Showing 1–50 of 111 results for author: MacFadyen, A

Eccentric Disks from Gaseous Rings around Equal-Mass, Circular Binaries

Authors: Leonardo Betancourt, Andrew MacFadyen, Jonathan Zrake

Abstract: We perform high-resolution, grid-based hydrodynamics simulations of gaseous rings viscously spreading into disks around equal-mass, circular binaries. We find that all systems suppress accretion onto the binary when the gas is relatively cold. Circumbinary rings (CBRs) display weak variability above the binary orbital frequency $Ω_b$ and a dominant spectral peak at $\sim0.1Ω_b$ (half the fiducial… ▽ More We perform high-resolution, grid-based hydrodynamics simulations of gaseous rings viscously spreading into disks around equal-mass, circular binaries. We find that all systems suppress accretion onto the binary when the gas is relatively cold. Circumbinary rings (CBRs) display weak variability above the binary orbital frequency $Ω_b$ and a dominant spectral peak at $\sim0.1Ω_b$ (half the fiducial lump frequency of $\sim0.2Ω_b$). The evolution of CBR eccentricity depends strongly on both the initial ring radius and gas temperature, with smaller, colder rings exhibiting higher eccentricity up to $e \simeq 0.7$. Cold, compact rings develop nearly radius-independent eccentricity profiles, maintaining large $e$ out to several times the initial gas semimajor axis. We find that eccentricity growth favors a stream impact mechanism, in which gas torqued by the binary at pericenter passage exerts a perturbative force on the cavity wall. We consider inefficiently-accreting, intermediate-mass ($\sim10^4 M_\odot$) black hole binaries as sources of quasi-periodic eruptions when rejected streams shock the cavity wall and radiate in the UV or soft X-ray. We discuss the implications of eccentric disks evolved from CBRs for quasar light curves and asymmetric, time-variable double-peaked line emission from disks in galactic nuclei. If binaries drive asymmetry in accretion disk line profiles, our study suggests that the progenitor CBR must have been very compact. △ Less

Submitted 6 May, 2026; v1 submitted 2 January, 2026; originally announced January 2026.

Comments: 16 pages, 9 figures

Journal ref: The Astrophysical Journal, Volume 1005, pg 80, June 26 (2026)

Thermal X-ray signatures in late-stage unequal-mass massive black hole binary mergers

Authors: Luke Major Krauth, Jordy Davelaar, Zoltán Haiman, John Ryan Westernacher-Schneider, Jonathan Zrake, Andrew MacFadyen

Abstract: The multi-messenger combination of gravitational waves (GWs) from merging massive black hole binaries (MBHBs) and the electromagnetic (EM) counterpart from the surrounding circumbinary disk (CBD) will open avenues to new scientific pursuits. In order to realize this science, we need to correctly localize the host galaxy of the merging MBHB. Multi-wavelength, time-dependent electromagnetic (EM) sig… ▽ More The multi-messenger combination of gravitational waves (GWs) from merging massive black hole binaries (MBHBs) and the electromagnetic (EM) counterpart from the surrounding circumbinary disk (CBD) will open avenues to new scientific pursuits. In order to realize this science, we need to correctly localize the host galaxy of the merging MBHB. Multi-wavelength, time-dependent electromagnetic (EM) signatures can greatly facilitate the identification of the unique EM counterpart among many sources in LISA's localization volume. To this end, we studied merging unequal-mass MBHBs embedded in a CBD using high-resolution 2D simulations, with a $Γ$-law equation of state, incorporating viscous heating, shock heating and radiative cooling. We simulate each binary starting from before it decouples from the CBD until just after the merger. We compute EM signatures and identify distinct features before, during, and after the merger. We corroborate previous findings of a several order of magnitude drop in the thermal X-ray luminosity near the time of merger, but with delayed timing compared to an equal-mass system. The source remains X-ray dark for hours post-merger. Our main results are a potential new signature of a sharp spike in the thermal X-ray emission just before the tell-tale steep drop occurs. This feature may further help to identify EM counterparts of LISA's unequal MBHBs before merger without the need for extensive pre-merger monitoring. Additionally, we find a role-reversal, in which the primary out-accretes the secondary during late inspiral, which may diminish signatures originating from Doppler modulation. △ Less

Submitted 3 March, 2025; originally announced March 2025.

Comments: 17 pages, 20 figures, submitted to journal

Journal ref: Mon Not R Astron Soc (2025) 2670-2685

Gravitational Wave Decoupling in Retrograde Circumbinary Disks

Authors: David O'Neill, Christopher Tiede, Daniel J. D'Orazio, Zoltan Haiman, Andrew MacFadyen

Abstract: We present a study of the late-time interaction between supermassive black hole binaries and retrograde circumbinary disks during the period of gravitational wave-driven inspiral. While mergers in prograde disks have received extensive study, retrograde disks offer distinct dynamics that could promote mergers and produce unique observational signatures. Through numerical simulations, we explore th… ▽ More We present a study of the late-time interaction between supermassive black hole binaries and retrograde circumbinary disks during the period of gravitational wave-driven inspiral. While mergers in prograde disks have received extensive study, retrograde disks offer distinct dynamics that could promote mergers and produce unique observational signatures. Through numerical simulations, we explore the process of binary-disk decoupling, where the binary's orbital decay rate is faster than the disk's viscous response rate. We find the point of decoupling to be comparable in prograde and retrograde disks, suggesting that any associated electromagnetic (EM) signatures will be produced at comparable times preceding merger. However, we find smaller central cavities for retrograde disks, likely leading to higher-frequency EM emissions and shorter post-merger rebrightening timescales compared to their prograde counterparts. Additionally, we identify quasi-periodic flaring due to instabilities unique to low-viscosity retrograde disks, which may produce distinctive EM signatures. △ Less

Submitted 20 January, 2025; originally announced January 2025.

Comments: Submitted to ApJ. Comments are welcome

Suppressed accretion onto massive black hole binaries surrounded by thin disks

Authors: Christopher Tiede, Jonathan Zrake, Andrew MacFadyen, Zoltan Haiman

Abstract: We demonstrate that gas disks around binary systems might deliver gas to the binary components only when the circumbinary disk is relatively warm. We present new grid-based hydrodynamics simulations, performed with the binary on the grid and a locally isothermal equation of state, in which the binary is seen to functionally ``stop accreting'' if the orbital Mach number in the disk exceeds a thresh… ▽ More We demonstrate that gas disks around binary systems might deliver gas to the binary components only when the circumbinary disk is relatively warm. We present new grid-based hydrodynamics simulations, performed with the binary on the grid and a locally isothermal equation of state, in which the binary is seen to functionally ``stop accreting'' if the orbital Mach number in the disk exceeds a threshold value of about 40. Above this threshold, the disk continues to extract angular momentum from the binary orbit, but it delivers very little mass to the black holes, and instead piles up mass in a ring surrounding the binary. This ring will eventually become viscously relaxed and deliver mass to the binary at the large-scale inflow rate. However we show that the timescale for such relaxation can far exceed the implied binary lifetime. We demonstrate that the ability of a binary-disk system to equilibrate is dependent on the efficiency at which accretion streams deposit mass onto the binary; which in turn is highly sensitive to the thermodynamic conditions of the inner disk. If disks around massive black hole binaries do operate in such non-accreting regimes, it suggests these systems may be dimmer than their single black hole counterparts, but could exhibit dramatic re-brightening after the black holes in-spiral and merge. This dimming begins in the UV/optical and could completely choke high-energy emission, such that these systems would likely be intrinsically X-ray weak with reddened continua, potentially resembling the spectra of `Little Red Dots'' recently identified in JWST observations. △ Less

Submitted 16 June, 2025; v1 submitted 4 October, 2024; originally announced October 2024.

Comments: 11 pages and 5 figures

Journal ref: The Astrophysical Journal, Volume 984, pg 144, May 7 (2025)

Did Binary Neutron Star Merger GW170817 Leave Behind A Long-lived Neutron Star?

Authors: Marcus DuPont, Andrew MacFadyen

Abstract: We consider the observational implications of the binary neutron star (BNS) merger GW170817 leaving behind a rapidly rotating massive neutron star that launches a relativistic, equatorial outflow as well as a jet. We show that if the equatorial outflow (ring) is highly beamed in the equatorial plane, its luminosity can be "hidden" from view until late times, even if carrying a significant fraction… ▽ More We consider the observational implications of the binary neutron star (BNS) merger GW170817 leaving behind a rapidly rotating massive neutron star that launches a relativistic, equatorial outflow as well as a jet. We show that if the equatorial outflow (ring) is highly beamed in the equatorial plane, its luminosity can be "hidden" from view until late times, even if carrying a significant fraction of the spin-down energy of the merger remnant. This hidden ring reveals itself as a re-brightening in the light curve once it slows down enough for Earth to be within the ring's relativistic beaming solid angle. We compute semi-analytic light curves using this model and find they are in agreement with the observations thus far, and we provide predictions for the ensuing afterglow. △ Less

Submitted 30 May, 2024; originally announced May 2024.

Relativistic Binary Precession: Impact on Eccentric Binary Accretion and Multi-Messenger Astronomy

Authors: Stanislav DeLaurentiis, Zoltan Haiman, John Ryan Westernacher-Schneider, Luke Major Krauth, Jordy Davelaar, Jonathan Zrake, Andrew MacFadyen

Abstract: Recent hydrodynamical simulations have shown that circumbinary gas disks drive the orbits of binary black holes to become eccentric, even when general relativistic corrections to the orbit are significant. Here, we study the general relativistic (GR) apsidal precession of eccentric equal-mass binary black holes in circumbinary disks (CBDs) via two-dimensional hydrodynamical simulations. We perform… ▽ More Recent hydrodynamical simulations have shown that circumbinary gas disks drive the orbits of binary black holes to become eccentric, even when general relativistic corrections to the orbit are significant. Here, we study the general relativistic (GR) apsidal precession of eccentric equal-mass binary black holes in circumbinary disks (CBDs) via two-dimensional hydrodynamical simulations. We perform a suite of simulations comparing precessing and non-precessing binaries across a range of eccentricities, semi-major axes, and precession rates. We find that the GR precession of the binary's semi-major axis can introduce a dominant modulation in the binary's accretion rate and the corresponding high-energy electromagnetic light-curves. We discuss the conditions under which this occurs and its detailed characteristics and mechanism. Finally, we discuss the potential to observe these precession signatures in electromagnetic and gravitational wave (GW) observations, as well as the precession signal's unique importance as a potential tool to constrain the mass, eccentricity, and semi-major axis of binary merger events. △ Less

Submitted 13 May, 2024; originally announced May 2024.

Comments: 19 pages, 17 figures. Submitted to ApJ

Strong Bow Shocks: Turbulence and An Exact Self-Similar Asymptotic

Authors: Marcus DuPont, Andrei Gruzinov, Andrew MacFadyen

Abstract: We show that strong bow shocks are turbulent and non-universal near the head, but asymptote to a universal steady, self-similar, and analytically solvable flow in the downstream. The turbulence is essentially 3D, and has been confirmed by a 3D simulation. The asymptotic behavior is confirmed with high resolution 2D and 3D simulations of a cold uniform wind encountering both a solid spherical obsta… ▽ More We show that strong bow shocks are turbulent and non-universal near the head, but asymptote to a universal steady, self-similar, and analytically solvable flow in the downstream. The turbulence is essentially 3D, and has been confirmed by a 3D simulation. The asymptotic behavior is confirmed with high resolution 2D and 3D simulations of a cold uniform wind encountering both a solid spherical obstacle and stellar wind. This solution is relevant in the context of: (i) probing the kinematic properties of observed high-velocity compact bodies -- e.g., runaway stars and/or supernova ejecta blobs -- flying through the interstellar medium; and (ii) constraining stellar bow shock luminosities invoked by some quasi-periodic eruption (QPE) models. △ Less

Submitted 20 July, 2024; v1 submitted 31 January, 2024; originally announced January 2024.

Comments: > updated to include focal length of the parabola in terms of the physical parameters. > updated to final publication-version while maintaining AG's style in Section 2

Explosions in Roche-Lobe Distorted Stars: Relativistic Bullets in Binaries

Authors: Marcus DuPont, Andrew MacFadyen, Selma de Mink

Abstract: State-of-the-art surveys reveal that most massive stars in the universe evolve in close binaries. Massive stars in such systems are expected to develop aspherical envelopes due to tidal interactions and/or rotational effects. Recently, it was shown that point explosions in oblate stars can produce relativistic equatorial ring-like outflows. Moreover, since stripped-envelope stars in binaries can e… ▽ More State-of-the-art surveys reveal that most massive stars in the universe evolve in close binaries. Massive stars in such systems are expected to develop aspherical envelopes due to tidal interactions and/or rotational effects. Recently, it was shown that point explosions in oblate stars can produce relativistic equatorial ring-like outflows. Moreover, since stripped-envelope stars in binaries can expand enough to fill their Roche lobes anew, it is likely that these stars die with a greater degree of asphericity than the oblate spheroid geometry previously studied. We investigate the effects of this asymmetry by studying the gas dynamics of axisymmetric point explosions in stars in various stages of filling their Roche lobes. We find that point explosions in these pear-shaped stars produce trans-relativistic ejecta that coalesces into bullets pointed both toward and away from the binary companion. We present this result and comment on key morphological differences between core-collapse explosions in spherical versus distorted stars in binary systems, effects on gravitational wave sources, and observational signatures that could be used to glean these explosion geometries from current and future surveys. △ Less

Submitted 31 October, 2023; originally announced October 2023.

Self-lensing flares from black hole binaries III: general-relativistic ray tracing of circumbinary accretion simulations

Authors: Luke Major Krauth, Jordy Davelaar, Zoltán Haiman, John Ryan Westernacher-Schneider, Jonathan Zrake, Andrew MacFadyen

Abstract: Self-lensing flares (SLFs) are expected to be produced once or twice per orbit by an accreting massive black hole binary (MBHB), if the eclipsing MBHBs are observed close to edge-on. SLFs can provide valuable electromagnetic (EM) signatures to accompany the gravitational waves (GWs) detectable by the upcoming Laser Interferometer Space Antenna (LISA). EM follow-ups are crucial for, e.g., sky-local… ▽ More Self-lensing flares (SLFs) are expected to be produced once or twice per orbit by an accreting massive black hole binary (MBHB), if the eclipsing MBHBs are observed close to edge-on. SLFs can provide valuable electromagnetic (EM) signatures to accompany the gravitational waves (GWs) detectable by the upcoming Laser Interferometer Space Antenna (LISA). EM follow-ups are crucial for, e.g., sky-localization, and constraining the Hubble constant and the graviton mass. We use high-resolution two-dimensional viscous hydrodynamical simulations of a circumbinary disk (CBD) embedding a MBHB. We then use very high-cadence output of these hydrodynamical simulation inputs for a general-relativistic ray-tracing code to produce synthetic spectra and phase-folded light curves. Our main results show a significant periodic amplification of the flux with the characteristic shape of a sharp flare with a central dip, as the foreground black hole (BH) transits across the minidisk and shadow of the background BH, respectively. These corroborate previous conclusions based on the microlensing approximation and analytical toy models of the emission geometry. We also find that at lower inclinations, without some occlusion of the minidisk emission by the CBD, shocks from quasi-periodic mass-trading between the minidisks can produce bright flares which can mimic SLFs and could hinder their identification. △ Less

Submitted 13 November, 2023; v1 submitted 30 October, 2023; originally announced October 2023.

Comments: 14 pages, 11 figures, submitted to journal, split Fig. 1 by frequency, fixed some typos

Stars Bisected by Relativistic Blades

Authors: Marcus DuPont, Andrew MacFadyen

Abstract: We consider the dynamics of an equatorial explosion powered by a millisecond magnetar formed from the core collapse of a massive star. We study whether these outflows -- generated by a priori magneto-centrifugally-driven, relativistic magnetar winds -- might be powerful enough to produce an ultra-relativistic blade ("lamina") that successfully carves its way through the dense stellar interior. We… ▽ More We consider the dynamics of an equatorial explosion powered by a millisecond magnetar formed from the core collapse of a massive star. We study whether these outflows -- generated by a priori magneto-centrifugally-driven, relativistic magnetar winds -- might be powerful enough to produce an ultra-relativistic blade ("lamina") that successfully carves its way through the dense stellar interior. We present high-resolution numerical special-relativistic hydrodynamic simulations of axisymmetric centrifugally-driven explosions inside a star and follow the blast wave propagation just after breakout. We estimate the engine requirements to produce ultra-relativistic lamina jets and comment on the physicality of the parameters considered. We find that sufficiently collimated -- half-opening angle $θ_r \leq 0.2^\circ$ -- laminas successfully break out of a compact progenitor at ultra-relativistic velocities ($Γ_{\rm core} \gtrsim 30$) and extreme isotropic energies ($E_{k,\rm iso} \sim 5 \times 10^{52}\text{erg}$) within a few percent of the typical spin-down period for a millisecond magnetar. The various phases of these ultra-thin outflows such as collimation shocks, Kelvin-Helmholtz instabilities, and lifetime are discussed and we speculate on the observational signatures echoed by this outflow geometry. △ Less

Submitted 26 September, 2023; originally announced September 2023.

Eccentric Minidisks in Accreting Binaries

Authors: John Ryan Westernacher-Schneider, Jonathan Zrake, Andrew MacFadyen, Zoltán Haiman

Abstract: We show that gas disks around the components of an orbiting binary system (so-called minidisks) may be susceptible to a resonant instability which causes the minidisks to become significantly eccentric. Eccentricity is injected by, and also induces, regular impacts between the minidisks at roughly the orbital period of the binary. Such eccentric minidisks are seen in vertically integrated, two-dim… ▽ More We show that gas disks around the components of an orbiting binary system (so-called minidisks) may be susceptible to a resonant instability which causes the minidisks to become significantly eccentric. Eccentricity is injected by, and also induces, regular impacts between the minidisks at roughly the orbital period of the binary. Such eccentric minidisks are seen in vertically integrated, two-dimensional simulations of a circular, equal-mass binary accreting from a circumbinary gas disk with a $Γ$-law equation of state. Minidisk eccentricity is suppressed by the use of an isothermal equation of state. However, the instability still operates, and can be revealed in a minimal disk-binary simulation by removing the circumbinary disk, and feeding the minidisks from the component positions. Minidisk eccentricity is also suppressed when the gravitational softening length is large ($\gtrsim 4\%$ of the binary semi-major axis), suggesting that its absence could be an artifact of widely adopted numerical approximations; a follow-up study in three dimensions with well-resolved, geometrically thin minidisks (aspect ratios $\lesssim 0.02$) may be needed to assess whether eccentric minidisks can occur in real astrophysical environments. If they can, the electromagnetic signature may be important for discriminating between binary and single black hole scenarios for quasi-periodic oscillations in active galactic nuclei; in turn, this might aid in targeted searches with pulsar timing arrays for individual supermassive black hole binary sources of low-frequency gravitational waves. △ Less

Submitted 13 February, 2024; v1 submitted 3 July, 2023; originally announced July 2023.

Comments: 5 Mb, 22 pages, video content URLs provided in the captions to Figures 5, 6, 9, 11, and 15. Version 2 is close to the published, open-access version (see the DOI to the journal version below)

Journal ref: The Astrophysical Journal, 962:76 (17pp), 2024 February 10

A standard siren measurement of the Hubble constant using GW170817 and the latest observations of the electromagnetic counterpart afterglow

Authors: A. Palmese, R. Kaur, A. Hajela, R. Margutti, A. McDowell, A. MacFadyen

Abstract: We present a new constraint on the Hubble constant $H_0$ using the latest measurements of the electromagnetic counterpart to the gravitational wave (GW) event GW170817. We use the latest optical, X-ray and radio observations of the afterglow up to $\sim 3.5$ years after the GW detection, and properly take into account the impact of the host galaxy peculiar velocity. We find… ▽ More We present a new constraint on the Hubble constant $H_0$ using the latest measurements of the electromagnetic counterpart to the gravitational wave (GW) event GW170817. We use the latest optical, X-ray and radio observations of the afterglow up to $\sim 3.5$ years after the GW detection, and properly take into account the impact of the host galaxy peculiar velocity. We find $75.46^{+5.34}_{-5.39}$ km s$^{-1}$ Mpc$^{-1}$ (68\% Credible Interval), a $\sim7\%$ precision measurement, which is a significant improvement compared to the $14\%$ precision of the first standard siren measurement. Our result is consistent within $1σ$ with the Cepheid-anchored Supernova and within $1.5 σ$ with the Cosmic Microwave Background measurements of the Hubble constant. We also explore the impact of the various assumptions made when fitting for the afterglow on the Hubble constant estimate. △ Less

Submitted 31 May, 2023; originally announced May 2023.

Comments: 9 pages, 5 figures

Disappearing thermal X-ray emission as a tell-tale signature of merging massive black hole binaries

Authors: Luke Major Krauth, Jordy Davelaar, Zoltán Haiman, John Ryan Westernacher-Schneider, Jonathan Zrake, Andrew MacFadyen

Abstract: The upcoming Laser Interferometer Space Antenna (LISA) is expected to detect gravitational waves (GWs) from massive black hole binaries (MBHB). Finding the electromagnetic (EM) counterparts for these GW events will be crucial for understanding how and where MBHBs merge, measuring their redshifts, constraining the Hubble constant and the graviton mass, and for other novel science applications. Howe… ▽ More The upcoming Laser Interferometer Space Antenna (LISA) is expected to detect gravitational waves (GWs) from massive black hole binaries (MBHB). Finding the electromagnetic (EM) counterparts for these GW events will be crucial for understanding how and where MBHBs merge, measuring their redshifts, constraining the Hubble constant and the graviton mass, and for other novel science applications. However, due to poor GW sky localisation, multi-wavelength, time-dependent electromagnetic (EM) models are needed to identify the right host galaxy among many candidates. We studied merging MBHBs embedded in a circumbinary disc using high-resolution two-dimensional simulations, with a $Γ$-law equation of state, incorporating viscous heating, shock heating, and radiative cooling. We simulate the binary from large separation until after merger, allowing us to model the decoupling of the binary from the circumbinary disc (CBD). We compute the EM signatures and identify distinct features before, during, and after the merger. Our main result is a multi-band EM signature: we find that the MBHB produces strong thermal X-ray emission until 1-2 days prior to the merger. However, as the binary decouples from the CBD, the X-ray-bright minidiscs rapidly shrink in size, become disrupted, and the accretion rate drops precipitously. As a result, the thermal X-ray luminosity drops by orders of magnitude, and the source remains X-ray dark for several days after the merger, regardless of any post-merger effects such as GW recoil or mass loss. Looking for the abrupt spectral change where the thermal X-ray disappears is a tell-tale EM signature of LISA mergers that does not require extensive pre-merger monitoring. △ Less

Submitted 6 October, 2023; v1 submitted 5 April, 2023; originally announced April 2023.

Comments: 14 pages, 16 figures, 1 table, accepted for publication in MNRAS

On The Theory of Ring Afterglows

Authors: Marcus DuPont, Andrew MacFadyen, Re'em Sari

Abstract: Synchrotron and inverse Compton emission successfully explain the observed spectra of gamma-ray burst (GRB) afterglows. It is thought that most GRBs are products of extremely relativistic outflows and the afterglow marks the interaction of that ejecta with the surrounding matter. Faster decay of afterglow light curves at late times is indicative of non-spherical geometries, and are usually interpr… ▽ More Synchrotron and inverse Compton emission successfully explain the observed spectra of gamma-ray burst (GRB) afterglows. It is thought that most GRBs are products of extremely relativistic outflows and the afterglow marks the interaction of that ejecta with the surrounding matter. Faster decay of afterglow light curves at late times is indicative of non-spherical geometries, and are usually interpreted as evidence for jet geometry. Recent numerical simulations have shown that ring-like geometries are also permissible for relativistic outflows. We therefore extend the standard theory of afterglow evolution to ring geometries. An analytic prescription for the light curves and spectra produced by relativistic toroidal blast waves is presented. We compare these to their spherical and jet-like counterparts, and show that ring afterglows decay faster than spherical outflows but not as fast as jets. △ Less

Submitted 31 March, 2023; originally announced April 2023.

Revisiting the Parameter Space of Binary Neutron Star Merger Event GW170817

Authors: Austin McDowell, Andrew MacFadyen

Abstract: Since the gravitational wave event GW170817 and gamma-ray burst GW170817A there have been numerous studies constraining the burst properties through analysis of the afterglow light curves. Most agree that the burst was viewed off-axis with a ratio of the observer angle to the jet angle ($θ_{obs}/θ_j$) between 4 - 6. We use a parameterized model and broadband synchrotron data up to $\sim 800$ days… ▽ More Since the gravitational wave event GW170817 and gamma-ray burst GW170817A there have been numerous studies constraining the burst properties through analysis of the afterglow light curves. Most agree that the burst was viewed off-axis with a ratio of the observer angle to the jet angle ($θ_{obs}/θ_j$) between 4 - 6. We use a parameterized model and broadband synchrotron data up to $\sim 800$ days post-merger to constrain parameters of the burst. To reproduce the hydrodynamics of a gamma-ray burst outflow we use a two-parameter "boosted fireball" model. The structure of a boosted fireball is determined by the specific internal energy, $η_0$, and the bulk Lorentz factor, $γ_B(\sim 1/θ_j)$ with shapes varying smoothly from a quasi-spherical outflow for low values of $γ_B$ to a highly collimated jet for high values. We run simulations with $γ_B$ in the range $1-20$ and $η_0$ in the range $2-15$. To calculate light curves we use a synchrotron radiation model characterized by $F_{peak}$, $ν_m$, and $ν_c$ and calculate millions of spectra at different times and $θ_{obs}$ values using the \texttt{boxfit} radiation code. We can tabulate the spectral parameter values from our spectra and rapidly generate arbitrary light curves for comparison to data in MCMC analysis. We find that our model prefers a gamma-ray burst with jet energy $E_j\sim10^{50}$ ergs and with an observer angle of $θ_{obs}=0.65^{+0.13}_{-0.14}$ radians and ratio to jet opening angle of ($θ_{obs}/θ_j$) = 5.4$^{+0.53}_{-0.38}$. △ Less

Submitted 22 February, 2023; originally announced February 2023.

Ellipsars: Ring-like explosions from flattened stars

Authors: Marcus DuPont, Andrew MacFadyen, Jonathan Zrake

Abstract: The stellar cataclysms producing astronomical transients have long been modeled as either a point-like explosion or jet-like engine ignited at the center of a spherically symmetric star. However, many stars are observed, or are expected on theoretical grounds, not to be precisely spherically symmetric, but rather to have a slightly flattened geometry similar to that of an oblate spheroid. Here we… ▽ More The stellar cataclysms producing astronomical transients have long been modeled as either a point-like explosion or jet-like engine ignited at the center of a spherically symmetric star. However, many stars are observed, or are expected on theoretical grounds, not to be precisely spherically symmetric, but rather to have a slightly flattened geometry similar to that of an oblate spheroid. Here we present axisymmetric two-dimensional hydrodynamical simulations of the dynamics of point-like explosions initiated at the center of an aspherical massive star with a range of oblateness. We refer to these exploding aspherical stars as "ellipsars" in reference to the elliptical shape of the iso-density contours of their progenitors in the two-dimensional axisymmetric case. We find that ellipsars are capable of accelerating expanding rings of relativistic ejecta which may lead to the production of astronomical transients including low-luminosity GRBs, relativistic supernovae, and Fast Blue Optical Transients (FBOTs.) △ Less

Submitted 24 February, 2022; v1 submitted 9 February, 2022; originally announced February 2022.

Comments: Edit: Update to remove typo introduced in the revision on version 2

Multi-band light curves from eccentric accreting supermassive black hole binaries

Authors: John Ryan Westernacher-Schneider, Jonathan Zrake, Andrew MacFadyen, Zoltán Haiman

Abstract: We use long-run, high-resolution hydrodynamics simulations to compute the multi-wavelength light curves (LCs) from thermal disk emission around accreting equal-mass supermassive black hole (BH) binaries, with a focus on revealing binary eccentricity. LCs are obtained by modeling the disk thermodynamics with an adiabatic equation of state, a local blackbody cooling prescription, and corrections to… ▽ More We use long-run, high-resolution hydrodynamics simulations to compute the multi-wavelength light curves (LCs) from thermal disk emission around accreting equal-mass supermassive black hole (BH) binaries, with a focus on revealing binary eccentricity. LCs are obtained by modeling the disk thermodynamics with an adiabatic equation of state, a local blackbody cooling prescription, and corrections to approximate the effects of radiation pressure. We find that modulation of multi-band LCs on the orbital time scale are generally in-phase (to within $\sim\,$2% of a binary orbital period), but they contain pulse substructure in the time domain that is not necessarily reflected in BH accretion rates $\dot M$. We thus predict that binary-hosting AGN will exhibit highly correlated, in-phase, periodic brightness modulations in their low-energy disk emission. However, detectability of these modulations in multi-wavelength observing campaigns could be seriously compromised because observed stochastic variability in AGNs typically has a higher amplitude than our proposed signal. It is possible that observations over temporal baselines of many binary periods may make the signal more prominent, but this would need to be analyzed carefully. If jet emission is predicted by $\dot{M}$, then we predict a weaker correlation with low-energy disk emission due to the differing sub-peak structure. For the binary parameters we explore, we show that LC variability due to hydrodynamics likely dominates Doppler brightening for all equal-mass binaries with disk Mach numbers $\lesssim 20$. A promising signature of eccentricity is weak or absent "lump" periodicity. We find hints that a significant lag exists between $\dot{M}$ and low-energy disk emission for circular binaries, but they are in-phase for eccentric binaries, which might explain some "orphan" blazar flares with no $γ$-ray counterpart. △ Less

Submitted 17 October, 2022; v1 submitted 12 November, 2021; originally announced November 2021.

Comments: v6: accepted version (PRD)

How Binaries Accrete: Hydrodynamic Simulations with Passive Tracer Particles

Authors: Christopher Tiede, Jonathan Zrake, Andrew MacFadyen, Zoltan Haiman

Abstract: Linear analysis of gas flows around orbiting binaries suggests that a centrifugal barrier ought to clear a low-density cavity around the binary and inhibit mass transfer onto it. Modern hydrodynamics simulations have confirmed the low-density cavity, but show that any mass flowing from large scales into the circumbinary disk is eventually transferred onto the binary components. Even though many nu… ▽ More Linear analysis of gas flows around orbiting binaries suggests that a centrifugal barrier ought to clear a low-density cavity around the binary and inhibit mass transfer onto it. Modern hydrodynamics simulations have confirmed the low-density cavity, but show that any mass flowing from large scales into the circumbinary disk is eventually transferred onto the binary components. Even though many numerical studies confirm this picture, it is still not understood precisely how gas parcels overcome the centrifugal barrier and ultimately accrete. We present a detailed analysis of the binary accretion process, using an accurate prescription for evolving grid-based hydrodynamics with Lagrangian tracer particles that track the trajectories of individual gas parcels. We find that binary accretion can be described in four phases: (1) gas is viscously transported through the circumbinary disk up to the centrifugal barrier at the cavity wall, (2) the cavity wall is tidally distorted into accretion streams consisting of near-ballistic gas parcels on eccentric orbits, (3) the portion of each stream moving inwards of an ``accretion horizon'' radius $\bar r \simeq a$ -- the radius beyond which no material is returned to the cavity wall -- becomes bound to a minidisk orbiting an individual binary component, and (4) the minidisk gas accretes onto the binary component through the combined effect of viscous and tidal stresses. △ Less

Submitted 13 June, 2022; v1 submitted 8 November, 2021; originally announced November 2021.

Comments: Published in ApJ; 18 pages, 16 figures

Journal ref: The Astrophysical Journal, Volume 932, Number 1, 2022 June 13

Evidence for X-ray Emission in Excess to the Jet Afterglow Decay 3.5 yrs After the Binary Neutron Star Merger GW 170817: A New Emission Component

Authors: A. Hajela, R. Margutti, J. S. Bright, K. D. Alexander, B. D. Metzger, V. Nedora, A. Kathirgamaraju, B. Margalit, D. Radice, C. Guidorzi, E. Berger, A. MacFadyen, D. Giannios, R. Chornock, I. Heywood, L. Sironi, O. Gottlieb, D. Coppejans, T. Laskar, Y. Cendes, R. Barniol Duran, T. Eftekhari, W. Fong, A. McDowell, M. Nicholl , et al. (12 additional authors not shown)

Abstract: For the first $\sim3$ years after the binary neutron star merger event GW 170817 the radio and X-ray radiation has been dominated by emission from a structured relativistic off-axis jet propagating into a low-density medium with n $< 0.01\,\rm{cm^{-3}}$. We report on observational evidence for an excess of X-ray emission at $δt>900$ days after the merger. With… ▽ More For the first $\sim3$ years after the binary neutron star merger event GW 170817 the radio and X-ray radiation has been dominated by emission from a structured relativistic off-axis jet propagating into a low-density medium with n $< 0.01\,\rm{cm^{-3}}$. We report on observational evidence for an excess of X-ray emission at $δt>900$ days after the merger. With $L_x\approx5\times 10^{38}\,\rm{erg\,s^{-1}}$ at 1234 days, the recently detected X-ray emission represents a $\ge 3.2\,σ$ (Gaussian equivalent) deviation from the universal post jet-break model that best fits the multi-wavelength afterglow at earlier times. In the context of JetFit afterglow models, current data represent a departure with statistical significance $\ge 3.1\,σ$, depending on the fireball collimation, with the most realistic models showing excesses at the level of $\ge 3.7\,σ$. A lack of detectable 3 GHz radio emission suggests a harder broad-band spectrum than the jet afterglow. These properties are consistent with the emergence of a new emission component such as synchrotron radiation from a mildly relativistic shock generated by the expanding merger ejecta, i.e. a kilonova afterglow. In this context, we present a set of ab-initio numerical-relativity BNS merger simulations that show that an X-ray excess supports the presence of a high-velocity tail in the merger ejecta, and argues against the prompt collapse of the merger remnant into a black hole. Radiation from accretion processes on the compact-object remnant represents a viable alternative. Neither a kilonova afterglow nor accretion-powered emission have been observed before, as detections of BNS mergers at this phase of evolution are unprecedented. △ Less

Submitted 5 March, 2022; v1 submitted 5 April, 2021; originally announced April 2021.

Comments: 30 pages, 13 figures, Accepted for publication in ApJL

Equilibrium eccentricity of accreting binaries

Authors: Jonathan Zrake, Christopher Tiede, Andrew MacFadyen, Zoltán Haiman

Abstract: Using high-resolution hydrodynamics simulations, we show that equal-mass binaries accreting from a circumbinary disk evolve toward an orbital eccentricity of $e \simeq 0.45$, unless they are initialized on a nearly circular orbit with $e \lesssim 0.08$, in which case they further circularize. The implied bi-modal eccentricity distribution resembles that seen in post-AGB stellar binaries. Large acc… ▽ More Using high-resolution hydrodynamics simulations, we show that equal-mass binaries accreting from a circumbinary disk evolve toward an orbital eccentricity of $e \simeq 0.45$, unless they are initialized on a nearly circular orbit with $e \lesssim 0.08$, in which case they further circularize. The implied bi-modal eccentricity distribution resembles that seen in post-AGB stellar binaries. Large accretion spikes around periapse impart a tell-tale, quasi-periodic, bursty signature on the light curves of eccentric binaries. We predict that intermediate-mass and massive black hole binaries at $z \lesssim 10$ entering the \emph{LISA} band will have measurable eccentricities in the range $e \simeq 10^{-3} - 10^{-2}$, if they have experienced a gas-driven phase. On the other hand, GW190521 would have entered the LIGO/Virgo band with undetectable eccentricity $\sim 10^{-6}$ if it had been driven into the gravitational wave regime by a gas disk. △ Less

Submitted 4 March, 2021; v1 submitted 19 October, 2020; originally announced October 2020.

Comments: Published in ApJL

Journal ref: The Astrophysical Journal Letters, Volume 909, Number 1, 2021 March 3

Evolution of gas disc-embedded intermediate mass ratio inspirals in the LISA band

Authors: A. Derdzinski, D. D'Orazio, P. Duffell, Z. Haiman, A. Macfadyen

Abstract: Among the potential milliHz gravitational wave (GW) sources for the upcoming space-based interferometer LISA are extreme- or intermediate-mass ratio inspirals (EMRI/IMRIs). These events involve the coalescence of supermassive black holes in the mass range $10^5 M_{\odot} \lesssim M \lesssim 10^7 M_{\odot}$ with companion BHs of much lower masses. A subset of E/IMRIs are expected to occur in the ac… ▽ More Among the potential milliHz gravitational wave (GW) sources for the upcoming space-based interferometer LISA are extreme- or intermediate-mass ratio inspirals (EMRI/IMRIs). These events involve the coalescence of supermassive black holes in the mass range $10^5 M_{\odot} \lesssim M \lesssim 10^7 M_{\odot}$ with companion BHs of much lower masses. A subset of E/IMRIs are expected to occur in the accretion discs of active galactic nuclei (AGN), where torques exerted by the disc can interfere with the inspiral and cause a phase shift in the GW waveform. Here we use a suite of two-dimensional hydrodynamical simulations with the moving-mesh code DISCO to present a systematic study of disc torques. We measure torques on an inspiraling BH and compute the corresponding waveform deviations as a function of the binary mass ratio $q\equiv M_2/M_1$, the disc viscosity ($α$), and gas temperature (or equivalently Mach number; $\mathcal{M}$). We find that the absolute value of the gas torques is within an order of magnitude of previously determined planetary migration torques, but their precise value and sign depends non-trivially on the combination of these parameters. The gas imprint is detectable by LISA for binaries embedded in AGN discs with surface densities above $Σ_0\ge10^{4-6} \rm \, g cm^{-2}$, depending on $q$, $α$ and $\mathcal{M}$. Deviations are most pronounced in discs with higher viscosities, and for E/IMRIs detected at frequencies where LISA is most sensitive. Torques in colder discs exhibit a noticeable dependence on the GW-driven inspiral rate as well as strong fluctuations at late stages of the inspiral. Our results further suggest that LISA may be able to place constraints on AGN disc parameters and the physics of disc-satellite interaction. △ Less

Submitted 8 March, 2021; v1 submitted 22 May, 2020; originally announced May 2020.

Journal ref: MNRAS, Volume 501, Issue 3, March 2021, Pages 3540-3557

Gas-driven inspiral of binaries in thin accretion disks

Authors: Christopher Tiede, Jonathan Zrake, Andrew MacFadyen, Zoltan Haiman

Abstract: Numerical studies of gas accretion onto supermassive black hole binaries (SMBHBs) have generally been limited to conditions where the circumbinary disk (CBD) is 10-100 times thicker than expected for disks in active galactic nuclei (AGN). This discrepancy arises from technical limitations, and also from publication bias toward replicating fiducial numerical models. Here we present the first system… ▽ More Numerical studies of gas accretion onto supermassive black hole binaries (SMBHBs) have generally been limited to conditions where the circumbinary disk (CBD) is 10-100 times thicker than expected for disks in active galactic nuclei (AGN). This discrepancy arises from technical limitations, and also from publication bias toward replicating fiducial numerical models. Here we present the first systematic study of how the binary's orbital evolution varies with disk scale height. We report three key results: (1) Binary orbital evolution switches from outspiralling for warm disks (aspect ratio ~0.1), to inspiralling for more realistic cooler, thinner disks at a critical aspect ratio ~0.04, corresponding to orbital Mach number ~25. (2) The net torque on the binary arises from a competition between positive torque from gas orbiting close to the black holes, and negative torque from the inner edge of the CBD, which is denser for thinner disks. This leads to increasingly negative net torques on the binary for increasingly thin disks. (3) The accretion rate is modestly suppressed with increasing Mach number. We discuss how our results may influence modeling of the nano-Hz gravitational wave background, as well as estimates of the LISA merger event rate. △ Less

Submitted 7 March, 2021; v1 submitted 19 May, 2020; originally announced May 2020.

Comments: Published in ApJ

Journal ref: The Astrophysical Journal Letters, Volume 900, Number 1, 2020 August 28

Circumbinary Disks: Accretion and Torque as a Function of Mass Ratio and Disk Viscosity

Authors: Paul C. Duffell, Daniel D'Orazio, Andrea Derdzinski, Zoltan Haiman, Andrew MacFadyen, Anna L. Rosen, Jonathan Zrake

Abstract: Using numerical hydrodynamics calculations and a novel method for densely sampling parameter space, we measure the accretion and torque on a binary system from a circumbinary disk. In agreement with previous studies, we find that the net torque on the binary is positive for mass ratios close to unity, and that accretion always drives the binary towards equal mass. Accretion variability depends sen… ▽ More Using numerical hydrodynamics calculations and a novel method for densely sampling parameter space, we measure the accretion and torque on a binary system from a circumbinary disk. In agreement with previous studies, we find that the net torque on the binary is positive for mass ratios close to unity, and that accretion always drives the binary towards equal mass. Accretion variability depends sensitively on the numerical sink prescription, but the torque and relative accretion onto each component do not depend on the sink timescale. Positive torque and highly variable accretion occurs only for mass ratios greater than around $0.05$. This means that for mass ratios below $0.05$, the binary would migrate inward until the secondary accreted sufficient mass, after which it would execute a U-turn and migrate outward. We explore a range of viscosities, from $α= 0.03$ to $α= 0.15$, and find that this outward torque is proportional to the viscous torque, simply proportional to viscosity in this range. Dependence of accretion and torque on mass ratio is explored in detail, densely sampling mass ratios between $0.01$ and unity. For mass ratio $q > 0.6$, accretion variability is found to exhibit a distinct sawtooth pattern, typically with a five-orbit cycle that provides a "smoking gun" prediction for variable quasars observed over long periods, as a potential means to confirm the presence of a binary. △ Less

Submitted 31 July, 2020; v1 submitted 13 November, 2019; originally announced November 2019.

Comments: ApJ Accepted

Two years of non-thermal emission from the binary neutron star merger GW170817: rapid fading of the jet afterglow and first constraints on the kilonova fastest ejecta

Authors: A. Hajela, R. Margutti, K. D. Alexander, A. Kathirgamaraju, A. Baldeschi, C. Guidorzi, D. Giannios, W. Fong, Y. Wu, A. MacFadyen, A. Paggi, E. Berger, P. K. Blanchard, R. Chornock, D. L. Coppejans, P. S. Cowperthwaite, T. Eftekhari, S. Gomez, G. Hosseinzadeh, T. Laskar, B. D. Metzger, M. Nicholl, K. Paterson, D. Radice, L. Sironi , et al. (5 additional authors not shown)

Abstract: We present Chandra and VLA observations of GW170817 at ~521-743 days post merger, and a homogeneous analysis of the entire Chandra data set. We find that the late-time non-thermal emission follows the expected evolution from an off-axis relativistic jet, with a steep temporal decay $F_ν\propto t^{-1.95\pm0.15}$ and a simple power-law spectrum $F_ν\propto ν^{-0.575\pm0.007}$. We present a new metho… ▽ More We present Chandra and VLA observations of GW170817 at ~521-743 days post merger, and a homogeneous analysis of the entire Chandra data set. We find that the late-time non-thermal emission follows the expected evolution from an off-axis relativistic jet, with a steep temporal decay $F_ν\propto t^{-1.95\pm0.15}$ and a simple power-law spectrum $F_ν\propto ν^{-0.575\pm0.007}$. We present a new method to constrain the merger environment density based on diffuse X-ray emission from hot plasma in the host galaxy and we find $n\le 9.6 \times 10^{-3}\,\rm{cm^{-3}}$. This measurement is independent from inferences based on the jet afterglow modeling and allows us to partially solve for model degeneracies. The updated best-fitting model parameters with this density constraint are a fireball kinetic energy $E_0 = 1.5_{-1.1}^{+3.6}\times 10^{49}\,\rm{erg}$ ($E_{iso}= 2.1_{-1.5}^{+6.4}\times10^{52}\, \rm{erg}$), jet opening angle $θ_{0}= 5.9^{+1.0}_{-0.7}\,\rm{deg}$ with characteristic Lorentz factor $Γ_j = 163_{-43}^{+23}$, expanding in a low-density medium with $n_0 = 2.5_{-1.9}^{+4.1} \times 10^{-3}\, \rm{cm^{-3}}$ and viewed $θ_{obs} = 30.4^{+4.0}_{-3.4}\, \rm{deg}$ off-axis. The synchrotron emission originates from a power-law distribution of electrons with $p=2.15^{+0.01}_{-0.02}$. The shock microphysics parameters are constrained to $ε_{\rm{e}} = 0.18_{-0.13}^{+0.30}$ and $ε_{\rm{B}}=2.3_{-2.2}^{+16.0} \times 10^{-3}$. We investigate the presence of X-ray flares and find no statistically significant evidence of $\ge2.5σ$ of temporal variability at any time. Finally, we use our observations to constrain the properties of synchrotron emission from the deceleration of the fastest kilonova ejecta with energy $E_k^{KN}\propto (Γβ)^{-α}$ into the environment, finding that shallow stratification indexes $α\le6$ are disfavored. △ Less

Submitted 8 November, 2019; v1 submitted 13 September, 2019; originally announced September 2019.

Comments: version accepted for publication in ApJL, 13 pages, 6 figures

The Optical Afterglow of GW170817: An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin

Authors: Wen-fai Fong, P. K. Blanchard, K. D. Alexander, J. Strader, R. Margutti, A. Hajela, V. A. Villar, Y. Wu, C. S. Ye, E. Berger, R. Chornock, D. Coppejans, P. S. Cowperthwaite, T. Eftekhari, D. Giannios, C. Guidorzi, A. Kathirgamaraju, T. Laskar, A. MacFadyen, B. D. Metzger, M. Nicholl, K. Paterson, G. Terreran, D. J. Sand, L. Sironi , et al. (3 additional authors not shown)

Abstract: We present a revised and complete optical afterglow light curve of the binary neutron star merger GW170817, enabled by deep Hubble Space Telescope (HST) F606W observations at $\approx\!584$ days post-merger, which provide a robust optical template. The light curve spans $\approx 110-362$ days, and is fully consistent with emission from a relativistic structured jet viewed off-axis, as previously i… ▽ More We present a revised and complete optical afterglow light curve of the binary neutron star merger GW170817, enabled by deep Hubble Space Telescope (HST) F606W observations at $\approx\!584$ days post-merger, which provide a robust optical template. The light curve spans $\approx 110-362$ days, and is fully consistent with emission from a relativistic structured jet viewed off-axis, as previously indicated by radio and X-ray data. Combined with contemporaneous radio and X-ray observations, we find no spectral evolution, with a weighted average spectral index of $\langle β\rangle = -0.583 \pm 0.013$, demonstrating that no synchrotron break frequencies evolve between the radio and X-ray bands over these timescales. We find that an extrapolation of the post-peak temporal slope of GW170817 to the luminosities of cosmological short GRBs matches their observed jet break times, suggesting that their explosion properties are similar, and that the primary difference in GW170817 is viewing angle. Additionally, we place a deep limit on the luminosity and mass of an underlying globular cluster of $L \lesssim 6.7 \times 10^{3}\,L_{\odot}$, or $M \lesssim 1.3 \times 10^{4}\,M_{\odot}$, at least 4 standard deviations below the peak of the globular cluster mass function of the host galaxy, NGC4993. This limit provides a direct and strong constraint that GW170817 did not form and merge in a globular cluster. As highlighted here, HST (and soon JWST) enables critical observations of the optical emission from neutron star merger jets and outflows. △ Less

Submitted 21 August, 2019; originally announced August 2019.

Comments: 11 pages, 6 figures, 2 tables. Accepted to ApJ Letters

GW170817 Afterglow Reveals that Short Gamma-Ray Bursts are Neutron Star Mergers

Authors: Yiyang Wu, Andrew MacFadyen

Abstract: We systematically investigate the outflow structure of GW170817 in comparison with a sample of 27 cosmological short GRBs by modelling their afterglow light curves. We find that cosmological short GRBs share the same outflow structures with GW170817, relativistic structured jets. The jet opening angle of GW170817 is $6.3^{+1.1}_{-0.6}{}^{\circ}$, which is consistent with that of cosmological short… ▽ More We systematically investigate the outflow structure of GW170817 in comparison with a sample of 27 cosmological short GRBs by modelling their afterglow light curves. We find that cosmological short GRBs share the same outflow structures with GW170817, relativistic structured jets. The jet opening angle of GW170817 is $6.3^{+1.1}_{-0.6}{}^{\circ}$, which is consistent with that of cosmological short GRBs ($θ_0 = 6.9^\circ \pm 2.3^{\circ}$). Our analysis indicates that GW170817 is viewed off-axis ($θ_{\rm obs} = 30^{+7}_{-4}{}^{\circ}$), while cosmological short GRBs are viewed on-axis ($θ_{\rm obs} \lesssim θ_0$). The exceptional properties of the GW170817 afterglow can be explained by the difference in observation angle alone. We demonstrate that the light curves of the GW170817 afterglow, if viewed on-axis, are consistent with those of cosmological short GRBs. Other properties of GW170817, such as Lorentz factor $Γ\approx 150$, spectral index $p \approx 2.15$, isotropic equivalent energy $E_{\rm iso} \approx 8 \times 10^{52}$ erg and interstellar medium density $n_0 \approx 10^{-2}$ proton cm$^{-3}$, fit well within the ranges of those of cosmological short GRBs. The similarity between the GW170817 outflow structure and those of cosmological short GRBs indicates that cosmological short GRBs are likely neutron star mergers. △ Less

Submitted 7 May, 2019; originally announced May 2019.

Off-axis synchrotron light curves from full-time-domain moving-mesh simulations of jets from massive stars

Authors: Xiaoyi Xie, Andrew MacFadyen

Abstract: We present full-time-domain, moving-mesh, relativistic hydrodynamic simulations of jets launched from the center of a massive progenitor star and compute the resulting synchrotron light curves for observers at a range of viewing angles. We follow jet evolution from ignition inside the stellar center, propagation in the stellar envelope and breakout from the stellar surface, then through the coasti… ▽ More We present full-time-domain, moving-mesh, relativistic hydrodynamic simulations of jets launched from the center of a massive progenitor star and compute the resulting synchrotron light curves for observers at a range of viewing angles. We follow jet evolution from ignition inside the stellar center, propagation in the stellar envelope and breakout from the stellar surface, then through the coasting and deceleration phases. The jet compresses into a thin shell, sweeps up the circumstellar medium, and eventually enters the Newtonian phase. The jets naturally develop angular and radial structure due to hydro-dynamical interaction with surrounding gas. The calculated synchrotron light curves cover the observed temporal range of prompt to late afterglow phases of long gamma-ray bursts (LGRBs). The on-axis light curves exhibit an early emission pulse originating in shock-heated stellar material, followed by a shallow decay and a later steeper decay. The off-axis light curves rise earlier than previously expected for top-hat jet models -- on a time scale of seconds to minutes after jet breakout, and decay afterwards. Sometimes the off-axis light curves have later re-brightening components that can be contemporaneous with SNe Ic-bl emission. Our calculations may shed light on the structure of GRB outflows in the afterglow stage. The off-axis light curves from full-time-domain simulations advocate new light curve templates for the search of off-axis/orphan afterglows. △ Less

Submitted 3 May, 2019; originally announced May 2019.

Multi-Messenger Astronomy with Extremely Large Telescopes

Authors: Ryan Chornock, Philip S. Cowperthwaite, Raffaella Margutti, Dan Milisavljevic, Kate D. Alexander, Igor Andreoni, Iair Arcavi, Adriano Baldeschi, Jennifer Barnes, Eric Bellm, Paz Beniamini, Edo Berger, Christopher P. L. Berry, Federica Bianco, Peter K. Blanchard, Joshua S. Bloom, Sarah Burke-Spolaor, Eric Burns, Dario Carbone, S. Bradley Cenko, Deanne Coppejans, Alessandra Corsi, Michael Coughlin, Maria R. Drout, Tarraneh Eftekhari , et al. (60 additional authors not shown)

Abstract: The field of time-domain astrophysics has entered the era of Multi-messenger Astronomy (MMA). One key science goal for the next decade (and beyond) will be to characterize gravitational wave (GW) and neutrino sources using the next generation of Extremely Large Telescopes (ELTs). These studies will have a broad impact across astrophysics, informing our knowledge of the production and enrichment hi… ▽ More The field of time-domain astrophysics has entered the era of Multi-messenger Astronomy (MMA). One key science goal for the next decade (and beyond) will be to characterize gravitational wave (GW) and neutrino sources using the next generation of Extremely Large Telescopes (ELTs). These studies will have a broad impact across astrophysics, informing our knowledge of the production and enrichment history of the heaviest chemical elements, constrain the dense matter equation of state, provide independent constraints on cosmology, increase our understanding of particle acceleration in shocks and jets, and study the lives of black holes in the universe. Future GW detectors will greatly improve their sensitivity during the coming decade, as will near-infrared telescopes capable of independently finding kilonovae from neutron star mergers. However, the electromagnetic counterparts to high-frequency (LIGO/Virgo band) GW sources will be distant and faint and thus demand ELT capabilities for characterization. ELTs will be important and necessary contributors to an advanced and complete multi-messenger network. △ Less

Submitted 11 March, 2019; originally announced March 2019.

Comments: White paper submitted to the Astro2020 Decadal Survey

Gravity and Light: Combining Gravitational Wave and Electromagnetic Observations in the 2020s

Authors: R. J. Foley, K. D. Alexander, I. Andreoni, I. Arcavi, K. Auchettl, J. Barnes, G. Baym, E. C. Bellm, A. M. Beloborodov, N. Blagorodnova, J. P. Blakeslee, P. R. Brady, M. Branchesi, J. S. Brown, N. Butler, M. Cantiello, R. Chornock, D. O. Cook, J. Cooke, D. L. Coppejans, A. Corsi, S. M. Couch, M. W. Coughlin, D. A. Coulter, P. S. Cowperthwaite , et al. (88 additional authors not shown)

Abstract: As of today, we have directly detected exactly one source in both gravitational waves (GWs) and electromagnetic (EM) radiation, the binary neutron star merger GW170817, its associated gamma-ray burst GRB170817A, and the subsequent kilonova SSS17a/AT 2017gfo. Within ten years, we will detect hundreds of events, including new classes of events such as neutron-star-black-hole mergers, core-collapse s… ▽ More As of today, we have directly detected exactly one source in both gravitational waves (GWs) and electromagnetic (EM) radiation, the binary neutron star merger GW170817, its associated gamma-ray burst GRB170817A, and the subsequent kilonova SSS17a/AT 2017gfo. Within ten years, we will detect hundreds of events, including new classes of events such as neutron-star-black-hole mergers, core-collapse supernovae, and almost certainly something completely unexpected. As we build this sample, we will explore exotic astrophysical topics ranging from nucleosynthesis, stellar evolution, general relativity, high-energy astrophysics, nuclear matter, to cosmology. The discovery potential is extraordinary, and investments in this area will yield major scientific breakthroughs. Here we outline some of the most exciting scientific questions that can be answered by combining GW and EM observations. △ Less

Submitted 11 March, 2019; originally announced March 2019.

Comments: submitted to Astro2020

An embedded X-ray source shines through the aspherical AT2018cow: revealing the inner workings of the most luminous fast-evolving optical transients

Authors: Raffaella Margutti, B. D. Metzger, R. Chornock, I. Vurm, N. Roth, B. W. Grefenstette, V. Savchenko, R. Cartier, J. F. Steiner, G. Terreran, G. Migliori, D. Milisavljevic, K. D. Alexander, M. Bietenholz, P. K. Blanchard, E. Bozzo, D. Brethauer, I. V. Chilingarian, D. L. Coppejans, L. Ducci, C. Ferrigno, W. Fong, D. GÖtz, C. Guidorzi, A. Hajela , et al. (19 additional authors not shown)

Abstract: We present the first extensive radio to gamma-ray observations of a fast-rising blue optical transient (FBOT), AT2018cow, over its first ~100 days. AT2018cow rose over a few days to a peak luminosity $L_{pk}\sim4\times 10^{44}$ erg/s exceeding those of superluminous supernovae (SNe), before declining as $\propto t^{-2}$. Initial spectra at $\lesssim 15$ days were mostly featureless and indicated l… ▽ More We present the first extensive radio to gamma-ray observations of a fast-rising blue optical transient (FBOT), AT2018cow, over its first ~100 days. AT2018cow rose over a few days to a peak luminosity $L_{pk}\sim4\times 10^{44}$ erg/s exceeding those of superluminous supernovae (SNe), before declining as $\propto t^{-2}$. Initial spectra at $\lesssim 15$ days were mostly featureless and indicated large expansion velocities v~0.1c and temperatures reaching 30000 K. Later spectra revealed a persistent optically-thick photosphere and the emergence of H and He emission features with v~sim 4000 km/s with no evidence for ejecta cooling. Our broad-band monitoring revealed a hard X-ray spectral component at $E\ge 10$ keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT2018cow showed bright radio emission consistent with the interaction of a blastwave with $v_{sh}$~0.1c with a dense environment ($\dot M\sim10^{-3}-10^{-4}\,M_{\odot}yr^{-1}$ for $v_w=1000$ km\s). While these properties exclude Ni-powered transients, our multi-wavelength analysis instead indicates that AT2018cow harbored a "central engine", either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released $\sim10^{50}-10^{51.5}$ erg over $\sim10^3-10^5$ s and resides within low-mass fast-moving material with equatorial-polar density asymmetry ($M_{ej,fast}\lesssim0.3\,\rm{M_{\odot}}$). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black-holes are disfavored by the large environmental density probed by the radio observations. △ Less

Submitted 25 October, 2018; originally announced October 2018.

Comments: 26 pages + tables. Submitted

Probing gas disc physics with LISA: simulations of an intermediate mass ratio inspiral in an accretion disc

Authors: A. Derdzinski, D. D'Orazio, P. Duffell, Z. Haiman, A. MacFadyen

Abstract: The coalescence of a compact object with a $10^{4}-10^{7} {\rm M_\odot}$ supermassive black hole (SMBH) produces mHz gravitational waves (GWs) detectable by the future Laser Interferometer Space Antenna (LISA). If such an inspiral occurs in the accretion disc of an active galactic nucleus (AGN), the gas torques imprint a small deviation in the GW waveform. Here we present two-dimensional hydrodyna… ▽ More The coalescence of a compact object with a $10^{4}-10^{7} {\rm M_\odot}$ supermassive black hole (SMBH) produces mHz gravitational waves (GWs) detectable by the future Laser Interferometer Space Antenna (LISA). If such an inspiral occurs in the accretion disc of an active galactic nucleus (AGN), the gas torques imprint a small deviation in the GW waveform. Here we present two-dimensional hydrodynamical simulations with the moving-mesh code DISCO of a BH inspiraling at the GW rate in a binary system with a mass ratio $q\!=\!M_2/M_1\!=\!10^{-3}$, embedded in an accretion disc. We assume a locally isothermal equation of state for the gas (with Mach number $\mathcal{M}=20$) and implement a standard $α$-prescription for its viscosity (with $α= 0.03$). We find disc torques on the binary that are weaker than in previous semi-analytic toy models, and are in the opposite direction: the gas disc slows down, rather than speeds up the inspiral. We compute the resulting deviations in the GW waveform, which scale linearly with the mass of the disc. The SNR of these deviations accumulates mostly at high frequencies, and becomes detectable in a 5-year LISA observation if the total phase shift exceeds a few radians. We find that this occurs if the disc surface density exceeds $Σ_0 \gtrsim 10^{2-3}\rm g\,cm^{-2}$, as may be the case in thin discs with near-Eddington accretion rates. Since the characteristic imprint on the GW signal is strongly dependent on disc parameters, a LISA detection of an intermediate mass ratio inspiral would probe the physics of AGN discs and migration. △ Less

Submitted 12 November, 2019; v1 submitted 8 October, 2018; originally announced October 2018.

Comments: Incorporates published erratum - revised figures in Section 5, results unchanged

Constraining the Outflow Structure of the Binary Neutron Star Merger Event GW170817 with Markov-Chain Monte Carlo Analysis

Authors: Yiyang Wu, Andrew MacFadyen

Abstract: The multi-wavelength non-thermal emission from the binary neutron star (BNS) merger GW170817 has raised a heated debate concerning the post-merger outflow structure. Both a relativistic structured jet viewed off-axis and a mildly relativistic quasi-spherical outflow can explain the observational data of GW170817 up to ~260 days. We utilize a physically motivated analytic two-parameter model called… ▽ More The multi-wavelength non-thermal emission from the binary neutron star (BNS) merger GW170817 has raised a heated debate concerning the post-merger outflow structure. Both a relativistic structured jet viewed off-axis and a mildly relativistic quasi-spherical outflow can explain the observational data of GW170817 up to ~260 days. We utilize a physically motivated analytic two-parameter model called the "boosted fireball" for the outflow structure after it has expanded far from the merger site and has entered the self-similar coasting phase. This model consists of a family of outflows with a structure varying smoothly between a highly collimated ultra-relativistic jet and an isotropic fireball. We simulate the dynamical evolution, starting with "boosted fireball" initial conditions, of 240 outflows using the moving-mesh relativistic hydrodynamics code JET to follow their evolution through the afterglow phase. We compute nearly 2,000,000 synchrotron spectra from the hydrodynamic simulations using the standard synchrotron radiation model. By making use of scaling relations in the hydrodynamic and radiation equations, we develop a synthetic light curve generator with an efficient sampling speed. This allows us to fit the observational data by performing Markov-Chain Monte Carlo (MCMC) analysis in a 8-dimensional parameter space, consisting of hydrodynamic parameters, radiation parameters and observational parameters. Our results favor the relativistic structured jet, with a jet opening angle ~5 deg and Lorentz factor ~175, viewed from an off-axis angle of 27(+9-3) deg. Due to parameter degeneracies, we find broad distributions for the explosion energy E_0, the circumburst density n_0, epsilon_e and epsilon_B. The combination of a high n_0 and a low epsilon_B can also produce a good fit, indicating that an extremely low n_0 may not be required for GW170817. △ Less

Submitted 18 September, 2018; originally announced September 2018.

Comments: 9 pages, 5 figures

Radio sky maps of the GRB 170817A afterglow from simulations

Authors: Jonathan Zrake, Xiaoyi Xie, Andrew MacFadyen

Abstract: We present synthetic radio images of the GRB170817A afterglow, computed from moving-mesh hydrodynamic simulations of binary neutron star merger outflows. Having expanded for nearly a year, the merger remnant is expected to subtend roughly 5 milli-arcseconds (mas) on the sky, potentially resolvable by very long baseline radio imaging techniques. Any observations revealing the radio centroid to be o… ▽ More We present synthetic radio images of the GRB170817A afterglow, computed from moving-mesh hydrodynamic simulations of binary neutron star merger outflows. Having expanded for nearly a year, the merger remnant is expected to subtend roughly 5 milli-arcseconds (mas) on the sky, potentially resolvable by very long baseline radio imaging techniques. Any observations revealing the radio centroid to be offset from the line-of-site to the merger would be the smoking gun of a jetted outflow. However, our results indicate that a measurement of the centroid position alone cannot independently determine whether that jet escaped successfully from the merger debris cloud, or was "choked," yielding a quasi-spherical explosion. We find that in both scenarios, the centroid exhibits superluminal proper motion away from the merger site at roughly 4 - 10 mas per day for at least the first 300 days. We argue that a successful strategy for differentiating among the explosion models will need to include multiple observations over the coming months - years. In particular, we find the time at which the centroid attains its maximum offset, and begins heading back toward the merger site, is considerably later if the jet was choked. Detecting a reversal of the centroid trajectory earlier than 600 days would uniquely identify a successful jet. Our results indicate the source might be resolved using VLBI radio observing techniques with roughly 1 mas resolution starting at roughly 400 days post-merger, and that the the angular extent of a successful jet is significantly smaller than that of a choked jet (4.5 versus 7 mas respectively). △ Less

Submitted 24 August, 2018; v1 submitted 18 June, 2018; originally announced June 2018.

Comments: Accepted to ApJL

Journal ref: The Astrophysical Journal Letters (2018), Volume 865, Number 1

A Decline in the X-ray through Radio Emission from GW170817 Continues to Support an Off-Axis Structured Jet

Authors: K. D. Alexander, R. Margutti, P. K. Blanchard, W. Fong, E. Berger, A. Hajela, T. Eftekhari, R. Chornock, P. S. Cowperthwaite, D. Giannios, C. Guidorzi, A. Kathirgamaraju, A. MacFadyen, B. D. Metzger, M. Nicholl, L. Sironi, V. A. Villar, P. K. G. Williams, X. Xie, J. Zrake

Abstract: We present new observations of the binary neutron star merger GW170817 at $Δt\approx 220-290$ days post-merger, at radio (Karl G. Jansky Very Large Array; VLA), X-ray (Chandra X-ray Observatory) and optical (Hubble Space Telescope; HST) wavelengths. These observations provide the first evidence for a turnover in the X-ray light curve, mirroring a decline in the radio emission at $\gtrsim5σ$ signif… ▽ More We present new observations of the binary neutron star merger GW170817 at $Δt\approx 220-290$ days post-merger, at radio (Karl G. Jansky Very Large Array; VLA), X-ray (Chandra X-ray Observatory) and optical (Hubble Space Telescope; HST) wavelengths. These observations provide the first evidence for a turnover in the X-ray light curve, mirroring a decline in the radio emission at $\gtrsim5σ$ significance. The radio-to-X-ray spectral energy distribution exhibits no evolution into the declining phase. Our full multi-wavelength dataset is consistent with the predicted behavior of our previously published models of a successful structured jet expanding into a low-density circumbinary medium, but pure cocoon models with a choked jet cannot be ruled out. If future observations continue to track our predictions, we expect that the radio and X-ray emission will remain detectable until $\sim 1000$ days post-merger. △ Less

Submitted 13 August, 2018; v1 submitted 8 May, 2018; originally announced May 2018.

Comments: Accepted to ApJL. Updated version includes new VLA observations extending through 2018 June 2

Numerical simulations of the jet dynamics and synchrotron radiation of binary neutron star merger event GW170817/GRB170817A

Authors: Xiaoyi Xie, Jonathan Zrake, Andrew MacFadyen

Abstract: We present numerical simulations of energetic flows propagating through the debris cloud of a binary neutron star (BNS) merger. Starting from the scale of the central engine, we use a moving-mesh hydrodynamics code to simulate the complete dynamical evolution of the produced relativistic jets. We compute synchrotron emission directly from the simulations and present multi-band light curves of the… ▽ More We present numerical simulations of energetic flows propagating through the debris cloud of a binary neutron star (BNS) merger. Starting from the scale of the central engine, we use a moving-mesh hydrodynamics code to simulate the complete dynamical evolution of the produced relativistic jets. We compute synchrotron emission directly from the simulations and present multi-band light curves of the early (sub-day) through late (weeks to years) afterglow stages. Our work systematically compares two distinct models for the central engine, referred to as the narrow and wide engine scenario, which is associated with a successful structured jet and a quasi-isotropic explosion respectively. Both engine models naturally evolve angular and radial structure through hydrodynamical interaction with the merger debris cloud. They both also result in a relativistic blast wave capable of producing the observed multi-band afterglow data. However, we find that the narrow and wide engine scenario might be differentiated by a new emission component that we refer to as a merger flash. This component is a consequence of applying the synchrotron radiation model to the shocked optically thin merger cloud. Such modeling is appropriate if injection of non-thermal electrons is sustained in the breakout relativistic shell, for example by internal shocks or magnetic reconnection. The rapidly declining signature may be detectable for future BNS mergers during the first minutes to day following the GW chirp. Furthermore, its non-detection for the GRB170817A event may disfavor the wide, quasi-isotropic explosion model. △ Less

Submitted 10 June, 2018; v1 submitted 25 April, 2018; originally announced April 2018.

The Binary Neutron Star event LIGO/VIRGO GW170817 a hundred and sixty days after merger: synchrotron emission across the electromagnetic spectrum

Authors: Raffaella Margutti, K. D. Alexander, X. Xie, L. Sironi, B. D. Metzger, A. Kathirgamaraju, W. Fong, P. K. Blanchard, E. Berger, A. MacFadyen, D. Giannios, C. Guidorzi, A. Hajela, R. Chornock, P. S. Cowperthwaite, T. Eftekhari, M. Nicholl, V. A. Villar, P. K. G. Williams, J. Zrake

Abstract: We report deep Chandra, HST and VLA observations of the binary neutron star event GW170817 at $t<160$ d after merger. These observations show that GW170817 has been steadily brightening with time and might have now reached its peak, and constrain the emission process as non-thermal synchrotron emission where the cooling frequency $ν_c$ is above the X-ray band and the synchrotron frequency $ν_m$ is… ▽ More We report deep Chandra, HST and VLA observations of the binary neutron star event GW170817 at $t<160$ d after merger. These observations show that GW170817 has been steadily brightening with time and might have now reached its peak, and constrain the emission process as non-thermal synchrotron emission where the cooling frequency $ν_c$ is above the X-ray band and the synchrotron frequency $ν_m$ is below the radio band. The very simple power-law spectrum extending for eight orders of magnitude in frequency enables the most precise measurement of the index $p$ of the distribution of non-thermal relativistic electrons $N(γ)\propto γ^{-p}$ accelerated by a shock launched by a NS-NS merger to date. We find $p=2.17\pm0.01$, which indicates that radiation from ejecta with $Γ\sim3-10$ dominates the observed emission. While constraining the nature of the emission process, these observations do \emph{not} constrain the nature of the relativistic ejecta. We employ simulations of explosive outflows launched in NS ejecta clouds to show that the spectral and temporal evolution of the non-thermal emission from GW170817 is consistent with both emission from radially stratified quasi-spherical ejecta traveling at mildly relativistic speeds, \emph{and} emission from off-axis collimated ejecta characterized by a narrow cone of ultra-relativistic material with slower wings extending to larger angles. In the latter scenario, GW170817 harbored a normal SGRB directed away from our line of sight. Observations at $t\le 200$ days are unlikely to settle the debate as in both scenarios the observed emission is effectively dominated by radiation from mildly relativistic material. △ Less

Submitted 25 February, 2018; v1 submitted 10 January, 2018; originally announced January 2018.

Comments: Updated with the latest VLA and Chandra data

The late inspiral of supermassive black hole binaries with circumbinary gas discs in the LISA band

Authors: Yike Tang, Zoltán Haiman, Andrew MacFadyen

Abstract: We present the results of 2D, moving-mesh, viscous hydrodynamical simulations of an accretion disc around a merging supermassive black hole binary (SMBHB). The simulation is pseudo-Newtonian, with the BHs modeled as point masses with a Paczynski-Wiita potential, and includes viscous heating, shock heating, and radiative cooling. We follow the gravitational inspiral of an equal-mass binary with a c… ▽ More We present the results of 2D, moving-mesh, viscous hydrodynamical simulations of an accretion disc around a merging supermassive black hole binary (SMBHB). The simulation is pseudo-Newtonian, with the BHs modeled as point masses with a Paczynski-Wiita potential, and includes viscous heating, shock heating, and radiative cooling. We follow the gravitational inspiral of an equal-mass binary with a component mass M_bh=10^6 M_sun from an initial separation of 60r_g (where r_g=GM_bh/c^2 is the gravitational radius) to the merger. We find that a central, low-density cavity forms around the binary, as in previous work, but that the BHs capture gas from the circumbinary disc and accrete efficiently via their own minidiscs, well after their inspiral outpaces the viscous evolution of the disc. The system remains luminous, displaying strong periodicity at twice the binary orbital frequency throughout the entire inspiral process, all the way to the merger. In the soft X-ray band, the thermal emission is dominated by the inner edge of the circumbinary disc with especially clear periodicity in the early inspiral. By comparison, harder X-ray emission is dominated by the minidiscs, and the light curve is initially more noisy but develops a clear periodicity in the late inspiral stage. This variability pattern should help identify the EM counterparts of SMBHBs detected by the space-based gravitational-wave detector LISA. △ Less

Submitted 7 January, 2018; originally announced January 2018.

Comments: submitted to MNRAS

The THESEUS space mission concept: science case, design and expected performances

Authors: L. Amati, P. O'Brien, D. Goetz, E. Bozzo, C. Tenzer, F. Frontera, G. Ghirlanda, C. Labanti, J. P. Osborne, G. Stratta, N. Tanvir, R. Willingale, P. Attina, R. Campana, A. J. Castro-Tirado, C. Contini, F. Fuschino, A. Gomboc, R. Hudec, P. Orleanski, E. Renotte, T. Rodic, Z. Bagoly, A. Blain, P. Callanan , et al. (187 additional authors not shown)

Abstract: THESEUS is a space mission concept aimed at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. These goals will be achieved through a unique combination of instruments allowing GRB and X-ray transient detection over a broad field of view (more than 1sr) with 0.5-1 arcmin localization, an energ… ▽ More THESEUS is a space mission concept aimed at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. These goals will be achieved through a unique combination of instruments allowing GRB and X-ray transient detection over a broad field of view (more than 1sr) with 0.5-1 arcmin localization, an energy band extending from several MeV down to 0.3 keV and high sensitivity to transient sources in the soft X-ray domain, as well as on-board prompt (few minutes) follow-up with a 0.7 m class IR telescope with both imaging and spectroscopic capabilities. THESEUS will be perfectly suited for addressing the main open issues in cosmology such as, e.g., star formation rate and metallicity evolution of the inter-stellar and intra-galactic medium up to redshift $\sim$10, signatures of Pop III stars, sources and physics of re-ionization, and the faint end of the galaxy luminosity function. In addition, it will provide unprecedented capability to monitor the X-ray variable sky, thus detecting, localizing, and identifying the electromagnetic counterparts to sources of gravitational radiation, which may be routinely detected in the late '20s / early '30s by next generation facilities like aLIGO/ aVirgo, eLISA, KAGRA, and Einstein Telescope. THESEUS will also provide powerful synergies with the next generation of multi-wavelength observatories (e.g., LSST, ELT, SKA, CTA, ATHENA). △ Less

Submitted 27 March, 2018; v1 submitted 12 October, 2017; originally announced October 2017.

Comments: Accepted for publication in Advances in Space Research. Partly based on the proposal submitted on October 2016 in response to the ESA Call for next M5 mission, with expanded and updated science sections

A GRB and Broad-lined Type Ic Supernova from a Single Central Engine

Authors: Jennifer Barnes, Paul C. Duffell, Yuqian Liu, Maryam Modjaz, Federica B. Bianco, Daniel Kasen, Andrew I. MacFadyen

Abstract: Unusually high velocities (< ~0.1c) and correspondingly high kinetic energies have been observed in a subset of Type Ic supernovae (so-called "broad-lined Ic" supernovae; SNe Ic-BL), prompting a search for a central engine model capable of generating such energetic explosions. A clue to the explosion mechanism may lie in the fact that all supernovae that accompany long-duration gamma-ray bursts be… ▽ More Unusually high velocities (< ~0.1c) and correspondingly high kinetic energies have been observed in a subset of Type Ic supernovae (so-called "broad-lined Ic" supernovae; SNe Ic-BL), prompting a search for a central engine model capable of generating such energetic explosions. A clue to the explosion mechanism may lie in the fact that all supernovae that accompany long-duration gamma-ray bursts belong to the SN Ic-BL class. Using a combination of two-dimensional relativistic hydrodynamics and radiation transport calculations, we demonstrate that the central engine responsible for long gamma-ray bursts can also trigger a SN Ic-BL. We find that a reasonable gamma-ray burst engine injected into a stripped Wolf-Rayet progenitor produces a relativistic jet with energy ~10^51 ergs, as well as a SN whose synthetic light curves and spectra are fully consistent with observed SNe Ic-BL during the photospheric phase. As a result of the jet's asymmetric energy injection, the SN spectra and light curves depend on viewing angle. The impact of viewing angle on the spectrum is particularly pronounced at early times, while the viewing angle dependence for the light curves (~10% variation in bolometric luminosity) persists throughout the photospheric phase. △ Less

Submitted 8 August, 2017; originally announced August 2017.

Comments: Submitted to ApJ; comments welcome

On the orbital evolution of supermassive black hole binaries with circumbinary accretion discs

Authors: Yike Tang, Andrew MacFadyen, Zoltan Haiman

Abstract: Gaseous circumbinary accretion discs provide a promising mechanism to facilitate the mergers of supermassive black holes (SMBHs) in galactic nuclei. We measure the torques exerted on accreting SMBH binaries, using 2D, isothermal, moving-mesh, viscous hydrodynamical simulations of circumbinary accretion discs. Our computational domain includes the entire inner region of the circumbinary disk with t… ▽ More Gaseous circumbinary accretion discs provide a promising mechanism to facilitate the mergers of supermassive black holes (SMBHs) in galactic nuclei. We measure the torques exerted on accreting SMBH binaries, using 2D, isothermal, moving-mesh, viscous hydrodynamical simulations of circumbinary accretion discs. Our computational domain includes the entire inner region of the circumbinary disk with the individual black holes (BHs) included as point masses on the grid and a sink prescription to model accretion onto each BH. The BHs each acquire their own well-resolved accretion discs ("minidiscs"). We explore a range of mass removal rates for the sink prescription removing gas from the central regions of the minidiscs. We find that the torque exerted on the binary is primarily gravitational, and dominated by the gas orbiting close behind and ahead of the individual BHs. The torques from the distorted circumbinary disc farther out and from the direct accretion of angular momentum are subdominant. The torques are sensitive to the sink prescription: slower sinks result in more gas accumulating near the BHs and more negative torques, driving the binary to merger more rapidly. For faster sinks, the torques are less negative and eventually turn positive (for unphysically fast sinks). When the minidiscs are modeled as standard alpha discs, our results are insensitive to the choice of sink radius. Scaling the simulations to a binary orbital period tbin = 1yr and background disc accretion rate Mdot = 0.3MEdd in Eddington units, the binary inspirals on a timescale of 3X10^6 years, irrespective of the SMBH masses. For binaries with total mass <10^7Msun, this is shorter than the inspiral time due to gravitational wave (GW) emission alone, implying that gas discs will have a significant impact on the SMBH binary population and can affect the GW signal for Pulsar Timing Arrays. △ Less

Submitted 11 March, 2017; originally announced March 2017.

Minidisks in Binary Black Hole Accretion

Authors: Geoffrey Ryan, Andrew MacFadyen

Abstract: Newtonian simulations have demonstrated that accretion onto binary black holes produces accretion disks around each black hole ("minidisks"), fed by gas streams flowing through the circumbinary cavity from the surrounding circumbinary disk. We study the dynamics and radiation of an individual black hole minidisk using two-dimensional hydrodynamical simulations performed with a new general relativi… ▽ More Newtonian simulations have demonstrated that accretion onto binary black holes produces accretion disks around each black hole ("minidisks"), fed by gas streams flowing through the circumbinary cavity from the surrounding circumbinary disk. We study the dynamics and radiation of an individual black hole minidisk using two-dimensional hydrodynamical simulations performed with a new general relativistic version of the moving mesh code Disco. We introduce a co-moving energy variable which enables highly accurate integration of these high Mach number flows. Tidally induced spiral shock waves are excited in the disk and propagate through the ISCO providing a Reynolds stress which causes efficient accretion by purely hydrodynamic means and producing a radiative signature brighter in hard X-rays than the Novikov-Thorne model. Disk cooling is provided by a local blackbody prescription that allows the disk to evolve self-consistently to a temperature profile where hydrodynamic heating is balanced by radiative cooling. We find that the spiral shock structure is in agreement with the relativistic dispersion relation for tightly-wound linear waves. We measure the shock induced dissipation and find outward angular momentum transport corresponding to an effective alpha parameter of order 0.01. We perform ray-tracing image calculations from the simulations to produce theoretical minidisk spectra and viewing angle dependent images for comparison with observations. △ Less

Submitted 31 January, 2017; v1 submitted 1 November, 2016; originally announced November 2016.

Comments: Accepted for publication in The Astronomical Journal, 16 pages, 19 figures

Journal ref: The Astrophysical Journal, Volume 835, Number 2, pp. 199 (2017)

A transition in circumbinary accretion discs at a binary mass ratio of 1:25

Authors: Daniel J. D'Orazio, Zoltan Haiman, Paul Duffell, Andrew I. MacFadyen, Brian D. Farris

Abstract: We study circumbinary accretion discs in the framework of the restricted three-body problem (R3Bp) and via numerically solving the height-integrated equations of viscous hydrodynamics. Varying the mass ratio of the binary, we find a pronounced change in the behaviour of the disc near mass ratio $q \equiv M_s/M_p \sim 0.04$. For mass ratios above $q=0.04$, solutions for the hydrodynamic flow transi… ▽ More We study circumbinary accretion discs in the framework of the restricted three-body problem (R3Bp) and via numerically solving the height-integrated equations of viscous hydrodynamics. Varying the mass ratio of the binary, we find a pronounced change in the behaviour of the disc near mass ratio $q \equiv M_s/M_p \sim 0.04$. For mass ratios above $q=0.04$, solutions for the hydrodynamic flow transition from steady, to strongly-fluctuating; a narrow annular gap in the surface density around the secondary's orbit changes to a hollow central cavity; and a spatial symmetry is lost, resulting in a lopsided disc. This phase transition is coincident with the mass ratio above which stable orbits do not exist around the L4 and L5 equilibrium points of the R3B problem. Using the DISCO code, we find that for thin discs, for which a gap or cavity can remain open, the mass ratio of the transition is relatively insensitive to disc viscosity and pressure. The $q=0.04$ transition has relevance for the evolution of massive black hole binary+disc systems at the centers of galactic nuclei, as well as for young stellar binaries and possibly planets around brown dwarfs. △ Less

Submitted 9 May, 2016; v1 submitted 17 December, 2015; originally announced December 2015.

Comments: published in MNRAS

Journal ref: Monthly Notices of the Royal Astronomical Society, Volume 459, Issue 3, p.2379-2393, 07/2016

A Narrow Short-Duration GRB Jet from a Wide Central Engine

Authors: Paul C. Duffell, Eliot Quataert, Andrew I. MacFadyen

Abstract: We use two-dimensional relativistic hydrodynamic numerical calculations to show that highly collimated relativistic jets can be produced in neutron star merger models of short-duration gamma ray bursts without the need for a highly directed engine or a large net magnetic flux. Even a hydrodynamic engine generating a very wide sustained outflow on small scales can in principle produce a highly coll… ▽ More We use two-dimensional relativistic hydrodynamic numerical calculations to show that highly collimated relativistic jets can be produced in neutron star merger models of short-duration gamma ray bursts without the need for a highly directed engine or a large net magnetic flux. Even a hydrodynamic engine generating a very wide sustained outflow on small scales can in principle produce a highly collimated relativistic jet, facilitated by a dense surrounding medium which provides a cocoon surrounding the jet core. An oblate geometry to the surrounding gas significantly enhances the collimation process. Previous numerical simulations have shown that the merger of two neutron stars produces an oblate, expanding cloud of dynamical ejecta. We show that this gas can efficiently collimate the central engine power much as the surrounding star does in long-duration GRB models. For typical short-duration GRB central engine parameters, we find jets with opening angles of order 10 degrees in which a large fraction of the total outflow power of the central engine resides in highly relativistic material. These results predict large differences in the opening angles of outflows from binary neutron star mergers versus neutron star-black hole mergers. △ Less

Submitted 20 May, 2015; originally announced May 2015.

A reduced orbital period for the supermassive black hole binary candidate in the quasar PG 1302-102?

Authors: Daniel J. D'Orazio, Zoltán Haiman, Paul Duffell, Brian D. Farris, Andrew I. MacFadyen

Abstract: Graham et al. (2015) have detected a 5.2 year periodic optical variability of the quasar PG 1302-102 at redshift $z=0.3$, which they interpret as the redshifted orbital period $(1+z)t_{\rm bin}$ of a putative supermassive black hole binary (SMBHB). Here we consider the implications of a $3-8$ times shorter orbital period, suggested by hydrodynamical simulations of circumbinary discs (CBDs) with ne… ▽ More Graham et al. (2015) have detected a 5.2 year periodic optical variability of the quasar PG 1302-102 at redshift $z=0.3$, which they interpret as the redshifted orbital period $(1+z)t_{\rm bin}$ of a putative supermassive black hole binary (SMBHB). Here we consider the implications of a $3-8$ times shorter orbital period, suggested by hydrodynamical simulations of circumbinary discs (CBDs) with nearly equal--mass SMBHBs ($q\equiv M_2/M_1\gtrsim 0.3$). With the corresponding $2-4$ times tighter binary separation, PG 1302 would be undergoing gravitational wave dominated inspiral, and serve as a proof that the BHs can be fueled and produce bright emission even in this late stage of the merger. The expected fraction of binaries with the shorter $t_{\rm bin}$, among bright quasars, would be reduced by 1-2 orders of magnitude, compared to the 5.2 year period, in better agreement with the rarity of candidates reported by Graham et al. (2015). Finally, shorter periods would imply higher binary speeds, possibly imprinting periodicity on the light curves from relativistic beaming, as well as measurable relativistic effects on the Fe K$α$ line. The CBD model predicts additional periodic variability on timescales of $t_{\rm bin}$ and $\approx 0.5 t_{\rm bin}$, as well as periodic variation of broad line widths and offsets relative to the narrow lines, which are consistent with the observations. Future observations will be able to test these predictions and hence the binary+CBD hypothesis for PG 1302. △ Less

Submitted 31 July, 2015; v1 submitted 10 February, 2015; originally announced February 2015.

Comments: Published in MNRAS

Journal ref: Monthly Notices of the Royal Astronomical Society 2015 452 (3): 2540-2545

Producing Magnetar Magnetic Fields in the Merger of Binary Neutron Stars

Authors: Bruno Giacomazzo, Jonathan Zrake, Paul Duffell, Andrew I. MacFadyen, Rosalba Perna

Abstract: The merger of binary neutron stars (BNSs) can lead to large amplifications of the magnetic field due to the development of turbulence and instabilities in the fluid, such as the Kelvin-Helmholtz shear instability, which drive small-scale dynamo activity. In order to properly resolve such instabilities and obtain the correct magnetic field amplification, one would need to employ resolutions that ar… ▽ More The merger of binary neutron stars (BNSs) can lead to large amplifications of the magnetic field due to the development of turbulence and instabilities in the fluid, such as the Kelvin-Helmholtz shear instability, which drive small-scale dynamo activity. In order to properly resolve such instabilities and obtain the correct magnetic field amplification, one would need to employ resolutions that are currently unfeasible in global general relativistic magnetohydrodynamic (GRMHD) simulations of BNS mergers. Here, we present a subgrid model that allows global simulations to take into account the small-scale amplification of the magnetic field which is caused by the development of turbulence during BNS mergers. Assuming dynamo saturation, we show that magnetar-level fields ($\sim 10^{16}\,{\rm G}$) can be easily reached, and should therefore be expected from the merger of magnetized BNSs. The total magnetic energy can reach values up to $\sim 10^{51}\,{\rm erg}$ and the post-merger remnant can therefore emit strong electromagnetic signals and possibly produce short gamma-ray bursts. △ Less

Submitted 8 September, 2015; v1 submitted 30 September, 2014; originally announced October 2014.

Comments: 5 pages, 4 figures, small changes in the text to match version published on ApJ

Journal ref: ApJ, 809, 39 (2015)

Binary Black Hole Accretion During Inspiral and Merger

Authors: Brian D. Farris, Paul Duffell, Andrew I. MacFadyen, Zoltan Haiman

Abstract: We present the results of 2D, moving mesh, viscous hydrodynamical simulations of accretion onto merging supermassive black hole (SMBH) binaries. We include viscous heating, shock heating, and radiative cooling, and simulate the transition from the "pre-decoupling" epoch, where the inspiral timescale is longer than the viscous timescale, to the "post-decoupling" epoch, where the inspiral timescale… ▽ More We present the results of 2D, moving mesh, viscous hydrodynamical simulations of accretion onto merging supermassive black hole (SMBH) binaries. We include viscous heating, shock heating, and radiative cooling, and simulate the transition from the "pre-decoupling" epoch, where the inspiral timescale is longer than the viscous timescale, to the "post-decoupling" epoch, where the inspiral timescale is shorter than the viscous timescale. We find that there is no abrupt halt to the accretion at decoupling, but rather the accretion shows a slow decay, with significant accretion well after the expected decoupling. Moreover, we find that the luminosity in X-rays is significantly higher prior to the merger, as orbital energy from the SMBH binary is converted to heat via strong shocks inside the cavity, and radiated away. Following the merger, the cavity refills viscously and the accretion rate relaxes to the Shakura-Sunyaev value, while the X-ray luminosity drops as the shocks quickly dissipate. △ Less

Submitted 17 September, 2014; originally announced September 2014.

From Engine to Afterglow: Collapsars Naturally Produce Top-Heavy Jets and Early-Time Plateaus in Gamma Ray Burst Afterglows

Authors: Paul C. Duffell, Andrew I. MacFadyen

Abstract: We demonstrate that the steep decay and long plateau in the early phases of gamma ray burst (GRB) X-ray afterglows are naturally produced in the collapsar model, by a means ultimately related to the dynamics of relativistic jet propagation through a massive star. We present two-dimensional axisymmetric hydrodynamical simulations which start from a collapsar engine and evolve all the way through th… ▽ More We demonstrate that the steep decay and long plateau in the early phases of gamma ray burst (GRB) X-ray afterglows are naturally produced in the collapsar model, by a means ultimately related to the dynamics of relativistic jet propagation through a massive star. We present two-dimensional axisymmetric hydrodynamical simulations which start from a collapsar engine and evolve all the way through the late afterglow phase. The resultant outflow includes a jet core which is highly relativistic after breaking out of the star, but becomes baryon-loaded after colliding with a massive outer shell, corresponding to mass from the stellar atmosphere of the progenitor star which became trapped in front of the jet core at breakout. The prompt emission produced before or during this collision would then have the signature of a high Lorentz factor jet, but the afterglow is produced by the amalgamated post-collision ejecta which has more inertia than the original highly relativistic jet core and thus has a delayed deceleration. This naturally explains the early light curve behavior discovered by Swift, including a steep decay and a long plateau, without invoking late-time energy injection from the central engine. The numerical simulation is performed continuously from engine to afterglow, covering a dynamic range of over ten orders of magnitude in radius. Light curves calculated from the numerical output demonstrate that this mechanism reproduces basic features seen in early afterglow data. Initial steep decays are produced by internal shocks, and the plateau corresponds to the coasting phase of the outflow. △ Less

Submitted 26 April, 2015; v1 submitted 30 July, 2014; originally announced July 2014.

Comments: ApJ Accepted

High-Frequency Voronoi Noise Reduced by Smoothed Mesh Motion

Authors: Paul C. Duffell, Andrew I. MacFadyen

Abstract: We describe a technique for improving the performance of hydrodynamics codes which employ a moving Voronoi mesh. Currently, such codes are susceptible to high-frequency noise produced by rapid adjustments in the grid topology on the smallest scales. The treatment for this grid noise is simple; instead of moving the mesh-generating marker points with the local fluid velocity, this velocity field is… ▽ More We describe a technique for improving the performance of hydrodynamics codes which employ a moving Voronoi mesh. Currently, such codes are susceptible to high-frequency noise produced by rapid adjustments in the grid topology on the smallest scales. The treatment for this grid noise is simple; instead of moving the mesh-generating marker points with the local fluid velocity, this velocity field is smoothed on small scales, so that neighboring marker points generally have similar velocities. We demonstrate significant improvement gained by this adjustment in several code tests relevant to the physics which moving-mesh codes are designed to capture. △ Less

Submitted 9 April, 2015; v1 submitted 27 July, 2014; originally announced July 2014.

Comments: MNRAS Accepted

An upper limit to the energy of gamma-ray bursts indicates that GRB/SNe are powered by magnetars

Authors: P. Mazzali, A. MacFadyen, S. Woosley, E. Pian, M. Tanaka

Abstract: The kinetic energy of supernovae (SNe) accompanied by gamma-ray bursts (GRBs) tends to cluster near E52 erg, with 2.E52 erg an upper limit to which no compelling exceptions are found (assuming a certain degree of asphericity), and it is always significantly larger than the intrinsic energy of the GRB themselves (corrected for jet collimation). This energy is strikingly similar to the maximum rotat… ▽ More The kinetic energy of supernovae (SNe) accompanied by gamma-ray bursts (GRBs) tends to cluster near E52 erg, with 2.E52 erg an upper limit to which no compelling exceptions are found (assuming a certain degree of asphericity), and it is always significantly larger than the intrinsic energy of the GRB themselves (corrected for jet collimation). This energy is strikingly similar to the maximum rotational energy of a neutron star rotating with period 1 ms. It is therefore proposed that all GRBs associated with luminous SNe are produced by magnetars. GRBs that result from black hole formation (collapsars) may not produce luminous SNe. X-ray Flashes (XRFs), which are associated with less energetic SNe, are produced by neutron stars with weaker magnetic field or lower spin. △ Less

Submitted 4 June, 2014; originally announced June 2014.

Comments: 6 pages, 1 figure, to appear in MNRAS

Characteristic Signatures in the Thermal Emission from Accreting Binary Black Holes

Authors: Brian D. Farris, Paul Duffell, Andrew I. MacFadyen, Zoltan Haiman

Abstract: We present the results of a calculation of the thermal spectrum from a 2D, moving mesh, high-accuracy, viscous hydrodynamical simulation of an accreting supermassive black hole binary. We include viscous heating, shock heating, and radiative cooling, evolving for longer than a viscous time so that we reach a quasi-steady accretion state. In agreement with previous work, we find that gas is efficie… ▽ More We present the results of a calculation of the thermal spectrum from a 2D, moving mesh, high-accuracy, viscous hydrodynamical simulation of an accreting supermassive black hole binary. We include viscous heating, shock heating, and radiative cooling, evolving for longer than a viscous time so that we reach a quasi-steady accretion state. In agreement with previous work, we find that gas is efficiently stripped from the inner edge of the circumbinary disk and enters the cavity along accretion streams, which feed persistent "mini-disks" surrounding each black hole. We also find that emission from the shock-heated mini-disks and accretion streams prevents any deficit in high-energy emission that may be expected inside the circumbinary cavity, and instead leads to a characteristic brightening of the spectrum beginning in soft X-rays. △ Less

Submitted 30 May, 2014; originally announced June 2014.