used to extract the 21 cm bias factor. Finally, we discuss the survey requirements for measuring the cross-bispectrum. Although we focus on the 21 cm–[C ii]–[C ii] bispectrum during reionization, our method may be of broader interest and can be applied to any two fields throughout cosmic history.

Previously we identified a new class of early galaxy that we estimate contributes up to 30% of the ionizing photons responsible for reionization. These are low-mass halos in the range  = 10^6.5–10⁸ that have been chemically enriched by supernova ejecta from prior Population III star formation. Despite their low star formation rates, these metal cooling halos (MCs) are significant sources of ionizing radiation, especially at the onset of reionization, owing to their high number density and ionizing escape fractions. Here we present a fully coupled radiation hydrodynamic simulation of reionization that includes these MCs, as well the more massive hydrogen atomic line cooling halos. Our method is novel: we perform halo finding in line with the radiation hydrodynamical simulation and assign escaping ionizing fluxes to halos using a probability distribution function (pdf) measured from the galaxy-resolving Renaissance Simulations. The pdf captures the mass dependence of the ionizing escape fraction, as well as the probability that a halo is actively forming stars. With MCs, reionization starts earlier than if only halos of 10⁸ and above are included; however, the redshift when reionization completes is only marginally affected, as this is driven by more massive galaxies. Because star formation is intermittent in MCs, the earliest phase of reionization exhibits a stochastic nature, with small H ii regions forming and recombining. Only later, once halos of mass ∼10⁹ and above begin to dominate the ionizing emissivity, does reionization proceed smoothly in the usual manner deduced from previous studies. This occurs at z ≈ 10 in our simulation.

Solar radio zebras belong to the most important radio fine structures used in diagnostics of solar flare plasmas. In the present paper, assuming the double plasma-resonance model of zebras, we study the relation between zebra-stripe frequencies and gyro-harmonic numbers. We artificially generated two possible types of zebras: the zebra with Sequence A and Sequence B, where an increase of the zebra-stripe frequency corresponds to decrease or increase of the gyro-harmonic number. Analyzing these ideal zebras, we found that the frequency ratios of the neighboring zebra stripes increase in zebras with Sequence A and decrease in zebras with Sequence B. This criterion and corresponding diagrams were applied for nine observed zebras. All these zebras were found to be with Sequence A. Then we checked and confirmed these results by using the new numerical method, where the gyro-harmonic numbers of the zebra stripes with the lowest frequency s₁ were also determined. We found that in all these zebras, the spatial scale of the magnetic field in the zebra-stripe sources was always shorter than that of the plasma density. Knowing the gyro-harmonic numbers and corresponding zebra-stripe frequencies, we determined the magnetic field and plasma density in zebra sources to be 0.84–37.31 G and 0.026 × 10¹⁰–16.03 × 10¹⁰ cm⁻³, respectively. Finally, we found that with increasing the gyro-harmonic number s₁, the ratio of perpendicular and parallel scales of the magnetic field and plasma density in the zebra-stripe sources also increases.

The HH 1 jet is a chain of knots extending out to ∼20″ from the VLA 1 source of the HH 1/2 system. Four epochs of [S ii] images obtained with the Hubble Space Telescope over a ∼20 yr period show that these knots have a time-evolving intensity as they travel away from the outflow source. We present an axisymmetric, gas-dynamic simulation of a two-sinusoidal-mode variable ejection velocity jet (including a treatment of the non-equilibrium ionization of the gas) from which we obtain predictions of the time evolution of the chain of knots close to the outflow source. Both the intensity versus position dependence (for the successive knots) and the time evolution of the [S ii] intensities of the individual knots obtained from the simulations agree in a very impressive way with the HH 1 jet observations. This is one of the most striking illustrations of the success of variable jet models at reproducing the observed properties of HH jets. Also, this work represents the first attempted comparison between models and observations of astrophysical jets with both time and spatial resolution.

The quasar lifetime t_Q is one of the most fundamental quantities for understanding quasar evolution and the growth of supermassive black holes (SMBHs), but remains uncertain by several orders of magnitude. In a recent study we uncovered a population of very young quasars (t_Q ≲ 10⁴–10⁵ yr), based on the sizes of their proximity zones, which are regions of enhanced Lyα forest transmission near the quasar resulting from its own ionizing radiation. The presence of such young objects poses significant challenges to models of SMBH formation, which already struggle to explain the existence of SMBHs (∼10⁹M_⊙) at such early cosmic epochs. We conduct the first comprehensive spectroscopic study of the youngest quasar known, SDSS J1335+3533 at z = 5.9012, whose lifetime is t_Q = 10^3.0±0.8 yr (95% confidence). A careful search of our deep optical and near-infrared spectra for H i and metal absorption lines allows us to convincingly exclude the possibility that its small proximity zone results from an associated absorption system rather than a short lifetime. From the Mg ii  emission line we measure a black hole mass , implying an Eddington ratio of —comparable to other co-eval quasars of similar luminosity. The only possible anomaly associated with SDSS J1335+3533's youth are its weak emission lines, but larger samples are needed to shed light on the potential causality. We discuss the implications of short lifetimes for various SMBH growth and formation scenarios, and argue that future observations of young quasars with the James Webb Space Telescope could distinguish between them.

The orbital properties of stars in the Milky Way disk are signatures of their formation, but they are also expected to change over time due to the dynamical evolution of the Galaxy. Stellar orbits can be quantified by the three dynamical actions, J_r, L_z, and J_z, which provide measures of the orbital eccentricity, guiding radius, and non-planarity, respectively. Changes in these dynamical actions over time reflect the strength and efficiency of the evolutionary processes that drive stellar redistributions. We examine how dynamical actions of stars are correlated with their age using two samples of stars with well-determined ages: 78 solar twin stars (with ages precise to ∼5%) and 4376 stars from the APOKASC2 sample (∼20%). We compute actions using spectroscopic radial velocities from previous surveys and parallax and proper motion measurements from Gaia DR2. We find weak gradients with significant scatter for all actions as a function of stellar age. These gradients and their associated variances provide strong constraints on the efficiency of the mechanisms that drive the redistribution of stellar orbits over time and demonstrate that actions are informative as to stellar age. However, the shallow action–age gradients combined with the large dispersion in each action at a given age render the prospect of age inference from orbits of individual stars bleak. Using the precision measurements of [Fe/H] and [α/Fe] we find that, similarly to our stellar age results, the dynamical actions afford little discriminating power between individual low- and high-α stars.

Jupiter-mass planets with large semimajor axes (a > 1.0 au) occur at a higher rate around evolved intermediate-mass stars. There is a pronounced paucity of close-in (a < 0.6 au), intermediate-period (5 < P < 100 days), low-mass (M_planet < 0.7M_Jup ) planets, known as the “Planet Desert.” Current radial velocity (RV) methods have yet to yield close-in, low-mass planets around these stars because the planetary signals could be hidden by the (5–10) m s⁻¹ RV variations caused by acoustic oscillations. We find that by implementing an observing strategy of taking three observations per night separated by an optimal Δt, which is a function of the oscillation periods and amplitudes, we can average over the stellar jitter and improve our sensitivity to low-mass planets. We find that Δt can be approximated using the stellar mass and radius given by the relationship Δt = 1.79 . We test our proposed method by injecting planets into well-sampled data of a subgiant star, γ Cep. We compare the fraction of planets recovered by our method to the fraction of planets recovered using current RV observational strategies. We find that our method decreases the rms of the stellar jitter due to acoustic oscillations by a factor of three over current single epoch observing strategies used for subgiant stars. Our observing strategy provides a means to test whether the Planet Desert extends to lower-mass planets.

Zirconium oxide (ZrO) is an important astrophysical molecule that defines the S-star classification class for cool giant stars. Accurate, empirical rovibronic energy levels, with associated labels and uncertainties, are reported for nine low-lying electronic states of the diatomic molecule. These 8088 empirical energy levels are determined using the Measured Active Rotational-Vibrational Energy Levels algorithm with 23,317 input assigned transition frequencies, 22,549 of which were validated during this study. A temperature-dependent partition function is presented alongside updated spectroscopic constants for the nine low-lying electronic states.

NGC 6791 is a unique stellar cluster, key to our understanding of both the multiple stellar population phenomenon and the evolution and assembly of the Galaxy. However, despite many investigations, its nature is still very controversial. Geisler et al. found evidence suggesting that it was the first open cluster to possess multiple populations, but several subsequent studies did not corroborate this. It has also been considered a member of the thin or thick disk or even the bulge, and either as an open or globular cluster or even the remnant of a dwarf galaxy. Here we present and discuss detailed abundances derived from high-resolution spectra obtained with UVES at VLT and HIRES at Keck of 17 evolved stars of this cluster. We obtained a mean [Fe/H] = +0.313 ± 0.005, in good agreement with recent estimates, and with no indication of star-to-star metallicity variation, as expected. We also did not find any variation in Na, in spite of having selected the very same stars as in Geisler et al., where an Na variation was claimed. This points to the presence of probable systematics in the lower-resolution spectra of this very high metallicity cluster analyzed in that work. In fact, we find no evidence for an intrinsic spread in any element, corroborating recent independent APOGEE data. The derived abundances indicate that NGC 6791 very likely formed in the Galactic bulge and that the proposed association with the thick disk is unlikely, despite its present Galactic location. We confirm the most recent hypothesis suggesting that the cluster could have formed in the bulge and radially migrated to its current location, which appears to be the best explanation for this intriguing object.

We present a multiwavelength analysis based on archival radio, optical, and X-ray data of the complex radio source 3C 196.1, whose host is the brightest cluster galaxy of a z = 0.198 cluster. HST data show Hα+[N ii] emission aligned with the jet 8.4 GHz radio emission. An Hα+[N ii] filament coincides with the brightest X-ray emission, the northern hotspot. Analysis of the X-ray and radio images reveals cavities located at galactic and cluster scales. The galactic-scale cavity is almost devoid of 8.4 GHz radio emission and the southwestern Hα+[N ii] emission is bounded (in projection) by this cavity. The outer cavity is cospatial with the peak of 147 MHz radio emission, and hence we interpret this depression in X-ray surface brightness as being caused by a buoyantly rising bubble originating from an active galactic nuclei outburst ∼280 Myr ago. A Chandra snapshot observation allowed us to constrain the physical parameters of the cluster, which has a cool core with a low central temperature ∼2.8 keV, low central entropy index ∼13 keV cm² and a short cooling time of ∼500 Myr, which is < 0.05 of the age of the universe at this redshift. By fitting jumps in the X-ray density, we found Mach numbers between 1.4 and 1.6, consistent with a shock origin. We also found compelling evidence of a past merger, indicated by a morphology reminiscent of gas sloshing in the X-ray residual image. Finally, we computed the pressures, enthalpies E_cav and jet powers P_jet associated with the cavities: E_cav ∼ 7 × 10⁵⁸ erg, P_jet ∼ 1.9 × 10⁴⁴ erg s⁻¹ for the inner cavity and E_cav ∼ 3 × 10⁶⁰ erg, P_jet ∼ 3.4 × 10⁴⁴ erg s⁻¹ for the outer cavity.

We study the mean tidal coherence of galaxy environments as a function of intrinsic luminosity determined by the absolute r-band magnitude. The tidal coherence of a galaxy environment is estimated as the cosine of the angle between two minor eigenvectors of the tidal field smoothed at the scales of 2 and 30 h⁻¹ Mpc, respectively, centered on each of the local galaxies from the Sloan Digital Sky Data Release 10. Creating four luminosity-selected samples of the Sloan galaxies, we control them to have identical density distributions in order to nullify the dominant effect of the local density. The samples containing more luminous wall and field galaxies were found to yield lower mean values of the tidal coherence, which is a trend that turns out to be robust against the variation of the smoothing scales. At a fixed morphology, the same trend is found for the late-type spiral and lenticular galaxies in both of the field and wall environments. The early-type spiral field galaxies show no significant dependence on the tidal coherence, while both of the least and most luminous elliptical wall galaxies are found to dwell in the regions with highest tidal coherence.

We report the results of a study aiming to detect signs of astrometric microlensing caused by an intermediate-mass black hole (IMBH) in the center of globular cluster M22 (NGC 6656). We used archival data from the Hubble Space Telescope (HST) taken between 1995 and 2014 to derive long-baseline astrometric time series for stars near the center of the cluster, using state-of-the-art software to extract high-precision astrometry from images. We then modeled these time-series data and compared microlensing model fits to simple linear proper-motion fits for each selected star. We find no evidence for astrometric microlensing in M22, in particular for bulge stars, which are much more likely to be lensed than cluster stars, due to the geometry of microlensing events. Although it is in principle possible to derive mass limits from such nondetections, we find that no useful mass limits can be derived for M22 with available data, mostly due to a 10 year gap in coverage. This is a result from difficulties with deriving precise enough astrometry from Wide Field Planetary Camera 2 observations for stars that do not fall on the planetary camera chip. However, this study shows that, for other HST instruments, we are able to reach precisions at which astrometric microlensing signals caused by IMBH in globular clusters could be detected and that this technique is a promising tool to make a first unambiguous detection of an IMBH.

Simultaneous XMM-Newton, NuSTAR, and HST observations, performed in 2017 March, of the nearby (z = 0.184) luminous quasar PDS 456 are presented. PDS 456 had a low X-ray flux compared to past observations, where the first of the two new XMM-Newton observations occurred during a pronounced dip in the X-ray light curve. The broadband X-ray spectrum is highly absorbed, attenuated by a soft X-ray absorber of column density N_H = 6 × 10²² cm⁻². An increase in obscuration occurs during the dip, which may be due to an X-ray eclipse. In addition, the persistent, fast Fe K outflow is present, with velocity components of −0.25c and −0.4c. The soft absorber is less ionized () compared to the iron K outflow () and is outflowing with a velocity of approximately −0.2c. A soft X-ray excess is present below 1 keV against the highly absorbed continuum and can be attributed to the re-emission from a wide-angle wind. The complex X-ray absorption present in PDS 456 suggests that the wind is inhomogeneous, whereby the soft X-ray absorber originates from denser clumps or filaments that may form farther out along the outflow. In contrast to the X-ray observations, the simultaneous UV spectrum of PDS 456 is largely unabsorbed, where only a very weak broad absorption trough is present blueward of Lyα, compared to a past observation in 2000 when the trough was significantly stronger. The relative weakness of the UV absorption may be due to the soft X-ray absorber being too highly ionized and almost transparent in the UV band.

In the faint short gamma-ray burst sGRB 170817A followed by the gravitational waves (GWs) from a merger of two neutron stars (NSs) GW170817, the spectral peak energy is too high to explain only by canonical off-axis emission. We investigate the off-axis appearance of an sGRB prompt emission scattered by a cocoon, which is produced through the jet–merger–ejecta interaction, with either subrelativistic or mildly relativistic velocities. We show that the observed properties of sGRB 170817A, in particular the high peak energy, can be consistently explained by the Thomson-scattered emission with a typical sGRB jet, together with its canonical off-axis emission, supporting that an NS–NS merger is the origin of sGRBs. The scattering occurs at ≲10¹⁰–10¹² cm not far from the central engine, implying the photospheric or internal shock origin of the sGRB prompt emission. The boundary between the jet and cocoon is sharp, which could be probed by future observations of off-axis afterglows. The scattering model predicts a distribution of the spectral peak energy that is similar to the observed one but with a cutoff around ∼MeV energy and its correlations with the luminosity, duration, and time lag from GWs, providing a way to distinguish it from alternative models.

The 2012 July 6 X1.1 flare at S13W59 and a halo coronal mass ejection (CME) with a space speed of ∼1900 km s⁻¹ were associated with type III and type II radio bursts. The metric-to-decametric type II radio burst extended down to ∼5 MHz. Simultaneously, a slowly drifting feature with a harmonic structure was observed by Wind and Solar Terrestrial Relations Observatory radio receivers around and below 1 MHz, above the strong type III radio burst at lower frequencies. The radio direction-finding analysis of this lower-frequency interplanetary (IP) type II radio burst indicates that the radio source was located near the nose and possibly toward the southern flank of the CME-driven shock. These results provide an independent confirmation of the previous suggestions that when the metric and IP type II bursts are overlapping, the lower-frequency IP type II radio burst originates near the shock nose, whereas the source of the higher-frequency metric type II burst is closer to the Sun in the shock flank region. These results further support the idea that the coronal and IP type II bursts are produced by the same CME-driven shock.

Lifetimes of protoplanetary disks (PPDs) are believed to be severely constrained by material-depleting mechanisms, including photoevaporative winds due to the host star radiation or external radiation sources. Most previous studies focused on exploring the role of the winds in the exposed PPDs with a single star; however, the evolution of the circumbinary disks with the photoevaporative winds driven by the host star radiation and external radiation sources deserves further exploration. In this study, we investigate the evolution of the circumbinary PPDs with the photoevaporative winds induced by the external far-ultraviolet radiation field. We show that this mass-loss process can significantly constrain properties of a circumbinary PPD, including its lifetime, mass, and radius. The lifetime of a circumbinary PPD, for instance, is found to be about a factor of two longer than a similar circumstellar disk, and this enhancement strongly depends on the viscosity parameter. But our model shows that viscosity dependence of the disk lifetime in the circumbinary case is more pronounced compared to the circumstellar case. We also show that dispersal of a circumbinary PPD occurs over a longer time as the disk temperature distribution becomes steeper. Our results also imply that a dead zone in a photoevaporative circumbinary PPD extends over a larger radial range in comparison to a circumstellar disk counterpart. We also show that our calculations are in agreement with the observed circumbinary PPDs orbiting equal-mass binaries.

In this paper, we employ a path-conservative HLLEM finite-volume method (FVM) to solve the solar wind magnetohydrodynamics (MHD) systems of extended generalized Lagrange multiplier (EGLM) formulation with Galilean invariance (G-EGLM MHD equations). The governing equations of single-fluid solar wind plasma MHD are advanced by using a one-step MUSCL-type time integration with the logarithmic spacetime reconstruction. The code is programmed in FORTRAN language with Message Passing Interface parallelization in spherical coordinates with a six-component grid system. Then, the large-scale solar coronal structures during Carrington rotations (CRs) 2048, 2069, 2097, and 2121 are simulated by inputting the line-of-sight magnetic field provided by the Global Oscillation Network Group (GONG). These four CRs belong to the declining, minimum, rising, and maximum phases of solar activity. Numerical results basically generate the observed characteristics of structured solar wind and thus show the code’s capability of simulating solar corona with complex magnetic topology.

We present Atacama Large Millimeter/submillimeter Array observations of multiple protostar systems in the Perseus molecular cloud, previously detected by the Karl G. Jansky Very Large Array. We observe 17 close (<600 au separation) multiple systems at 1.3 mm in continuum and five molecular lines (i.e., ¹²CO, C¹⁸O, ¹³CO, H₂CO, SO) to characterize the circum-multiple environments in which these systems are forming. We detect at least one component in the continuum for the 17 multiple systems. In three systems one companion is not detected, and for two systems the companions are unresolved at our observed resolution. We also detect circum-multiple dust emission toward eight out of nine Class 0 multiples. Circum-multiple dust emission is not detected toward any of the eight Class I multiples. Twelve systems are detected in the dense gas tracers toward their disks/inner envelopes. For these 12 systems, we use the dense gas observations to characterize their formation mechanism. The velocity gradients in the circum-multiple gas are clearly orthogonal to the outflow directions in eight out of the 12 systems, consistent with disk fragmentation. Moreover, only two systems with separations <200 au are inconsistent with disk fragmentation, in addition to the two widest systems (>500 au). Our results suggest that disk fragmentation via gravitational instability is an important formation mechanism for close multiple systems, but further statistics are needed to better determine the relative fraction formed via this method.

Galaxy formation simulations demonstrate that cosmic-ray (CR) feedback may be important in the launching of galactic-scale winds. CR protons dominate the bulk of the CR population, yet most observational constraints of CR feedback come from synchrotron emission of CR electrons. In this paper, we analyze 105 months of Fermi Gamma-ray Space Telescope observations of the Small Magellanic Cloud (SMC), with the aim of exploring CR feedback and transport in an external galaxy. We produce maps of the 2–300 GeV emission and detect statistically significant extended emission along the “Bar” and the “Wing,” where active star formation is occurring. Gamma-ray emission is not detected above 13 GeV, and we set stringent upper limits on the flux above this energy. We find the best fit to the gamma-ray spectrum is a single-component model with a power law of index and an exponential cutoff energy of  GeV. We assess the relative contribution of pulsars and CRs to the emission, and we find that pulsars may produce up to 10% of the flux above 100 MeV. Thus, we attribute most of the gamma-ray emission (based on its spectrum and morphology) to CR interactions with the interstellar medium. We show that the gamma-ray emissivity of the SMC is at least five times smaller than that of the Milky Way and that the SMC is far below the “calorimetric limit,” where all CR protons experience pion losses. We interpret these findings as evidence that CRs are escaping the SMC via advection and diffusion.

We present results from an analytical model for magnetic buoyancy and rotational instabilities in full spherical shell stellar tachoclines that include rotation, differential rotation of either solar or antisolar type, and toroidal field. We find that in all cases, for latitudes where the tachocline vertical rotation gradient is positive, toroidal fields can be stored against magnetic buoyancy up to a limit that is proportional to the square root of the local vertical rotation gradient. For solar magnitude differential rotation, this limit is about 9 kG. For fixed percentage differential rotation, storage capacity varies linearly with the rotation rate. Faster rotators with the same percentage differential rotation can store larger fields, and slower rotators can store smaller fields. At latitudes where the vertical rotation gradient is negative, vigorous magnetorotational instability for even weak (≪1 kG) toroidal fields prevents such storage. We infer from these results that for stars with solar-type latitudinal differential rotation (fast equator, slow poles), any starspots present should be found in low latitudes, similar to the Sun. For antisolar differential rotation, any spots present should be found in mid- and high latitudes, perhaps with a peak of occurrence near 55°. These results hopefully provide some guidance for making and interpreting observations of stellar activity and differential rotation on stars with convection zones and tachoclines.

We present the Lyα luminosity function (LF) derived from 34 Lyα emitters (LAEs) at z = 7.0 on an area of sky of 3.1 deg², the largest sample of those in the literature to date obtained at a redshift z ≳ 7. The LAE sample is compiled from deep large-area narrowband observations with Subaru conducted by the Cosmic HydrOgen Reionization Unveiled with Subaru (CHORUS) project. The z = 7.0 Lyα LF of our project is consistent with those of the previous Dark Energy Camera and Subaru studies at the bright and faint ends, respectively, while having uncertainties that are significantly smaller than those of the previous study results. Exploiting the small errors of our measurements, we investigate the shape of the Lyα LF from the faint end to the bright end. We find that the shape of the z = 7.0 Lyα LF can be explained by the steep slope of α ≃ −2.5 suggested at z = 6.6, and that there is no clear signature of a bright-end excess at z ≃ 7 claimed by the previous work, which was thought to be created by the ionized bubbles around bright LAEs, whose Lyα photons could easily escape from the partly neutral intergalactic medium (IGM) at z ≃ 7. We estimate the Lyα luminosity densities (LDs) with Lyα LFs at z ≃ 6–8 given by our studies and the previous ones, and compare the evolution of the UV-continuum LD estimated with dropouts. The Lyα LD monotonically decreases from z ∼ 6 to 8, and evolves more strongly than the UV-continuum LD, which is indicative of the Lyα damping wing absorption of the IGM toward the heart of the reionization epoch.

The slowly pulsating B-type (SPB) stars are the upper main-sequence stars on the HR diagram. Their oscillations are high-order, low-degree g-mode and can be used to probe the structure of the radiative zone near the outer boundary of the convective core and constrain the chemical mixing in stellar interiors. In SPB stars, the period spacing periodically varies with periods. It has been regarded as a signature of the chemical composition gradient beyond the convective core. Based on theoretical calculations, we find that the variation frequency of the period spacings (f_ΔP) is related to the width of the μ-gradient region on the buoyancy radius (Λ_μ) with the relation of f_ΔP ∼ 0.5Λ_μ. This indicates that the variation frequency f_ΔP is sensitive to the central hydrogen mass fraction X_C (i.e., the evolutionary status). Finally, we find that the variation frequency f_ΔP and the means of the period spacings can be used to construct a new C–D-like diagram (f_ΔP versus ), which can be used to roughly decide the stellar evolutionary stages and to approximately determine stellar mass for SPB stars.

We used the Atacama Large Millimeter/Submillimeter Array (ALMA) to map the CO(3–2) and [C i](1–0) lines, as well as their underlying continuum emission, from the central ∼200 pc region of the Circinus galaxy that hosts the nearest type 2 Seyfert-class active galactic nucleus (AGN), with a spatial resolution of ∼6–15 pc. The lines and continuum-emitting regions consist of a circumnuclear disk (CND; 74 pc × 34 pc) and spiral arms. The distribution of the continuum emission revealed a temperature-dependent dust geometry and possibly polar dust elongation in the torus region. The molecular mass of the CND is , with a beam-averaged H₂ column density of ∼5 × 10²³ cm⁻² toward the AGN position, which contributes significantly to the nuclear obscuration. The [C i](1–0)/CO(3–2) ratio at the AGN position is unusually high, suggesting an X-ray-dominated region-type chemistry. We decomposed the observed velocity fields into rotational and dispersion components, and revealed a multiphase dynamic nature in the r ≲ 10 pc torus region, i.e., the diffuse atomic gas is more spatially extended along the vertical direction of the disk than the dense molecular gas. Through comparisons with our model predictions based on the radiation-driven fountain scheme, we indicate that atomic outflows are the driver of the geometrical thickness of the atomic disk. This supports the validity of the radiation-driven fountain scheme in the vicinity of this AGN, which would explain the longstanding mystery of the physical origin of the AGN torus.

We investigate the properties of the ionized gas irradiated by an active galactic nucleus (AGN) based on our “radiation-driven fountain” model for the nearest type-2 Seyfert galaxy, the Circinus galaxy. We conducted “quasi-three-dimensional” spectral analysis using the spectral synthesis code Cloudy and obtained the surface brightness distributions of lines, such as Hα, Hβ, [O iii], [N ii], and [S ii] for the central 16 pc region. The ionized regions observed based on these lines show a conical morphology around the rotation axis, even if we do not phenomenologically postulate the presence of an optically thick “torus.” This region also shows non-uniform internal structures, reflecting the inhomogeneous structure of fountain flows. Using ionization diagnostic diagrams, we investigated the spectral properties of the ionized gas. The diagrams based on the line ratios of [N ii]/Hα and [S ii]/Hα show that most regions of the cone have the same properties as those in the narrow line regions (NLRs) in AGNs, whereas using [O i]/Hα, the central 10 pc regions are rather LINER-like. The gas density, temperature, and ionizing parameter in regions identified as “NLR” are typically n ∼ 300–1500 cm⁻³, T ∼ (1–3) × 10⁴ K, and U ∼ 0.01, respectively. The morphology and [O iii] intensity are similar to the base of the observed [O iii] cone in the Circinus galaxy, implying some physical connections with the origin of the ∼100 pc scale NLR.

Probing the speed of light is an important test of general relativity, but the measurements of c using objects in the distant universe have been almost completely unexplored. In this paper, we propose an idea to use the multiple measurements of galactic-scale strong gravitational lensing systems with Type Ia supernovae acting as background sources to estimate the speed of light. This provides an original method to measure the speed of light using objects located at different redshifts that emitted their light in a distant past. Moreover, we predict that strongly lensed Type Ia supernovae observed by the Large Synoptic Survey Telescope (LSST) would produce robust constraints on Δc/c at the level of 10⁻³. We also discuss whether future surveys such as LSST may succeed in detecting any hypothetical variation of c predicted by theories in which fundamental constants have a dynamical nature.

Radio observations grant access to a wide range of physical processes through different emission mechanisms. These processes range from thermal and quiescent to eruptive phenomena, such as shock waves and particle beams. We present a new synthetic radio imaging tool that calculates and visualizes the bremsstrahlung radio emission. This tool works concurrently with state-of-the-art magnetohydrodynamic simulations of the solar corona using the code Block-Adaptive Tree Solarwind Roe Upwind Scheme (BATS-R-US). Our model produces results that are in good agreement with both high- and low-frequency observations of the solar disk. In this study, a ray-tracing algorithm is used, and the radio intensity is computed along the actual curved ray trajectories. We illustrate the importance of refraction in locating the radio-emitting source by comparison of the radio imaging illustrations when the line of sight is considered instead of the refracted paths. We are planning to incorporate nonthermal radio emission mechanisms in a future version of the radio imaging tool.

In this paper, we revisit the scenario that an internal gradual magnetic dissipation taking place within the wind from a newborn millisecond magnetar can be responsible for gamma-ray burst (GRB) production. We show that a combination of two emission components in this model, i.e., the photospheric emission from the wind and the synchrotron radiation within the magnetic reconnection region, can give a reasonable fit to the observed spectrum of the prompt emission phase of GRB 160804A. We obtain the physical parameters through a Monte Carlo procedure and deduce the initial spin period and magnetic field of the central magnetar. Furthermore, the independent afterglow fitting analysis gives a consistent result, adding great credibility to this scenario. In addition, we predict a subclass of GRBs from such Magnetar wind Internal Gradual MAgnetic Dissipation (abbreviated as “MIGMAD bursts”) that have several distinctive properties.

The mechanism for producing gamma-ray quasi-periodic oscillation (QPO) in blazars is unknown. One possibility is the geometric model, in which without the need for intrinsic quasi-periodic variation, the relativistic Doppler factor changes periodically, resulting in observed gamma-ray QPO. We propose a method to test this geometric model. We analyze the Fermi-LAT data of PG 1553+113 spanning from 2008 August until 2018 February. According to 29 four-month average spectral energy distributions in the energy range of 0.1–300 GeV, we split the Fermi-LAT energy range into three bands: 0.1–1 GeV, 1–10 GeV, and 10–300 GeV. The spectrum in each energy range can be successfully fitted by a power law. The light curves and photon indices in the three energy ranges are obtained. Then, light curves in three narrow energy ranges, i.e., 0.2–0.5 GeV, 2–5 GeV, and 20–40 GeV, are constructed, and the relative variability amplitudes in the three narrow energy ranges are calculated. A discrete-correlation analysis is performed for the light curves. Our results indicate that (i) the light curves in the different energy ranges follow the same pattern showed in the light curve above 0.1 GeV; (ii) the three groups of photon indices in the energy ranges of 0.1–1 GeV, 1–10 GeV, and 10–300 GeV keep nearly constant; and (iii) the ratio between relative variability amplitudes in different narrow energy ranges are equal (within their errors) to the prediction by the Doppler effect. Our results support the scenario of the relativistic boost producing the gamma-ray QPO for PG 1553+113.

The distribution of a pure condensible planetary atmosphere in equilibrium with a surface reservoir is revisited, employing the energy budget of the climate system and emphasizing the atmospheric horizontal latent heat transport. This configuration is applicable to icy solar system bodies such as Triton, as well as a range of possible exoplanet atmospheres, including water or CO₂ iceballs or ocean worlds, and lava planets with mineral vapor atmospheres. Climate regimes for slowly rotating planets with the hotspot near the substellar point and for rapidly rotating planets with a warm equatorial belt are both treated. A nondimensional parameter controlling the fractional variation of the surface pressure is derived; it measures whether the pure condensible atmosphere is global or localized. The global pure condensible atmosphere with the nondimensional parameter much less than order of unity is maintained by the strong horizontal latent heat transport associated with an “evaporation/sublimation-driven flow” from warm to cold places that compensates for the incoming differential radiative forcing. We show that the variation of surface temperature can be estimated in terms of this nondimensional parameter if it is not too large. In the case of a pure water-vapor atmosphere with an ice or liquid surface, we show that the atmosphere is thick enough to maintain nearly isothermal surface conditions even when the substellar surface temperature is around the freezing point. Finally, it is proposed that the evaporation/sublimation-driven flow regime for global atmospheres could be detected via its effect on the inhomogeneous distribution of minor noncondensible components in the atmosphere.

HESS J1640-465 is an extended TeV γ-ray source, and whether its γ-ray emission is from the shell of a supernova remnant (SNR) or a pulsar wind nebula (PWN) is still under debate. We reanalyze the GeV γ-ray data in the field of HESS J1640-465 using eight years of Pass 8 data recorded by the Fermi Large Area Telescope. An extended GeV γ-ray source positionally coincident with HESS J1640-465 is found. Its photon spectrum can be described by a power law with an index of 1.42 ± 0.19 in the energy range of 10–500 GeV and smoothly connects with the TeV spectrum of HESS J1640-465. The broadband spectrum of HESS J1640-465 can be well fit by a leptonic model with a broken power-law spectrum of electrons with an exponential cut off at ∼300 TeV. The spectral properties of HESS J1640-465 are broadly consistent with the characteristics of other sources identified as PWNe, such as the correlations between high-energy luminosity ratios and the physical parameters of pulsar, including the spin-down luminosity and the characteristic age τ_c. All of these pieces of evidence support that the γ-ray emission of HESS J1640-465 may originate from the PWN powered by PSR J1640-4631 rather than the shell of the SNR G338.3-0.0.

We compare ultraviolet (UV) and optical colors of a sample of 29 type Ia supernovae (SNe Ia) observed with the Swift satellite’s UltraViolet Optical Telescope with theoretical models of an asymmetric explosion viewed from different angles from Kasen & Plewa. This includes mid-UV (1600–2700 Å; uvw2 and uvm2) and near-UV (2700–4000 Å; uvw1 and u) filters. We find the observed colors to be redder than the model predictions, and that these offsets are unlikely to be caused by dust reddening. We confirm that high-velocity SNe Ia have red UV-optical observed colors. After correcting the colors for dust reddening by assuming a constant b − v color, we find no correlation between the uvw1 − v or u − v colors and the ejecta velocities for 25 SNe Ia with published velocities and/or spectra. When assuming an optical color–velocity relation, weak correlations of 2 and 3.6σ are found for uvw1 − v and u − v. However, we find that weak correlations can be reproduced with shuffled velocities and colors that are corrected for reddening. The slope and significance of a correlation between the UV colors and the velocity is thus dependent on the slope of the optical color–velocity relation. Even with a correction, a significant scatter still remains in the uvw1 − v colors including a large spread at low velocities, demonstrating that the NUV-blue/red spread is not caused by the photospheric velocity. The uvm2 − uvw1 colors also show a large dispersion uncorrelated with the velocity.

The short gamma-ray burst (GRB) 170817A was the first GRB associated with a gravitational-wave event. Due to the exceptionally low luminosity of the prompt γ-ray and the afterglow emission, the origin of both radiation components is highly debated. The most discussed models for the burst and the afterglow include a regular GRB jet seen off-axis and the emission from the cocoon encompassing a “choked” jet. Here, we report low radio frequency observations at 610 and 1390 MHz obtained with the Giant Metrewave Radio Telescope. Our observations span a range of ∼7 to ∼152 days after the burst. The afterglow started to emerge at these low frequencies about 60 days after the burst. The 1390 MHz light curve barely evolved between 60 and 150 days, but its evolution is also marginally consistent with an F_ν ∝ t^0.8 rise seen in higher frequencies. We model the radio data and archival X-ray, optical, and high-frequency radio data with models of top-hat and Gaussian structured GRB jets. We performed a Markov Chain Monte Carlo analysis of the structured-jet parameter space. Though highly degenerate, useful bounds on the posterior probability distributions can be obtained. Our bounds of the viewing angle are consistent with that inferred from the gravitational-wave signal. We estimate the energy budget in prompt emission to be an order of magnitude lower than that in the afterglow blast wave.

Most studies on low-frequency electromagnetic cyclotron waves have assumed a small wave amplitude, which ensures the reasonable application of linear and quasi-linear theories. However, the topic of large-amplitude electromagnetic cyclotron waves has not received much attention. Using Magnetospheric Multiscale measurements, this study observes low-frequency, left-hand circularly polarized electromagnetic waves with magnetic fluctuation ∼1–2 nT in the dusk flank side of the Earth’s magnetosheath. Considering the ambient magnetic field ∼15 nT therein, the relative wave amplitude is of the order of 0.1. These large magnetic field fluctuations result in a periodic variation of the ion pitch angle. The electron pitch angle exhibits a localized distribution feature with a timescale approximating the wave period. Moreover, some electrons are trapped at a pitch angle ∼90°, and the trapping is more remarkable as strong waves arise. These two features of the electron pitch angle distribution imply that the trapping of electrons (partly) results from large-amplitude electromagnetic cyclotron fluctuations. Our results illustrate the important role of large-amplitude electromagnetic cyclotron waves on the dynamics of charged particles.

Fast and accurate integration of geodesics in Kerr spacetimes is an important tool in modeling the orbits of stars and the transport of radiation in the vicinities of black holes. Most existing integration algorithms employ Boyer–Lindquist (BL) coordinates, which have coordinate singularities at the event horizon and along the poles. Handling the singularities requires special numerical treatment in these regions, often slows down the calculations, and may lead to inaccurate geodesics. We present here a new general-purpose geodesic integrator, GRay2, that overcomes these issues by employing the Cartesian form of Kerr–Schild (KS) coordinates. By performing particular mathematical manipulations of the geodesic equations and several optimizations, we develop an implementation of the Cartesian KS coordinates that outperforms calculations that use the seemingly simpler equations in BL coordinates. We also employ the OpenCL framework, which allows GRay2 to run on multicore CPUs as well as on a wide range of graphics processing units hardware accelerators, making the algorithm more versatile. We report numerous convergence tests and benchmark results for GRay2 for both time-like (particle) and null (photon) geodesics.

Glitch size and waiting time probability density functions (PDFs) are estimated for the five pulsars that have glitched the most using the nonparametric kernel density estimator. Two objects exhibit decreasing size and waiting time PDFs. Their activity is Poisson-like, and their size statistics are approximately scale-invariant. Three objects exhibit a statistically significant local maximum in the PDFs, including one (PSR J1341−6220), which was classified as Poisson-like in previous analyses. Their activity is quasiperiodic, although the dispersion in waiting times is relatively broad. The classification is robust: it is preserved across a wide range of bandwidth choices. There is no compelling evidence for multimodality, but this issue should be revisited when more data become available. The implications for superfluid vortex avalanche models of pulsar glitches are explored briefly.

We simulate the evolution of the stellar wind and the supernova remnant (SNR) originating from a runaway massive star in a uniform Galactic environment based on three-dimensional magnetohydrodynamics models. Taking the stellar wind into consideration, we can explain the radio morphologies of many SNRs. The directions of the kinematic velocity of the progenitor, the magnetic field, and the line of sight are the most important factors influencing the morphologies. If the velocity is perpendicular to the magnetic field, the simulation will give us two different unilateral SNRs and a bilateral symmetric SNR. If the velocity is parallel to the magnetic field, we obtain a bilateral asymmetric SNR and a quasi-circular SNR. Our simulations show the stellar wind plays a key role in the radio evolution of an SNR, which implies that the Galactic global density and magnetic field distribution play a secondary role.

White dwarfs (WDs) are excellent forensic tools for studying end-of-life issues surrounding low- and intermediate-mass stars, and the old, solar metallicity open star cluster Messier 67 is a proven laboratory for the study of stellar evolution for solar-type stars. In this paper, we present a detailed spectroscopic study of brighter (M_g ≤ 12.4) WDs in Messier 67, and in combination with previously published proper motion membership determinations, we identify a clean, representative sample of cluster WDs, including 13 members with hydrogen-dominated atmospheres, at least one of which is a candidate double degenerate, and 5 members with helium-dominated atmospheres. Using this sample we test multiple predictions surrounding the final stages of stellar evolution in solar-type stars. In particular, the stochasticity of the integrated mass lost by ∼1.5 solar mass stars is less than 7% of the WD remnant mass. We identify WDs likely resulting from binary evolution, including at least one blue straggler remnant and two helium-core WDs. We observe no evidence of a significant population of helium-core WDs formed by enhanced mass loss on the red giant branch of the cluster. The distribution of WD atmospheric compositions is fully consistent with that in the field, limiting proposed mechanisms for the suppression of helium atmosphere WD formation in star clusters. In short, the WD population of Messier 67 is fully consistent with basic predictions of single- and multiple-star stellar evolution theories for solar metallicity stars.

Extensive calculations are reported for electron collision strengths and rate coefficients for a wide range of transitions in Fe i. The calculations were carried out with the B-spline R-matrix method. A multiconfiguration Hartree–Fock method with nonorthogonal, term-dependent orbitals was employed to generate accurate initial- and final-state wave functions. The close-coupling expansion contained 221 LS states of Fe and included all levels of the , , , , and configurations. Effective collision strengths were obtained by averaging the electron collision strengths over a Maxwellian speed distribution at electron temperatures ranging from 10² to 10⁵ K. They were tabulated for 24,531 transitions between all LS-terms included in the close-coupling expansion. The present results considerably expand on the few existing data sets for Fe i. They enable more detailed treatments of the available measured spectra from various observatories than previously possible. In particular, nonlocal thermodynamic equilibrium modeling of late-type stars, where large amounts of collisional data for the atomic species of interest are required, can be performed. The same close-coupling expansion was used to study low-energy photodetachment of Fe⁻, where the cross sections exhibit prominent resonance features. Good agreement with the few existing experimental values for partial cross sections to specific final target states of Fe was obtained.

Although the most luminous class of neutron star (NS) low-mass X-ray binaries, known as Z sources, have been well studied, their behavior is not fully understood. In particular, what causes these sources to trace out the characteristic Z-shaped pattern on color–color or hardness–intensity diagrams (HIDs) is not well known. By studying the physical properties of the different spectral states of these sources, we may better understand such variability. With that goal in mind, we present a recent NuSTAR observation of the Z source GX 349+2, which spans approximately 2 days and covers all its spectral states. By creating an HID we were able to extract five spectra and trace the change in spectral parameters throughout the Z-track. GX 349+2 shows a strong, broad Fe Kα line in all states, regardless of the continuum model used. Through modeling of the reflection spectrum and Fe Kα line we find that in most states the inner disk radius is consistent with remaining unchanged at an average radius of 17.5 R_g or 36.4 km for a canonical 1.4 M_⊙ NS. During the brightest flaring branch, however, the inner disk radius from reflection is not well constrained.

We report on updated radio imaging observations of the radio remnant of SN 1987A at 9 GHz, taken with the Australia Telescope Compact Array (ATCA), covering a 25 yr period (1992–2017). We use Fourier modeling of the supernova remnant to model its morphology, using both a torus model and a ring model, and find that both models show an increasing flux density and have shown a continuing expansion of the remnant. As found in previous studies, we find that the torus model most accurately fits our data and has shown a change in the remnant expansion at day 9300 ± 210 from 2300 ± 200 km s⁻¹ to 3610 ± 240 km s⁻¹. We have also seen an increase in brightness in the western lobe of the remnant, although the eastern lobe is still the dominant source of emission, unlike what has been observed at contemporary optical and X-ray wavelengths. We expect to observe a reversal in this asymmetry by the year ∼2020, and we note that the southeastern side of the remnant is now beginning to fade, as has also been seen in optical and X-ray data. Our data indicate that high-latitude emission has been present in the remnant from the earliest stages of the shock wave interacting with the equatorial ring around day 5000. However, we find that the emission has become increasingly dominated by the low-lying regions by day 9300, overlapping with the regions of X-ray emission. We conclude that the shock wave is now leaving the equatorial ring, exiting first from the southeast region of the remnant, and is reaccelerating as it begins to interact with the circumstellar medium beyond the dense inner ring.

Double-peaked narrow emission lines in active galactic nucleus (AGN) spectra can be produced by AGN outflows, rotation, or dual AGNs, which are AGN pairs in ongoing galaxy mergers. Consequently, double-peaked narrow AGN emission lines are useful tracers of the coevolution of galaxies and their supermassive black holes, as driven by AGN feedback and AGN fueling. We investigate this concept further with follow-up optical longslit observations of a sample of 95 Sloan Digital Sky Survey (SDSS) galaxies that have double-peaked narrow AGN emission lines. Based on a kinematic analysis of the longslit spectra, we confirm previous work that finds that the majority of double-peaked narrow AGN emission lines are associated with outflows. We also find that eight of the galaxies have companion galaxies with line-of-sight velocity separations <500 km s⁻¹ and physical separations <30 kpc. Since we find evidence of AGNs in both galaxies, all eight of these systems are compelling dual AGN candidates. Galaxies with double-peaked narrow AGN emission lines occur in such galaxy mergers at least twice as often as typical active galaxies. Finally, we conclude that at least 3% of SDSS galaxies with double-peaked narrow AGN emission lines are found in galaxy mergers where both galaxies are resolved in SDSS imaging.

A fraction of active galactic nuclei (AGN) exhibit dramatic variability, which is observed on timescales down to minutes in the X-ray band. We introduce the case study of 1H 1934-063 (z = 0.0102), a Narrow-line Seyfert 1 among the brightest and most variable AGN ever observed with XMM-Newton. This work includes spectral and temporal analyses of a concurrent XMM-Newton and NuSTAR 2015 observation lasting 130 kiloseconds, during which the X-ray source exhibited a steep (factor of ∼6) plummet and subsequent full recovery of the flux level, accompanied by deviation from a single log-normal flux distribution. We rule out Compton-thin obscuration as the cause for this dramatic variability observed even at NuSTAR energies. In order to constrain coronal geometry, dynamics, and emission/absorption processes, we compare a detailed spectral fitting with a Fourier-based timing analysis. Similar to other well-studied, highly variable Seyfert 1s, this AGN is X-ray bright and displays strong reflection features. We find a narrower broad iron line component compared to most Seyfert 1s, and constrain the black hole spin to be <0.1, one of the lowest yet discovered for such systems. Combined spectral and timing results are consistent with a dramatic change in the continuum on timescales as short as a few kiloseconds dictating the nature of this variability. We also discover a Fe–K time lag, measuring a delay of 20 s between relativistically blurred reflection off the inner accretion flow and the hard X-ray continuum emission.

Utilizing all the 16 yr RXTE observations, we analyze the X-ray spectra of 32 TeV blazars and perform a systematic investigation of X-ray spectral variability for the five brightest sources during their major flares that lasted several days. We obtain photon spectral index (α), flux and synchrotron radiation peak energy (E_p) from empirical spectral fitting, and electron spectral index (p) from theoretical synchrotron radiation modeling. We find that both α and p generally display a harder-when-brighter trend, confirming the results of many previous works. Furthermore, we confirm and strengthen the result that p must vary in order to explain the observed X-ray spectral variability during flares, which would have useful implications for interpreting the associated higher-energy spectral variability. We see apparent electron spectral hysteresis in many but not all p-flux plots that takes a form of “loop” or oblique “8.” We obtain a tight p–hardness ratio (HR) relation and a tighter p–α relation using spectra of flaring periods, both of which are also applicable to stacked data of quiescent periods. We demonstrate that these two empirical relations can be used efficiently to estimate p from HR or α that is readily achieved. Finally, we find that, when considering TeV blazars as a whole, α and X-ray luminosity are positively correlated, E_p is negatively correlated with p and α, and E_p is positively correlated with HR; all these correlations are in line with the blazar sequence. However, after correcting for the Doppler boosting effect, α and intrinsic X-ray luminosity follow an anticorrelation.

Cold dark matter scenarios of hierarchical large-scale structure formation predict the existence of abundant subhalos around large galaxies. However, the number of observed dwarf galaxies is far from this theoretical prediction, suggesting that most of the subhalos could be dark or quite faint. Gravitational lensing is a powerful tool to probe the mass distribution directly irrespective of whether it is visible or dark. Time delay anomalies in strongly lensed quasar systems are complementary to flux-ratio anomalies in probing dark matter substructure in galaxies. Here we propose that lensed gravitational waves detected by the third-generation ground detectors with quite accurate time delay measurements could be a much better tool for this study than conventional techniques. Combined with good quality images of lensed host galaxies identified by the electromagnetic counterpart measurements, lensed gravitational wave signals could make the systematic errors caused by dark matter substructures detectable at levels of several percent, depending on their mass functions, internal distribution of subhalos, and lensing system configuration.

M stars are powerful emitters of far-ultraviolet light. Over long timescales, a significant, possibly dominant, fraction of this emission is produced by stellar flares. Characterizing this emission is critical to understanding the atmospheres of the stars producing it and the atmospheric evolution of the orbiting planets subjected to it. Ultraviolet emission is known to be elevated for several hundred million years after M stars form. Whether or not the same is true of ultraviolet flare activity is a key concern for the evolution of exoplanet atmospheres. Hubble Space Telescope (HST) observations by the HAZMAT program (HAbitable Zones and M dwarf Activity across Time) detected 18 flares on young (40 Myr) early M stars in the Tucana–Horologium association over 10 hr of observations, 10 having energy >10³⁰ erg. These imply that flares on young M stars are 100–1000× more energetic than those occurring at the same rate on “inactive,” field age M dwarfs. However, when energies are normalized by quiescent emission, there is no statistical difference between the young and field age samples. The most energetic flare observed, dubbed the “Hazflare,” emitted an energy of 10^32.1 erg in the FUV, 30× more energetic than any stellar flare previously observed in the FUV with HST’s COS or STIS spectrographs. It was accompanied by 15,500 ± 400 K blackbody emission bright enough to designate it as a superflare (E > 10³³ erg), with an estimated bolometric energy of erg. This blackbody emitted % of its flux in the FUV (912–1700 Å), where molecules are generally most sensitive to photolysis. Such hot superflares in young, early M stars could play an important role in the evolution of nascent planetary atmospheres.

M dwarf stars are known for their vigorous flaring. This flaring could impact the climate of orbiting planets, making it important to characterize M dwarf flares at the short wavelengths that drive atmospheric chemistry and escape. We conducted a far-ultraviolet flare survey of six M dwarfs from the recent MUSCLES (Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems) observations, as well as four highly active M dwarfs with archival data. When comparing absolute flare energies, we found the active-M-star flares to be about 10× more energetic than inactive-M-star flares. However, when flare energies were normalized by the star’s quiescent flux, the active and inactive samples exhibited identical flare distributions, with a power-law index of (cumulative distribution). The rate and distribution of flares are such that they could dominate the FUV energy budget of M dwarfs, assuming the same distribution holds to flares as energetic as those cataloged by Kepler and ground-based surveys. We used the observed events to create an idealized model flare with realistic spectral and temporal energy budgets to be used in photochemical simulations of exoplanet atmospheres. Applied to our own simulation of direct photolysis by photons alone (no particles), we find that the most energetic observed flares have little effect on an Earth-like atmosphere, photolyzing ∼0.01% of the total O₃ column. The observations were too limited temporally (73 hr cumulative exposure) to catch rare, highly energetic flares. Those that the power-law fit predicts occur monthly would photolyze ∼1% of the O₃ column and those it predicts occur yearly would photolyze the full O₃ column. Whether such energetic flares occur at the rate predicted is an open question.

By implementing a dynamic wind tunnel model in a smoothed particle chemodynamic/hydrodynamic simulation suite, we have investigated the effects of ram pressure and tidal forces on dwarf galaxies similar to the Magellanic Clouds, within host galaxies with gas and dark matter halos that are varied, to compare the relative effects of tides and ram pressure. We concentrate on how the distributions of metals are affected by interactions. We find that while ram pressure and tidal forces have some effect on dwarf galaxy outflows, these effects do not produce large differences in the metal distributions of the dwarf disks, other than truncation in the outer regions in some cases, and that confinement from the host galaxy gas halo appears to be more significant than ram pressure stripping. We find that stochastic variations in the star formation rate can explain the remaining variations in disk metal properties. This raises questions on the cause of low metallicities in dwarf galaxies.

We use high-resolution cosmological hydrodynamical simulations of Milky-Way-sized galaxies with varying supernova feedback strengths and merger histories to investigate the formation of their gaseous halos and especially their hot (>10⁶ K) X-ray-luminous coronae. Our simulations predict the presence of significant hot gas in the halos as early as z = 3–4, well before the halos ought to be able to sustain hot mode accretion in the conventional picture. The nascent coronae grow inside-out and initially do so primarily as a result of outflows from the central galaxies powered by merger-induced shock heating and strong supernova feedback, both of which are elemental features of today’s successful galaxy formation models. Furthermore, the outflows and the forming coronae also accelerate the transition from cold to hot mode accretion by contributing to the conditions for sustaining stable accretion shocks. They also disrupt the filamentary streams funneling cold gas onto the central galaxies by causing their mouths to fray into a broad delta, detach from the galaxies, and be pushed away to larger radii. And even though at early times the filaments repeatedly re-form, the hot gas and the outflows act to weaken the filaments and accelerate their ultimate disruption. Although galactic outflows are generally thought of as ejective feedback, we find that their action on the filaments suggests a preventive role as well.

We propose a new internal linear combination (ILC) method in the pixel space, applicable on large angular scales of the sky, to estimate a foreground-minimized cosmic microwave background (CMB) temperature anisotropy map by incorporating prior knowledge about the theoretical CMB covariance matrix. The usual ILC method in pixel space, on the contrary, does not use any information about the underlying CMB covariance matrix. The new approach complements the usual pixel space ILC technique specifically at low-multipole regions, using global information available from the theoretical CMB covariance matrix and from the data. Since we apply our method over the large scale on the sky containing low multipoles, we perform foreground minimization globally. We apply our methods on low-resolution Planck and WMAP foreground-contaminated CMB maps and validate the methodology by performing detailed Monte Carlo simulations. Our cleaned CMB map and its power spectrum have significantly less error than those obtained following the usual ILC technique at low resolution that does not use CMB covariance information. Another very important advantage of our method is that the cleaned power spectrum does not have any negative bias at the low multipoles because of effective suppression of CMB–foreground chance correlations on large angular scales of the sky. Our cleaned CMB map and its power spectrum match well with those estimated by other research groups.

We run a suite of dissipative N-body simulations to determine which regions of phase space for smooth disk migration are consistent with the GJ876 system, an M-dwarf hosting three planets orbiting in a chaotic 4:2:1 Laplace resonance. We adopt adaptive mesh refinement (AMR) methods that are commonly used in hydrodynamical simulations to efficiently explore the parameter space defined by the semimajor axis and eccentricity damping timescales. We find that there is a large region of phase space that produces systems in the chaotic Laplace resonance and a smaller region consistent with the observed eccentricities and libration amplitudes for the resonant angles. Under the assumptions of Type I migration for the outer planet, we translate these damping timescales into constraints on the protoplanetary disk surface density and thickness. When we strongly (weakly) damp the eccentricities of the inner two Laplace planets, these timescales correspond to disk surface densities around ten thousand (a few hundred) grams per square centimeter and disk aspect ratios between 1% and 10%. Additionally, smooth migration produces systems with a range of chaotic timescales, from decades and centuries to upward of thousands of years. In agreement with previous studies, the less chaotic regions of phase space coincide with the system being in a low-energy double apsidal corotation resonance. Our detailed modeling of multiplanetary systems coupled with our AMR exploration method enhances our ability to map out the parameter space of planet formation models and is well suited to study other resonant chain systems such as Trappist-1, Kepler-60, and others.

The TRAPPIST-1 planetary system provides an unprecedented opportunity to study terrestrial exoplanet evolution with the James Webb Space Telescope (JWST) and ground-based observatories. Since M dwarf planets likely experience extreme volatile loss, the TRAPPIST-1 planets may have highly evolved, possibly uninhabitable atmospheres. We used a versatile, 1D terrestrial planet climate model with line-by-line radiative transfer and mixing length convection (VPL Climate) coupled to a terrestrial photochemistry model to simulate environmental states for the TRAPPIST-1 planets. We present equilibrium climates with self-consistent atmospheric compositions and observational discriminants of postrunaway, desiccated, 10–100 bar O₂- and CO₂-dominated atmospheres, including interior outgassing, as well as for water-rich compositions. Our simulations show a range of surface temperatures, most of which are not habitable, although an aqua planet TRAPPIST-1 e could maintain a temperate surface given Earth-like geological outgassing and CO₂. We find that a desiccated TRAPPIST-1 h  may produce habitable surface temperatures beyond the maximum greenhouse distance. Potential observational discriminants for these atmospheres in transmission and emission spectra are influenced by photochemical processes and aerosol formation and include collision-induced oxygen absorption (O₂–O₂), and O₃,  CO, SO₂, H₂O, and CH₄ absorption features, with transit signals of up to 200 ppm. Our simulated transmission spectra are consistent with K2, Hubble Space Telescope, and Spitzer observations of the TRAPPIST-1 planets. For several terrestrial atmospheric compositions, we find that TRAPPIST-1 b is unlikely to produce aerosols. These results can inform JWST observation planning and data interpretation for the TRAPPIST-1 system and other M dwarf terrestrial planets.

We present a novel technique of imaging a 2D region such that the image formed has both spectral and spatial information in a single frame. This technique uses a combination of a microlenslet array and a Fabry–Pérot etalon to generate multiple images of the region at different wavelength positions. This results in the formation of a hyperspectral image in a single snapshot. The instrument contains no moving parts and uses the property of blueshift of the transmitted wavelength as the angle of incidence increases on an etalon. The spectral band covered, field of view of the region, and the hyperspectral bands needed can be selected and altered by a simple change in the combination of focal length of imaging and collimating lenses used in the setup. Results obtained from solar observation of two regions in absorption and emission lines, respectively, are presented. A conceptual design of extending the capability of this instrument to perform hyperspectropolarimetry is also presented.

From a study of the light curves of the M dwarfs observed by the Kepler space telescope in its primary mission, a number of flare events with the peak flux increases reaching more than the nominal stellar luminosity have been found. One of them, KIC 9201463, produced an extreme flare with the peak flux increase jumping to five times the quiet-time value. In relative terms, this class of hyperflares is much stronger than the superflares of the solar-type stars and could have a very important influence on the atmospheric evolution and the potential development of biospheres of habitable super-Earths orbiting around M dwarf stars. A cross-correlation of the flare activities of some of these M dwarf stars and their Hα equivalent width (EW) values derived from the LAMOST project indicates that the Hα EW values can be used to monitor the occurrence of hyperflares as well as the level of flare activity of different classes of M dwarfs with fast to slow rotations, and hence the long-term environmental effects of star–planet interaction of exoplanets.

The orientation of the magnetic field (B field) in the filamentary dark cloud GF 9 was traced from the periphery of the cloud into the L1082C dense core that contains the low-mass, low-luminosity Class 0 young stellar object (YSO) GF 9-2 (IRAS 20503+6006). This was done using SOFIA HAWC+ dust thermal emission polarimetry (TEP) at 216 μm in combination with Mimir near-infrared background starlight polarimetry (BSP) conducted in the H band (1.6 μm) and K band (2.2 μm). These observations were augmented with published I-band (0.77 μm) BSP and Planck 850 μm TEP to probe B-field orientations with offset from the YSO in a range spanning 6000 au to 3 pc. No strong B-field orientation change with offset was found, indicating remarkable uniformity of the B-field from the cloud edge to the YSO environs. This finding disagrees with weak-field models of cloud core and YSO formation. The continuity of inferred B-field orientations for both TEP and BSP probes is strong evidence that both are sampling a common B field that uniformly threads the cloud, core, and YSO region. Bayesian analysis of Gaia DR2 stars matched to the Mimir BSP stars finds a distance to GF 9 of 270 ± 10 pc. No strong wavelength dependence of B-field orientation angle was found, contrary to previous claims.

We analyzed the spectral shape of the Compton shoulder around the neutral Fe–K_α line of the Compton-thick type II Seyfert nucleus of the Circinus galaxy. The characteristics of this Compton shoulder with respect to the reflected continuum and Fe–K_α line core intensity are powerful diagnostics tools for analyzing the structure of the molecular tori, which obscures the central engine. We applied our Monte-Carlo-based X-ray reflection spectral model to the Chandra High Energy Transmission Grating data and successfully constrained the various spectral parameters independently, using only the spectral data only around the Fe–K_α emission line. The obtained column density and inclination angle are consistent with previous observations and the Compton-thick type II Seyfert picture. In addition, we determined the metal abundance of the molecular torus for the case of the smooth and clumpy torus to be and 1.74 ± 0.16 solar abundance, respectively. Such slightly over-solar abundance can be useful information for discussing the star formation rate in the molecular tori of active galactic nuclei.

The spectrum of gyrosynchrotron emission from solar flares generally peaks in the microwave range. Its optically thin, high-frequency component, above the spectral peak, is often used for diagnostics of the nonthermal electrons and the magnetic field in the radio source. Under favorable conditions, its low-frequency counterpart brings additional, complementary information about these parameters as well as thermal plasma diagnostics, either through gyrosynchrotron self-absorption, free–free absorption by the thermal plasma, or the suppression of emission through the so-called Razin effect. However, their effect on the low-frequency spectrum is often masked by spatial nonuniformity. To disentangle the various contributions to low-frequency gyrosynchrotron emission, a combination of spectral and imaging data is needed. To this end, we have investigated Owens Valley Solar Array (OVSA) multi-frequency images for 26 solar bursts observed jointly with RHESSI during the first half of 2002. For each, we examined dynamic spectra, time- and frequency-synthesis maps, RHESSI images with overlaid OVSA contours, and a few representative single-frequency snapshot OVSA images. We focus on the frequency dependence of microwave source sizes derived from the OVSA images and their effect on the low-frequency microwave spectral slope. We succeed in categorizing 18 analyzed events into several groups. Four events demonstrate clear evidence of being dominated by gyrosynchrotron self-absorption, with an inferred brightness temperature of ≥10⁸ K. The low-frequency spectra in the remaining events are affected to varying degrees by Razin suppression. We find that many radio sources are rather large at low frequencies, which can have important implications for solar energetic particle production and escape.

This paper reports on the re-analysis of solar flares in which the hard X-rays (HXRs) come predominantly from the corona rather than from the more usual chromospheric footpoints. All of the 26 previously analyzed event time intervals, over 13 flares, are re-examined for consistency with a flare model in which electrons are accelerated near the top of a magnetic loop which has a sufficiently high density to stop most of the electrons by Coulomb collisions before they can reach the footpoints. Of particular importance in the previous analysis was the finding that the length of the coronal HXR source increased with energy in the 20–30 keV range. However, after allowing for the possibility that footpoint emission at the higher energies affects the inferred length of the coronal HXR source, and using analysis techniques that suppress the possible influence of such footpoint emission, we conclude that there is no longer evidence that the length of the HXR coronal sources increase with increasing energy. In fact, for the six flares and 12 time intervals that satisfied our selection criteria, the loop lengths decreased on average by 1.0 ± 0.2 arcsec between 20 and 30 keV, with a standard deviation of 3.5 arcsec. We find strong evidence that the peak of the coronal HXR source increases in altitude with increasing energy. For the thermal component of the emission, this is consistent with the standard CHSKP flare model in which magnetic reconnection in a coronal current sheet results in new hot loops being formed at progressively higher altitudes. The explanation for the nonthermal emission is not so clear.

On 2017 September 6, the solar active region 12673 produced an X9.3 flare, regarded to be the largest to have occurred in solar cycle 24. In this work we have performed a magnetohydrodynamic (MHD) simulation in order to reveal the three-dimensional (3D) dynamics of the magnetic fields associated with the X9.3 solar flare. We first performed an extrapolation of the 3D magnetic field based on the observed photospheric magnetic field prior to the flare and then used this as the initial condition for the MHD simulation, which revealed a dramatic eruption. In particular, we found that a large coherent flux rope composed of highly twisted magnetic field lines formed during the eruption. A series of small flux ropes were found to lie along a magnetic polarity inversion line prior to the flare. Reconnection occurring between each flux rope during the early stages of the eruption formed the large, highly twisted flux rope. Furthermore, we observed a writhing motion of the erupting flux rope. Understanding these dynamics is important in the drive to increase the accuracy of space weather forecasting. We report on the detailed dynamics of the 3D eruptive flux rope and discuss the possible mechanisms of the writhing motion.

Sudden jets of collimated plasma arise from many locations on the Sun, including active regions. The magnetic field along which a jet emerges is often open to interplanetary space, offering a clear “escape route” for any flare-accelerated electrons, making jets lucrative targets for studying particle acceleration and the solar sources of transient heliospheric events. Bremsstrahlung hard X-rays (HXRs) could, in principle, trace the accelerated electrons that escape along the paths of the jets, but measurements of the escaping electron beams are customarily difficult due to the low densities of the corona. In this work, we augment HXR observations with gyrosynchrotron emission observed in microwaves, as well as extreme ultraviolet (EUV) emission and modeling to investigate flare-accelerated electrons in a coronal jet. HXR and microwave data from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Owens Valley Solar Array (OVSA), respectively, give complementary insight into electron spectra and locations, including the presence of accelerated electrons in the jet itself. High-time-resolution HXR data from the Konus-Wind instrument suggest electron acceleration timescales on the order of 1 s or shorter. We model the energetic electron distributions in the GX Simulator framework using the Solar and Heliospheric Observatory (SOHO)/Michelson Doppler Imager (MDI), the Transition Region and Coronal Explorer (TRACE), RHESSI, and OVSA data as constraints. The result is a modeled distribution, informed and constrained by measurements, of accelerated electrons as they escape the Sun. Combining the detection of microwave gyrosynchrotron emission from an open, rather than closed, magnetic configuration, with realistic 3D modeling constrained by magnetograms, EUV, and X-ray emission, we obtain the most stringent constraints to date on the accelerated electrons within a solar jet.