Showing 1–50 of 328 results for author: Zhao, X

The Centennial Evolution of Solar Chromospheric Rotation

Authors: N. B. Xiang, X. H. Zhao, L. H. Deng, F. Y. Li, M. Wan, S. Y. Qi

Abstract: Rotation is a prominent feature of the Sun, and it plays a crucial role in the generation and dynamic evolution of solar magnetic fields. The daily composite time series of Ca II K plage areas from 1907 February 1 to 2023 December 31 is used to analyze its periodicity and examine the temporal variation of its rotation period lengths (RPLs) using continuous wavelet transform. Wavelet analysis revea… ▽ More Rotation is a prominent feature of the Sun, and it plays a crucial role in the generation and dynamic evolution of solar magnetic fields. The daily composite time series of Ca II K plage areas from 1907 February 1 to 2023 December 31 is used to analyze its periodicity and examine the temporal variation of its rotation period lengths (RPLs) using continuous wavelet transform. Wavelet analysis reveals that over a time span of more than a century, chromospheric rotation exhibits a dominant synodic period of approximately 26.62 days, with complex temporal variations. The long-term trend of chromospheric rotation is well-characterized by a statistically significant quadratic polynomial, showing a gradual deceleration from solar cycles 15 to 19, followed by a gradual acceleration from cycles 19 to 24. The RPLs exhibit a negative correlation between the rotation rate of the chromosphere and solar magnetic activity. Their behavior follows a distinct pattern within a Schwabe cycle: the rotation period progressively lengthens during the initial approximately 3 years, then maintains a relatively long value from year 3 to approximately 7.5, and finally shortens during the declining phase, returning to a minimum near the subsequent solar minimum. The variations of chromospheric RPLs show significant periods of 3.2, 5.7, 7.7, 10.3, and 12.3 years, with cross-correlation analysis pointing to a complex relationship with solar activity. The possible mechanisms for the temporal variation of the chromospheric rotation are discussed. △ Less

Submitted 17 December, 2025; originally announced December 2025.

Comments: Accepted for publication in The Astrophysical Journal

Measurement of the cosmic ray nickel energy spectrum from 10 GeV/n to 2 TeV/n with the DAMPE

Authors: F. Alemanno, Q. An, P. Azzarello, F. C. T. Barbato, P. Bernardini, X. J. Bi, H. V. Boutin, I. Cagnoli, M. S. Cai, E. Casilli, J. Chang, D. Y. Chen, J. L. Chen, Z. F. Chen, Z. X. Chen, P. Coppin, M. Y. Cui, T. S. Cui, I. De Mitri, F. de Palma, A. Di Giovanni, T. K. Dong, Z. X. Dong, G. Donvito, J. L. Duan , et al. (123 additional authors not shown)

Abstract: Nickel, one of the most stable elements alongside iron, is the most abundant heavy element beyond iron in cosmic rays. With DAMPE's excellent charge resolution and broad energy range, a high-precision energy spectrum provides valuable insights into the acceleration sources of heavy nuclei and their propagation through the interstellar medium. In this analysis, we report the direct measurement of c… ▽ More Nickel, one of the most stable elements alongside iron, is the most abundant heavy element beyond iron in cosmic rays. With DAMPE's excellent charge resolution and broad energy range, a high-precision energy spectrum provides valuable insights into the acceleration sources of heavy nuclei and their propagation through the interstellar medium. In this analysis, we report the direct measurement of cosmic-ray nickel spectrum from 10 GeV/n to 2 TeV/n with nine years of flight data. The nickel spectrum is consistent with a single power law with spectral index -2.60 +/- 0.03 from 40 GeV/n to 1 TeV/n. This work provides an accurate measurement of differential flux of nickel with kinetic energy extending to TeV/n for the first time. △ Less

Submitted 12 December, 2025; originally announced December 2025.

Lateral Deformation of Large-scale Coronal Mass Ejections during the Transition from Non-radial to Radial Propagation

Authors: Huidong Hu, Chong Chen, Yiming Jiao, Bei Zhu, Rui Wang, Xiaowei Zhao, Liping Yang

Abstract: Many coronal mass ejections (CMEs) initially propagate non-radially, and then transition to radial propagation in the corona. This directional transition is a significant process that determines a CME's space weather effects but remains poorly understood. Based on multi-wavelength observations, we investigate the transition from non-radial to radial propagation in the low corona for two large-scal… ▽ More Many coronal mass ejections (CMEs) initially propagate non-radially, and then transition to radial propagation in the corona. This directional transition is a significant process that determines a CME's space weather effects but remains poorly understood. Based on multi-wavelength observations, we investigate the transition from non-radial to radial propagation in the low corona for two large-scale CMEs from the same active region on the solar limb. In the beginning, both CMEs move in a non-radial direction, beneath a system of overlying loops that are roughly parallel to the flux-rope axis. The CMEs laterally deform by bulging their upper flanks in the non-radial stage toward the higher corona, which results in the transition to a radial propagation direction approximately 25$^\circ$ away from the eruption site. After the directional transition, the non-radial-stage upper flank becomes the leading edge in the radial stage. Although the overlying loops do not strap over the flux rope, their strong magnetic tension force constrains the radial expansion of part of the CME during the transition by acting on the flux-rope legs. A major portion of the filament is displaced to the southern part of a CME in the radial stage, which implies the complexity of observational CME features. This study presents the first investigation of the lateral deformation during the transition of CMEs from non-radial to radial in the low corona, and makes an essential contribution to the complete CME evolution picture. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Comments: 25 pages, 9 figures, 1 table. Accepted for publication in ApJ. Supplementary animations available at https://doi.org/10.5281/zenodo.17776047

Multi-Wavelength Afterglows as Diagnostic Probes of Dense Circumburst Medium in GRBs

Authors: Xiao-Hong Zhao

Abstract: Gamma-ray bursts (GRBs) are generally believed to occur in environments where the surrounding medium is either a uniform interstellar medium (ISM) or, in some cases, a dense stellar wind from a massive progenitor. Recently, GRB 191019A has been proposed to originate within the accretion disk of an active galactic nucleus (AGN), suggesting that some GRBs may occur in extremely dense environments, a… ▽ More Gamma-ray bursts (GRBs) are generally believed to occur in environments where the surrounding medium is either a uniform interstellar medium (ISM) or, in some cases, a dense stellar wind from a massive progenitor. Recently, GRB 191019A has been proposed to originate within the accretion disk of an active galactic nucleus (AGN), suggesting that some GRBs may occur in extremely dense environments, although this interpretation remains under debate. This scenario has drawn considerable attention, as AGN disks are promising sites that can host progenitors of both long and short GRBs, and whose dense, gas-rich environment could significantly influence jet propagation and afterglow emission. Yet, our theoretical understanding of the resulting afterglow signatures in such environments is limited, and further systematic exploration is required. In this study, we investigate how multi-wavelength afterglow light curves can be utilized as diagnostic tools to probe the nature of the circumburst environment. Our results show that in dense environments, GRB afterglows exhibit distinct frequency-dependent behaviors. For jets with large opening angles, the X-ray light curve displays a shallow decay or bump due to a transition from synchrotron to SSC dominance, while the optical and high-energy (GeV) light curves follow typical power-law decays. On the other hand, for small opening angles, the light curves exhibit wavelength-dependent jet breaks: the GeV and optical bands break simultaneously, while the X-ray break is delayed as the SSC component gradually compensates for the fading synchrotron component. These signatures provide potential diagnostics of GRBs occurring in dense media such as AGN disks. △ Less

Submitted 9 December, 2025; v1 submitted 8 December, 2025; originally announced December 2025.

Comments: 15 pages, 6 figures, accepted for publication in ApJ

Fast X-ray Variability from the Coronae of Supermassive Black Holes

Authors: Xiurui Zhao, Luca Comisso, Stefano Marchesi, Marco Ajello, Elias Kammoun, Yue Shen, Qiaoya Wu

Abstract: We present the first systematic study of short-timescale X-ray variability in radio-quiet active galactic nuclei (AGN), utilizing archival Chandra observations of approximately 3,000 broad-line AGN selected from the SDSS and DESI spectroscopic surveys. We identify 14 AGN exhibiting rapid (on timescales of tens of kiloseconds) X-ray flux variations by factors of two or more that are statistically s… ▽ More We present the first systematic study of short-timescale X-ray variability in radio-quiet active galactic nuclei (AGN), utilizing archival Chandra observations of approximately 3,000 broad-line AGN selected from the SDSS and DESI spectroscopic surveys. We identify 14 AGN exhibiting rapid (on timescales of tens of kiloseconds) X-ray flux variations by factors of two or more that are statistically significant ($p\le6\times10^{-4}$), indicative of fast coronal variability. By converting minimum variability timescales to light-crossing times, we place upper limits on the sizes of the variable coronal regions, finding typical scales of $\lesssim10^{-4}$~pc. The coronal variable region size upper limits of an AGN in our sample are found to be much smaller than the typical coronal sizes inferred from microlensing, suggesting that its corona is composed of localized, transient structures rather than smooth, homogeneous plasmas. Such efficient magnetic energy dissipation in compact volumes is consistent with expectations for magnetically dominated coronae and is supported by recent general relativistic magnetohydrodynamic simulations. Future high-throughput X-ray observatories will enable the detection of even faster coronal variability, providing direct constraints on the physical mechanisms driving plasma energization and flux fluctuation near supermassive black holes. Our results suggest that luminous AGN hosting massive black holes are prime targets for probing the small-scale structure and dynamics of AGN coronae. △ Less

Submitted 6 December, 2025; originally announced December 2025.

Comments: 13 pages, 5 figures, 2 tables, accepted to ApJL

GRB 240715A: Revealing Novel Intrinsic Mechanism by Different Individual Pulse

Authors: Wen-Jun Tan, Chen-Wei Wang, Shao-Lin Xiong, Shi-Jie Zheng, Jiang He, Xiao-Yun Zhao, Yue Huang, Shu-Xu Yi, Bing Li, He Gao, Bo-bing Wu, Bing Zhang, Frederic Daigne, Maria-Grazia Bernardini, Bin-Bin Zhang, Stephane Basa, Bertrand Cordier, Jin-Song Deng, Yong-Wei Dong, Damien Dornic, Olivier Godet, Xu-Hui Han, Mao-Hai Huang, Cyril Lachaud, Hua-Li Li , et al. (15 additional authors not shown)

Abstract: The Space-based multiband astronomical Variable Objects Monitor (SVOM), detected its first short gamma-ray burst, GRB 240715A, in-flight, which was jointly observed by Fermi. Based on observational data of SVOM/GRM and Fermi/GBM, we perform a comprehensive temporal and spectral analysis for individual pulse in the prompt emission of this burst, and novel characteristics are revealed. Firstly, oppo… ▽ More The Space-based multiband astronomical Variable Objects Monitor (SVOM), detected its first short gamma-ray burst, GRB 240715A, in-flight, which was jointly observed by Fermi. Based on observational data of SVOM/GRM and Fermi/GBM, we perform a comprehensive temporal and spectral analysis for individual pulse in the prompt emission of this burst, and novel characteristics are revealed. Firstly, opposite evolutions of spectral lag are found in the first and third pulse of this burst. Second, the large negative lag of the first pulse is an outlier in short GRB sample, especially when the pulse duration is considered. Spectral analysis shows that the negative lag of the first pulse is caused by the evolution of spectrum index, and is irrelevant to Epeak, which is inconsistent with the previous study. The intrinsic mechanism is probably attributed to electron cooling in the decaying magnetic field, which leads to the continuous hardening of the spectrum index and results in negative lag. Furthermore, spectral analysis also shows that the third pulse is more likely to be described by a quasi-thermal spectrum, indicating the existence of photospheric emission. It is difficult to explain how the synchrotron radiation appears before photospheric emission in a single GRB and some assumptions are discussed. △ Less

Submitted 23 November, 2025; originally announced November 2025.

Comments: 21 pages, 4 figures, accepted by The Astrophysical Journal Letters

A Method for Gamma-Ray Energy Spectrum Inversion and Correction

Authors: Zhi-Qiang Ding, Xin-Qiao Li, Da-Li Zhang, Zheng-Hua An, Zhen-Xia Zhang, Roberto Battiston, Roberto Iuppa, Zhuo Li, Yan-Qiu Zhang, Yan Huang, Chao Zheng, Yan-Bing Xu, Xiao-Yun Zhao, Lu Wang, Ping Wang, Hong Lu

Abstract: Accurate spectral analysis of high-energy astrophysical sources often relies on comparing observed data to incident spectral models convolved with the instrument response. However, for Gamma-Ray Bursts and other high-energy transient events observed at high count rates, significant distortions (e.g., pile-up, dead time, and large signal trailing) are introduced, complicating this analysis. We pres… ▽ More Accurate spectral analysis of high-energy astrophysical sources often relies on comparing observed data to incident spectral models convolved with the instrument response. However, for Gamma-Ray Bursts and other high-energy transient events observed at high count rates, significant distortions (e.g., pile-up, dead time, and large signal trailing) are introduced, complicating this analysis. We present a method framework to address the model dependence problem, especially to solve the problem of energy spectrum distortion caused by instrument signal pile-up due to high counting rate and high-rate effects, applicable to X-ray, gamma-ray, and particle detectors. Our approach combines physics-based Monte Carlo (MC) simulations with a model-independent spectral inversion technique. The MC simulations quantify instrumental effects and enable correction of the distorted spectrum. Subsequently, the inversion step reconstructs the incident spectrum using an inverse response matrix approach, conceptually equivalent to deconvolving the detector response. The inversion employs a Convolutional Neural Network, selected for its numerical stability and effective handling of complex detector responses. Validation using simulations across diverse input spectra demonstrates high fidelity. Specifically, for 27 different parameter sets of the brightest gamma-ray bursts, goodness-of-fit tests confirm the reconstructed spectra are in excellent statistical agreement with the input spectra, and residuals are typically within $\pm 2σ$. This method enables precise analysis of intense transients and other high-flux events, overcoming limitations imposed by instrumental effects in traditional analyses. △ Less

Submitted 19 November, 2025; originally announced November 2025.

Comments: The Astrophysical Journal has accepted

How to evaluate the sufficiency and complementarity of summary statistics for cosmic fields: an information-theoretic perspective

Authors: Ce Sui, Yi Mao, Xiaosheng Zhao, Tao Jing, Benjamin D. Wandelt

Abstract: The advent of increasingly advanced surveys and cosmic tracers has motivated the development of new inference techniques and novel approaches to extracting information from cosmic fields. A central challenge in this endeavor is to quantify the information content carried by these summary statistics in cosmic fields. In particular, how should we assess which statistics are more informative than oth… ▽ More The advent of increasingly advanced surveys and cosmic tracers has motivated the development of new inference techniques and novel approaches to extracting information from cosmic fields. A central challenge in this endeavor is to quantify the information content carried by these summary statistics in cosmic fields. In particular, how should we assess which statistics are more informative than others and assess the exact degree of complementarity of the information from each statistic? Here, we introduce mutual information (MI) that provides, from an information-theoretic perspective, a natural framework for assessing the sufficiency and complementarity of summary statistics in cosmological data. We demonstrate how MI can be applied to typical inference tasks to make information-theoretic evaluations, using two representative examples: the cosmic microwave background map, from which the power spectrum extracts almost all information as is expected for a Gaussian random field, and the 21~cm brightness temperature map, from which the scattering transform extracts the most non-Gaussian information but is complementary to power spectrum and bispectrum. Our results suggest that MI offers a robust theoretical foundation for evaluating and improving summaries, thereby enabling a deeper understanding of cosmic fields from an information-theoretic perspective. △ Less

Submitted 11 November, 2025; originally announced November 2025.

Comments: Submitted to ApJL. Extended version of the conference paper arXiv:2307.04994. Comments welcome

SVOM Follow-up Observation Coordinating Service

Authors: Xu-hui Han, Pin-pin Zhang, Yu-jie Xiao, Ruo-song Zhang, Chao Wu, Li-ping Xin, Hong-bo Cai, Hai Cao, Hui-jun Chen, Jin-song Deng, Wen-long Dong, Guo-wang Du, Lei Huang, Lin Lan, Hua-li Li, Guang-wei Li, Xiao-meng Lu, Yu-lei Qiu, Jian-feng Tian, Jing Wang, Wen-jin Xie, Da-wei Xu, Yang Xu, Zhu-heng Yao, Xue-ying Zhao , et al. (5 additional authors not shown)

Abstract: The Sino-French SVOM (Space Variable Objects Monitor) mission is a space-based astronomy mission complemented with ground-based dedicated instrumentation. It aims to explore and study high-energy cosmic phenomena, such as gamma-ray bursts (GRBs). This unprecedented combination of space-based and ground-based instruments will provide leading multi-wavelength observational capabilities in gamma-rays… ▽ More The Sino-French SVOM (Space Variable Objects Monitor) mission is a space-based astronomy mission complemented with ground-based dedicated instrumentation. It aims to explore and study high-energy cosmic phenomena, such as gamma-ray bursts (GRBs). This unprecedented combination of space-based and ground-based instruments will provide leading multi-wavelength observational capabilities in gamma-rays, X-rays, optical, and near-infrared bands. The complete observation sequence of each GRB triggered by the SVOM mission consists of three stages, the GRB detections, followed by the on-board and grounded automatic follow-ups, and rapid deep multi-band photometry and spectroscopy re-visit observations. To efficiently organize all grounded instruments performing automatic follow-ups and re-visit observations, we develop a follow-up observation coordinating service (FOCS), which is capable of performing GRB trigger distributing, automatic observation scheduling and observation coordination supporting by providing a user support platform. The FOCS also facilitates the provision of observational planning for ground-based telescopes to conduct synchronized observations of identical celestial regions as SVOM. The FOCS is utilized for the SVOM-dedicated ground-based telescopes as well as for associated partner telescopes. Since the launch of SVOM in June 2024, as the FOCS system joining the operations of SVOM, multiple successful observations have been made for SVOM GRBs. In this paper, we present the goals of the FOCS system as well as the principle and workflow developed to achieve these goals. The structure, technical design, implementation, and performance of the FOCS system are also described in detail. We conclude with a summary of the current status of the FOCS system and our near-future development plan. △ Less

Submitted 9 November, 2025; originally announced November 2025.

Comments: 10 pages, 5 figures

Charge-dependent spectral softenings of primary cosmic-rays from proton to iron below the knee

Authors: DAMPE Collaboration, Francesca Alemanno, Qi An, Philipp Azzarello, Felicia-Carla-Tiziana Barbato, Paolo Bernardini, Xiao-Jun Bi, Hugo Valentin Boutin, Irene Cagnoli, Ming-Sheng Cai, Elisabetta Casilli, Jin Chang, Deng-Yi Chen, Jun-Ling Chen, Zhan-Fang Chen, Zi-Xuan Chen, Paul Coppin, Ming-Yang Cui, Tian-Shu Cui, Ivan De Mitri, Francesco de Palma, Adriano Di Giovanni, Tie-Kuang Dong, Zhen-Xing Dong, Giacinto Donvito , et al. (124 additional authors not shown)

Abstract: In most particle acceleration mechanisms, the maximum energy of the cosmic rays can achieve is charge dependent. However, the observational verification of such a fundamental relation is still lack due to the difficulty of measuring the spectra of individual particles from one (kind of) source(s) up to very high energies. This work reports direct measurements of the carbon, oxygen, and iron spectr… ▽ More In most particle acceleration mechanisms, the maximum energy of the cosmic rays can achieve is charge dependent. However, the observational verification of such a fundamental relation is still lack due to the difficulty of measuring the spectra of individual particles from one (kind of) source(s) up to very high energies. This work reports direct measurements of the carbon, oxygen, and iron spectra from ~ 20 gigavolts to ~ 100 teravolts (~ 60 teravolts for iron) with 9 years of on-orbit data collected by the Dark Matter Particle Explorer (DAMPE). Distinct spectral softenings have been directly detected in these spectra for the first time. Combined with the updated proton and helium spectra, the spectral softening appears universally at a rigidity of ~ 15 teravolts. A nuclei mass dependent softening is rejected at a confidence level of > 99.999%. Taking into account the correlated structures at similar energies in the large-scale anisotropies of cosmic rays, one of the most natural interpretations of the spectral structures is the presence of a nearby cosmic ray source. In this case, the softening energies correspond to the acceleration upper limits of such a source, forming the so-called Peters cycle of the spectra. The results thus offer observational verification of the long-standing prediction of the charge-dependent energy limit of cosmic ray acceleration. △ Less

Submitted 10 November, 2025; v1 submitted 7 November, 2025; originally announced November 2025.

Comments: This update corrects typos in the tables

Constraining gravity with the decay rate of cosmological gravitational potential

Authors: Xinyi Zhao, Pengjie Zhang, Fuyu Dong

Abstract: A key task in cosmology is to test the validity of general relativity (GR) at cosmological scales and, therefore, to distinguish between dark energy and modified gravity (MG) as the driver of the late-time cosmic acceleration. The decay rate ($DR$) of cosmological gravitational potential, being sensitive to gravity and being immune to various astrophysical uncertainties, enables GR tests independe… ▽ More A key task in cosmology is to test the validity of general relativity (GR) at cosmological scales and, therefore, to distinguish between dark energy and modified gravity (MG) as the driver of the late-time cosmic acceleration. The decay rate ($DR$) of cosmological gravitational potential, being sensitive to gravity and being immune to various astrophysical uncertainties, enables GR tests independent to other structure growth probes. Recently we have measured $DR$ at $0.2\leq z\leq 1.4$, combining the DR9 galaxy catalog from the DESI imaging surveys and Planck cosmic microwave background maps \citep{arXiv:2411.12594}. Here we use this measurement to test gravity, and restrict the analysis to one-parameter extensions to the standard $Λ$CDM cosmology. We consider four one-parameter MG parameterizations. One is $f(a)=Ω_m^γ(a)$. The other three adopt the gravitational slip parameter $η=1$ and consider variations in the effective gravitational constant $G_{\rm eff}/G$ with the parameterization $Σ(a)=Σ_ΛΩ_Λ(a)/Ω_Λ$, $Σ(a)=Σ_1 a$ or $Σ(a)=Σ_2 a^2$. We find $γ=0.47^{+0.22}_{-0.15}$, consistent with the GR prediction $γ\simeq 0.55$. We also find $Σ_Λ=0.018^{+0.052}_{-0.053}$, $Σ_1=0.020^{+0.065}_{-0.062}$, and $Σ_2=0.027^{+0.067}_{-0.069}$, fully consistent with the GR case of $Σ=0$, regardless of parameterizations of $Σ(a)$. The constraining power is already competitive, while a factor of 2 further improvement is expected for the upcoming full-sky galaxy surveys. △ Less

Submitted 6 November, 2025; originally announced November 2025.

Comments: 6 pages, 4 figures, 2 tables. Submitted to ApJ

The Advanced X-ray Imaging Satellite Community Science Book

Authors: Michael Koss, Nafisa Aftab, Steven W. Allen, Roberta Amato, Hongjun An, Igor Andreoni, Timo Anguita, Riccardo Arcodia, Thomas Ayres, Matteo Bachetti, Maria Cristina Baglio, Arash Bahramian, Marco Balboni, Ranieri D. Baldi, Solen Balman, Aya Bamba, Eduardo Banados, Tong Bao, Iacopo Bartalucci, Antara Basu-Zych, Rebeca Batalha, Lorenzo Battistini, Franz Erik Bauer, Andy Beardmore, Werner Becker , et al. (373 additional authors not shown)

Abstract: The AXIS Community Science Book represents the collective effort of more than 500 scientists worldwide to define the transformative science enabled by the Advanced X-ray Imaging Satellite (AXIS), a next-generation X-ray mission selected by NASA's Astrophysics Probe Program for Phase A study. AXIS will advance the legacy of high-angular-resolution X-ray astronomy with ~1.5'' imaging over a wide 24'… ▽ More The AXIS Community Science Book represents the collective effort of more than 500 scientists worldwide to define the transformative science enabled by the Advanced X-ray Imaging Satellite (AXIS), a next-generation X-ray mission selected by NASA's Astrophysics Probe Program for Phase A study. AXIS will advance the legacy of high-angular-resolution X-ray astronomy with ~1.5'' imaging over a wide 24' field of view and an order of magnitude greater collecting area than Chandra in the 0.3-12 keV band. Combining sharp imaging, high throughput, and rapid response capabilities, AXIS will open new windows on virtually every aspect of modern astrophysics, exploring the birth and growth of supermassive black holes, the feedback processes that shape galaxies, the life cycles of stars and exoplanet environments, and the nature of compact stellar remnants, supernova remnants, and explosive transients. This book compiles over 140 community-contributed science cases developed by five Science Working Groups focused on AGN and supermassive black holes, galaxy evolution and feedback, compact objects and supernova remnants, stellar physics and exoplanets, and time-domain and multi-messenger astrophysics. Together, these studies establish the scientific foundation for next-generation X-ray exploration in the 2030s and highlight strong synergies with facilities of the 2030s, such as JWST, Roman, Rubin/LSST, SKA, ALMA, ngVLA, and next-generation gravitational-wave and neutrino networks. △ Less

Submitted 31 October, 2025; originally announced November 2025.

Comments: 595 pages, 225 figures

The $N_H$ Distribution of Hard X-ray Selected AGN in the NEP Field

Authors: Samantha Creech, Francesca Civano, Daniel R. Wik, Ross Silver, Xiurui Zhao, Rafael Ortiz III, Tonima Ananna, Normal A. Grogin, Rolf Jansen, Anton M. Koekemoer, Christopher N. A. Willmer, Rogier A. Windhorst

Abstract: X-ray surveys are one of the most unbiased methods for detecting Compton Thick (CT; $N_{\mathrm{H}} \geq 10^{24}$ cm$^{-2}$) AGN, which are thought to comprise up to $60\%$ of AGN within $z \lesssim 1.0$. These CT AGN are often difficult to detect with current instruments, but the X-ray data within the JWST-North Ecliptic Pole (NEP) Time Domain Field (TDF) present a unique opportunity to study fai… ▽ More X-ray surveys are one of the most unbiased methods for detecting Compton Thick (CT; $N_{\mathrm{H}} \geq 10^{24}$ cm$^{-2}$) AGN, which are thought to comprise up to $60\%$ of AGN within $z \lesssim 1.0$. These CT AGN are often difficult to detect with current instruments, but the X-ray data within the JWST-North Ecliptic Pole (NEP) Time Domain Field (TDF) present a unique opportunity to study faint and obscured AGN. The NEP contains the deepest NuSTAR survey to date, and Zhao et al. (2024) detected 60 hard X-ray sources from the combined exposure of NuSTAR's Cycle 5 and 6 observations. In this work, we utilize the NuSTAR Cycle 5+6+8+9 data and simultaneous XMM-Newton observations in order to perform the first spectroscopic analysis of the 60-source catalog. We present this analysis and measure the $N_{\mathrm{H}}$ distribution of the sample. We measure an observed CT fraction of $0.13_{-0.04}^{+0.15}$ down to an observed $8-24$ keV flux of $6.0 \times 10^{-14}$ erg/s/cm$^{2}$, and we correct our analysis for absorption bias to estimate an underlying CT fraction of $0.32_{-0.08}^{+0.23}$. The derived obscuration distribution and CT fraction are consistent with population synthesis models and previous surveys. △ Less

Submitted 30 October, 2025; originally announced October 2025.

Comments: 24 pages, 14 figures, 7 tables

Evidence of cosmic-ray acceleration up to sub-PeV energies in the supernova remnant IC 443

Authors: Zhen Cao, F. Aharonian, Y. X. Bai, Y. W. Bao, D. Bastieri, X. J. Bi, Y. J. Bi, W. Bian, A. V. Bukevich, C. M. Cai, W. Y. Cao, Zhe Cao, J. Chang, J. F. Chang, A. M. Chen, E. S. Chen, G. H. Chen, H. X. Chen, Liang Chen, Long Chen, M. J. Chen, M. L. Chen, Q. H. Chen, S. Chen, S. H. Chen , et al. (291 additional authors not shown)

Abstract: Supernova remnants (SNRs) have been considered as the primary contributors to cosmic rays (CRs) in our Galaxy. However, the maximum energy of particles that can be accelerated by shocks of SNRs is uncertain observationally and theoretically, and the role of contribution to CRs around PeV energies by SNRs is unclear. In this study, we present observations of high-energy $γ$-ray emission from the SN… ▽ More Supernova remnants (SNRs) have been considered as the primary contributors to cosmic rays (CRs) in our Galaxy. However, the maximum energy of particles that can be accelerated by shocks of SNRs is uncertain observationally and theoretically, and the role of contribution to CRs around PeV energies by SNRs is unclear. In this study, we present observations of high-energy $γ$-ray emission from the SNR IC 443 using the Large High Altitude Air Shower Observatory (LHAASO). The morphological analysis reveals a pointlike source whose location and spectrum are consistent with those of the Fermi-LAT-detected compact source with $π^0$-decay signature, and a more extended source which is consistent with a newly discovered source, previously unrecognized by Fermi-LAT. The spectrum of the point source can be described by a power-law function with an index of $\sim3.0$, extending beyond $\sim 30$ TeV without apparent cutoff. Assuming a hadronic origin of the $γ$-ray emission, the $95\%$ lower limit of accelerated protons reaches about 300 TeV. The extended source might be coincident with IC 443, SNR G189.6+3.3 or the putative pulsar wind nebula CXOU J061705.3+222127, and can be explained by either a hadronic or leptonic model. The LHAASO results provide compelling evidence that CR protons up to sub-PeV energies can be accelerated by the SNR. △ Less

Submitted 29 October, 2025; originally announced October 2025.

Fast-Cooling Synchrotron Prompt Emission from Internal Shocks in GRB 241030A

Authors: Varun, Bin-Bin Zhang, Xiao-Hong Zhao, Jun Yang, Run-Chao Chen, Vikas Chand

Abstract: We present a time-resolved, joint Swift-Fermi spectral study of GRB 241030A (z=1.411) that cleanly isolates the synchrotron origin of its prompt emission and favors a matter-dominated, internal-shock scenario. The light curve shows two episodes separated by a quiescent gap. Episode I (0-45 s) is well described by a single power law with photon index $\simeq -3/2$, consistent with the fast-cooling… ▽ More We present a time-resolved, joint Swift-Fermi spectral study of GRB 241030A (z=1.411) that cleanly isolates the synchrotron origin of its prompt emission and favors a matter-dominated, internal-shock scenario. The light curve shows two episodes separated by a quiescent gap. Episode I (0-45 s) is well described by a single power law with photon index $\simeq -3/2$, consistent with the fast-cooling synchrotron slope below the peak. Episode II (100-200 s), exhibits two robust spectral breaks: a low-energy break at $E_{b}$$\sim$$2-3$ keV that remains nearly constant in time, and a spectral peak $E_{p}$ that tracks the flux within pulses but steps down between them. The photon indices below and above $E_{b}$ cluster around -2/3 and -3/2, respectively, as expected for fast-cooling synchrotron emission. The burst displays an unusually small (consistent with zero) spectral lag across GBM bands. At later times ($\geq 230$ s), the spectrum softens toward $\sim-2.7$, as expected when the observing band lies above both $ν_m$ and $ν_c$. These behaviors are difficult to reconcile with a globally magnetized outflow with a decaying field, which naturally produces hard-to-soft Ep evolution, growing $ν_c$, and appreciable lags. By contrast, internal shocks with a roughly steady effective magnetic field and a time-variable minimum electron Lorentz factor (equivalently, e.g., a varying fraction of accelerated electrons simultaneously account for (i) the stable $E_{b}$, (ii) the intensity-tracking yet step-down $E_{p}$, (iii) the canonical -2/3 and -3/2 slopes, and (iv) the near-zero lag. △ Less

Submitted 28 October, 2025; originally announced October 2025.

Comments: 16 pages, 8 Figures, 3 Tables. Submitted to APJ

PEARLS: NuSTAR and XMM-Newton Extragalactic Survey of the JWST North Ecliptic Pole Time-domain Field III

Authors: Ross Silver, Francesca Civano, Xiurui Zhao, Samantha Creech, Christopher N. A. Willmer, S. P. Willner, Rogier A. Windhorst, Haojing Yan, Anton M. Koekemoer, Rosalia O'Brien, Rafael Ortiz III, Rolf A. Jansen, W. Peter Maksym, Nico Cappelluti, Francesca Fornasini, Timothy Carleton, Seth H. Cohen, Rachel Honor, Jake Summers, Jordan C. J. D'Silva, Sibasish Laha, Dan Coe, Christopher J. Conselice, Jose M. Diego, Simon P. Driver , et al. (6 additional authors not shown)

Abstract: The James Webb Space Telescope (JWST) North Ecliptic Pole (NEP) Time-Domain Field (TDF) has been monitored by NuSTAR and XMM-Newton with a regular cadence for five years starting in 2019. The survey has accumulated 3.5Ms of NuSTAR exposure and 228 ks quasi-simultaneous XMM-Newton observations covering 0.31 deg^2. This paper presents the results from the most recent two-years' 2Ms NuSTAR and 166 ks… ▽ More The James Webb Space Telescope (JWST) North Ecliptic Pole (NEP) Time-Domain Field (TDF) has been monitored by NuSTAR and XMM-Newton with a regular cadence for five years starting in 2019. The survey has accumulated 3.5Ms of NuSTAR exposure and 228 ks quasi-simultaneous XMM-Newton observations covering 0.31 deg^2. This paper presents the results from the most recent two-years' 2Ms NuSTAR and 166 ks XMM observations in NuSTAR cycles 8 and 9. These observations reached a 20%-area flux of 2.20 x 10^-14 erg cm^-2 s^-1 in the 8-24 keV band. 75 NuSTAR sources and 274 XMM-Newton sources are detected at 99% reliability level. The logN-logS measured in cycles 8+9 are consistent with those measured in the previous cycle 5+6 NuSTAR NEP survey, but in a larger area (0.3 deg^2 compared with 0.19 deg^2). The slope of the cycles 8+9 8-24 keV logN-logS curve is flatter than other works (α89 = 1.13 +/- 0.46), but is consistent with the Euclidean value of α = 1.50. In addition, we found ~36% of the NuSTAR sources to be heavily obscured (NH >= 10^23 cm^-2). The Compton-thick (NH >= 10^24 cm^-2) (CT-) AGN fraction is 9+18-8% in the NEP-TDF, which is consistent with the measurements in previous surveys. △ Less

Submitted 28 October, 2025; originally announced October 2025.

Robustness Analysis and Controller Design of Arm-locking System in Space-based Gravitational Wave Detectors

Authors: Yongbin Shao, Xinyi Zhao, Long Ma, Ming Xin

Abstract: Arm-locking frequency stabilization is a key technique for suppressing laser frequency noise in space-based gravitational-wave detectors. The robustness of the arm-locking control loop is crucial for maintaining laser frequency stability, which directly impacts the accuracy of gravitational-wave measurements. In this work, a parametric stability analysis framework is developed by combining the D-s… ▽ More Arm-locking frequency stabilization is a key technique for suppressing laser frequency noise in space-based gravitational-wave detectors. The robustness of the arm-locking control loop is crucial for maintaining laser frequency stability, which directly impacts the accuracy of gravitational-wave measurements. In this work, a parametric stability analysis framework is developed by combining the D-subdivision theory with the Semi-Discretization method to map the stability regions of arm-locking systems in the parameter space and identify their critical stability boundaries. Based on the frequency-domain characteristics, a robust arm-locking controller is designed to enhance loop stability under parameter perturbations. Theoretical analysis and time-domain simulations confirm that the proposed controller maintains closed-loop stability and realize suppression of laser frequency noise against parameter perturbation. △ Less

Submitted 20 October, 2025; originally announced October 2025.

BREAKFAST: A Framework for general joint BA duty and follow-up guidance of multiple $γ$-ray monitors

Authors: Chen-Wei Wang, Peng Zhang, Shao-Lin Xiong, Yue Huang, Wen-Jun Tan, Zheng-Hang Yu, Yue Wang, Wang-Chen Xue, Chao Zheng, Hao-Xuan Guo, Ce Cai, Yong-Wei Dong, Jiang He, Cheng-Kui Li, Xiao-Bo Li, Jia-Cong Liu, Xing-Hao Luo, Xiang Ma, Rahim Moradi, Yang-Zhao Ren, Li-Ming Song, Ping Wang, Jin Wang, Bo-Bing Wu, Shuo Xiao , et al. (8 additional authors not shown)

Abstract: With the growing number of gamma-ray monitors in operation, several research teams have adopted a strategy of joint operation and scientific duty to improve efficiency. A successful example is the GECAM-HXMT-SVOM (GHS) constellation collaboration, which sets a precedent for other gamma-ray monitor constellations. However, joint duty also presents challenges to Burst Advocates (BAs), including the… ▽ More With the growing number of gamma-ray monitors in operation, several research teams have adopted a strategy of joint operation and scientific duty to improve efficiency. A successful example is the GECAM-HXMT-SVOM (GHS) constellation collaboration, which sets a precedent for other gamma-ray monitor constellations. However, joint duty also presents challenges to Burst Advocates (BAs), including the increased number of triggers and, more importantly, the frequent switching between various systems due to incompatibilities among different missions, which complicates the situation. To address the current requirements of multi-wavelength and multi-messenger astronomy, we developed a customized framework for unified trigger processing within the GHS joint duty, named "BA's Rapid Evaluation and Analysis Kit for Formulating Alerts and Summary Tools" (BREAKFAST). This framework incorporates a series of automated, semi-automated, and manual pipelines designed to rapidly process triggers of prompt emissions in the gamma-ray band from different instruments, while maintaining flexible compatibility for future missions. The pursuit of BREAKFAST goes beyond merely providing trigger processing for BAs. BREAKFAST also aims to filtering high-value targets and guiding follow-up telescopes through rapid analysis and reporting, thus serving as an important bridge between prompt emission observations and afterglow observations. To this end, a suite of comprehensive analysis modules is included in BREAKFAST, particularly the specially designed module that predicts X-ray afterglow brightness based on prompt emission properties. The framework's effectiveness has already been demonstrated in recent observational campaigns, and it is expected to play a significant role in the discovery and observation of peculiar transients in the future. △ Less

Submitted 28 October, 2025; v1 submitted 17 October, 2025; originally announced October 2025.

QFP Waves Driven by the Tuning-Fork Effect during Magnetic Reconnecion

Authors: Jialiang Hu, Xiaozhou Zhao, Guiping Zhou, Yuhao Chen, Chunlan Jin, Mijie Shi, Guanchong Cheng, Xiaoxia Yu, Jing Ye, Xinping Zhou, Hanxian Fang

Abstract: Through three-dimensional MHD simulations, we have uncovered a kind of fast coronal wave originating from both ends of a current sheet (CS) during a solar eruption. These waves are observed to appear near the top and bottom ends of the reconnection-related CS. The simulations demonstrate the presence of termination shock regions above the two ends of the CS. As the reconnection outflows escape fro… ▽ More Through three-dimensional MHD simulations, we have uncovered a kind of fast coronal wave originating from both ends of a current sheet (CS) during a solar eruption. These waves are observed to appear near the top and bottom ends of the reconnection-related CS. The simulations demonstrate the presence of termination shock regions above the two ends of the CS. As the reconnection outflows escape from the vertical CS and encounter these termination shocks, they undergo partial reflection, redirecting towards the CS terminal fork walls. The identified waves propagate rapidly at a speed of approximately 1400 km/s with a period of just 2 s. Concurrently, the time-evolution of intensity within a small region of the CS terminal fork structures, exhibits a similar oscillation period of 2 s. All these evidence supports the notion that these QFP (Quasi-periodic Fast-Propagating) waves were excited by tuning fork effects within the CS system. Essentially, the rapid reconnection outflows are reflected by the terminal shocks, striking the fork walls at the CS ends. Moreover, parts of the oscillations along the tuning fork handle are transformed into thermal energy, accumulating in the CS center and elevating the temperature. This is the first time to report such QFP waves resulting from tuning fork effects within the CS during a solar eruption. These waves are anticipated to manifest closely following the propagation of CMEs and adjacent to the related post-flare loops in observations, with partial confirmation in current observations. △ Less

Submitted 16 October, 2025; originally announced October 2025.

Comments: 9 pages, 5 figures

Unveil A Peculiar Light Curve Pattern of Magnetar Burst with GECAM observations of SGR J1935+2154

Authors: Yue Wang, Chen-Wei Wang, Shaolin Xiong, Xiao Xiao, Yanqiu Zhang, Sheng-Lun Xie, Lin Lin, Yuan-Pei Yang, Haoxuan Guo, Ce Cai, Yue Huang, Cheng-Kui Li, Bing Li, Xiaobo Li, Jiacong Liu, Xiang Ma, Liming Song, Wen-Jun Tan, Ping Wang, Wang-Chen Xue, Shu-Xu Yi, Yun-Wei Yu, Zheng-Hang Yu, Jin-Peng Zhang, Peng Zhang , et al. (6 additional authors not shown)

Abstract: Magnetar X-ray Burst (MXB) is usually composed of a single pulse or multiple pulses with rapid rise and brief duration mostly observed in hard X-ray (soft gamma-ray) band. Previous work studied the temporal behavior of some magnetar bursts and employed the Fast Rise Exponential Decay (FRED) model to fit pulses of MXB. However, whether there is other kind of pulse shape has not been explored. In th… ▽ More Magnetar X-ray Burst (MXB) is usually composed of a single pulse or multiple pulses with rapid rise and brief duration mostly observed in hard X-ray (soft gamma-ray) band. Previous work studied the temporal behavior of some magnetar bursts and employed the Fast Rise Exponential Decay (FRED) model to fit pulses of MXB. However, whether there is other kind of pulse shape has not been explored. In this study, we systematically examined light curve of MXBs from SGR J1935+2154 detected by GECAM between 2021 and 2022. We find that there are different light curve morphologies. Especially, we discover a peculiar and new pattern, Exponential Rise and Cut-Off Decay (ERCOD), which is significantly different from FRED and could be well described by a mathematical function we proposed. We find that MXBs with ERCOD shape are generally longer in duration, brighter in the peak flux, and harder in spectrum. We note that the ERCOD shape is not unique to SGR J1935+2154 but also present in other magnetars. This new light curve pattern may imply a special burst and radiation mechanism of magnetar. △ Less

Submitted 13 October, 2025; originally announced October 2025.

Comments: 13 pages, 5 figures, accepted to publication on ApJ

A Giant Peanut-shaped Ultra-High-Energy Gamma-Ray Emitter Off the Galactic Plane

Authors: Zhen Cao, Felix Aharonian, Yunxiang Bai, Yiwei Bao, Denis Bastieri, Xiaojun Bi, YuJiang Bi, Mr Bian WenYi, A. Butkevich, Chengmiao Cai, Wenyu Cao, Zhe Cao, Jin Chang, Jinfan Chang, Mr Aming Chen, Ensheng Chen, Mr Guo-Hai Chen, Mr Huaxi Chen, Liang Chen, Long Chen, Mingjun Chen, Mali Chen, Qihui Chen, Shi Chen, Suhong Chen , et al. (291 additional authors not shown)

Abstract: Ultra-high-energy (UHE), exceeding 100 TeV (10^12 electronvolts), γ-rays manifests extreme particle acceleration in astrophysical sources. Recent observations by γ-ray telescopes, particularly by the Large High Altitude Air Shower Observatory (LHAASO), have revealed a few tens of UHE sources, indicating numerous Galactic sources capable of accelerating particles to PeV (10^15 electronvolts) energi… ▽ More Ultra-high-energy (UHE), exceeding 100 TeV (10^12 electronvolts), γ-rays manifests extreme particle acceleration in astrophysical sources. Recent observations by γ-ray telescopes, particularly by the Large High Altitude Air Shower Observatory (LHAASO), have revealed a few tens of UHE sources, indicating numerous Galactic sources capable of accelerating particles to PeV (10^15 electronvolts) energies. However, discerning the dominant acceleration mechanisms (leptonic versus hadronic), the relative contributions of specific source classes, and the role of particle transport in shaping their observed emission are central goals of modern UHE astrophysics. Here we report the discovery of a giant UHE γ-ray emitter at -17.5° off the Galactic plane - a region where UHE γ-ray sources are rarely found. The emitter exhibits a distinctive asymmetric shape, resembling a giant "Peanut" spanning 0.45° \times 4.6°, indicative of anisotropic particle distribution over a large area. A highly aged millisecond pulsar (MSP) J0218+4232 is the sole candidate accelerator positionally coincident with the Peanut region. Its association with UHE γ-rays extending to 0.7 PeV, if confirmed, would provide the first evidence of a millisecond pulsar powering PeV particles. Such a finding challenges prevailing models, which posit that millisecond pulsars cannot sustain acceleration to PeV energies. The detection reveals fundamental gaps in understanding particle acceleration, cosmic-ray transport, and interstellar magnetic field effects, potentially revealing new PeV accelerator (PeVatron) classes. △ Less

Submitted 25 October, 2025; v1 submitted 8 October, 2025; originally announced October 2025.

Analysis of the plane of satellites around Milky Way-like galaxies in $Λ$CDM cosmology

Authors: Xinghai Zhao, Guobao Tang, Paola Gonzalez, Grant J. Mathews, Lara Arielle Phillips

Abstract: It has been suggested that the Plane of Satellites (PoS) phenomenon may imply a tension with current $Λ$CDM cosmology since a Milky-Way (MW)-like PoS is very rare in simulations. In this study, we analyze a large sample of satellite systems of MW-like galaxies in the IllustrisTNG simulations. We analyze their spatial aspect ratio, orbital pole dispersion, Gini coefficient, radial distribution, and… ▽ More It has been suggested that the Plane of Satellites (PoS) phenomenon may imply a tension with current $Λ$CDM cosmology since a Milky-Way (MW)-like PoS is very rare in simulations. In this study, we analyze a large sample of satellite systems of MW-like galaxies in the IllustrisTNG simulations. We analyze their spatial aspect ratio, orbital pole dispersion, Gini coefficient, radial distribution, and bulk satellite velocity relative to the host galaxy. These are compared to the observed Milky~Way PoS. We identified galaxy samples in two mass ranges ($0.1 - 0.8 \times 10^{12} $ M$_\odot$ and $0.8 - 3.0 \times 10^{12}$ M$_\odot$). We find for both mass ranges that only $\sim$ 1 percent of MW-like galaxies contain a PoS similar to that of the MW. Nevertheless, these outliers occur naturally in $Λ$CDM cosmology. We analyze the formation, environment, and evolution of the PoS for nine systems that are most MW-like. We suggest that a PoS can form from one or more of at least five different processes. A massive Magellanic~Cloud (MC)-like satellite is found in 1/3 of the systems and probably plays an important role in the PoS formation. We find a tendency for about half of the satellites to have recently arrived at $z < 0.5$, indicating that a MW-like PoS is a recent and transient phenomenon. We also find that a spin up of the angular momentum amplitude of the most massive satellites is an indicator of the recent in-fall of the PoS satellites. △ Less

Submitted 2 October, 2025; originally announced October 2025.

Comments: Submitted to ApJ. 27 pages, 11 figures

Unveiling Obscured Accretion in the Local Universe

Authors: Indrani Pal, Stefano Marchesi, Ross Silver, Marco Ajello, Vittoria Gianolli, Núria Torres-Albà, Isaiah Cox, Xiurui Zhao, Dhrubojyoti Sengupta, Anuvab Banerjee, Kouser Imam, Andrealuna Pizzetti

Abstract: Heavily obscured Active Galactic Nuclei (AGN), especially Compton-thick sources with line-of-sight column density ($N_{\rm H,los}$) $>$ 10$^{24}$ cm$^{-2}$, are critical to understanding supermassive black hole (SMBH) growth and the origin of the Cosmic X-ray Background (CXB). However, their observed fraction remains significantly below model predictions, due to strong absorption bias, even in the… ▽ More Heavily obscured Active Galactic Nuclei (AGN), especially Compton-thick sources with line-of-sight column density ($N_{\rm H,los}$) $>$ 10$^{24}$ cm$^{-2}$, are critical to understanding supermassive black hole (SMBH) growth and the origin of the Cosmic X-ray Background (CXB). However, their observed fraction remains significantly below model predictions, due to strong absorption bias, even in the hard X-ray (i.e., above 10 keV) band. We analyze a sample of 26 nearby ($z < 0.1$) AGN from the Swift-BAT 150-month catalog, selected via mid-IR to X-ray diagnostics and observed with NuSTAR and soft X-ray telescopes (Xmm-Newton, Chandra, or Swift-xrt). Using self-consistent torus models (MyTorus, Borus02, and UXCLUMPY), we aim to constrain $N_{\rm H,los}$, the average torus column density, and other geometrical parameters of the obscuring medium. A comparative analysis among the three torus models showed that while estimates of $N_{\rm{H,los}}$ were generally in agreement, Borus02 tended to classify a slightly larger number of sources as Compton-thick AGN (CT-AGN). Building on this comparison, we benchmark two prediction schemes -- a mid-IR/X-ray relation and a machine-learning model -- against our broadband best-fit $N_{\rm H,los}$ measurements to assess which approach more effectively bridges the gap between predicted and measured obscuration, finding that while the former works effectively in the heavily obscured region (log$\rm{N_H} \gtrsim$ 23.5 $\rm{cm^{-2}}$), the latter provides improved accuracy, particularly for Compton-thin to moderately thick regimes (log$\rm{N_H} \lesssim$ 23.5 $\rm{cm^{-2}}$). △ Less

Submitted 25 September, 2025; originally announced September 2025.

Comments: 16 pages, 13 Figures, submitted to the journal

Investigation of hadronic cross sections of cosmic ray carbon and oxygen on BGO from 200 GeV to 10 TeV energy at the DAMPE experiment

Authors: F. Alemanno, Q. An, P. Azzarello, F. C. T. Barbato, P. Bernardini, X. J. Bi, H. Boutin, I. Cagnoli, M. S. Cai, E. Casilli, E. Catanzani, J. Chang, D. Y. Chen, J. L. Chen, Z. F. Chen, Z. X. Chen, P. Coppin, M. Y. Cui, T. S. Cui, Y. X. Cui, I. De Mitri, F. de Palma, A. Di Giovanni, T. K. Dong, Z. X. Dong , et al. (122 additional authors not shown)

Abstract: The Dark Matter Particle Explorer (DAMPE) has made significant progress in measuring the fluxes of cosmic rays. These new measurements are pivotal in advancing our understanding of the origins and propagation mechanisms of cosmic rays. The bismuth germanium oxide (BGO) calorimeter plays a crucial role in these measurements, particularly in the precise determination of cosmic ray fluxes. However, f… ▽ More The Dark Matter Particle Explorer (DAMPE) has made significant progress in measuring the fluxes of cosmic rays. These new measurements are pivotal in advancing our understanding of the origins and propagation mechanisms of cosmic rays. The bismuth germanium oxide (BGO) calorimeter plays a crucial role in these measurements, particularly in the precise determination of cosmic ray fluxes. However, for a calorimetric experiment like DAMPE, uncertainties in hadronic models persist as a major barrier in achieving more accurate measurements of fluxes of cosmic ray nuclei. This study centers on the measurement of the inelastic hadronic cross sections of carbon and oxygen nuclei interacting with BGO crystals target over an extensive energy range, spanning from 200 GeV to 10 TeV. For carbon nuclei interacting with the BGO target, the measurements of the cross sections have achieved a total relative uncertainty of less than 10% below 8 TeV for carbon, and below 3 TeV for oxygen. For oxygen nuclei, the same level of precision was attained below 3 TeV. Additionally, we compare the experimental results with Geant4 and FLUKA simulations to validate the accuracy and consistency of these simulation tools. Through comprehensive analysis of the inelastic hadronic interaction cross sections, this research provides validation for the hadronic interaction models used in DAMPE's cosmic-ray flux measurements. △ Less

Submitted 21 September, 2025; originally announced September 2025.

Cosmic Ray Magnetohydrodynamics: A New Two-Moment Framework with Numerical Implementation

Authors: Xihui Zhao, Xue-Ning Bai, Eve C. Ostriker

Abstract: Cosmic rays (CRs) play a pivotal role in various astrophysical systems, delivering feedback over a broad range of scales. However, modeling CR transport remains challenging due to its inherently multi-scale nature and complex microphysics. Recent advances in two-moment CR hydrodynamics have alleviated some of these challenges, improving understanding of CR feedback. Yet, current two-moment methods… ▽ More Cosmic rays (CRs) play a pivotal role in various astrophysical systems, delivering feedback over a broad range of scales. However, modeling CR transport remains challenging due to its inherently multi-scale nature and complex microphysics. Recent advances in two-moment CR hydrodynamics have alleviated some of these challenges, improving understanding of CR feedback. Yet, current two-moment methods may not be able to directly incorporate all relevant CR transport processes, while the outcome of CR feedback sensitively depends on these underlying microphysics. Furthermore, numerical challenges persist, including instabilities from streaming terms and ambiguities in solver design for coupled CR-MHD systems. In this work, we develop a two-moment description for CR hydrodynamics from first principles. Beyond canonical CR streaming, our formulation accounts for CR pressure anisotropy and Alfvén waves propagating in both directions along the magnetic field, providing a general framework to incorporate more CR transport physics. We implement this framework as a new CR fluid module in the \textit{Athena}++ code, and validate it through a suite of benchmark tests. In particular, we derive the full dispersion relation of the two-moment CR-MHD system, identifying the CR-acoustic instability as well as other wave branches. These CR-MHD waves serve as rigorous benchmarks and also enable the use of realistic signal speeds in our Riemann solver. We propose a time step guideline to mitigate numerical instabilities arising from streaming source terms. △ Less

Submitted 4 September, 2025; originally announced September 2025.

Comments: 32 pages, 12 figures, submitted to ApJS, comments welcomed

Hard X-ray Emission in AU Mic Flares: A Minor Contributor to Planetary Atmospheric Escape

Authors: Yifan Hu, Murray Brightman, Fabio Favata, Haiwu Pan, Brian Grefenstette, Fiona A. Harrison, Daniel Stern, Weimin Yuan, Yuk L. Yung, Xiurui Zhao

Abstract: Stellar flares are potent drivers of atmospheric evolution on orbiting exoplanets, primarily through extreme ultraviolet (EUV) and soft X-ray (XUV) irradiation. However, the contribution of hard X-rays (HXR; 3--20 keV)-which penetrate deeper into planetary atmospheres-to mass loss and particle acceleration has remained poorly understood. To quantify the HXR share of the total radiative budget, we… ▽ More Stellar flares are potent drivers of atmospheric evolution on orbiting exoplanets, primarily through extreme ultraviolet (EUV) and soft X-ray (XUV) irradiation. However, the contribution of hard X-rays (HXR; 3--20 keV)-which penetrate deeper into planetary atmospheres-to mass loss and particle acceleration has remained poorly understood. To quantify the HXR share of the total radiative budget, we conducted quasi-simultaneous observations of the active M-dwarf AU Mic using NuSTAR, Swift, and the Einstein Probe. Our analysis detected two major flares, and we performed an empirical check by deriving a quiescent-phase soft X-ray (SXR; 0.3--3 keV)-HXR relation and then applying it to the flares. By combining this with the quiescent coronal SXR-EUV relations conversion of J. Sanz-Forcada et al. (2011), we computed the total high-energy flux (EUV + SXR + HXR) and assessed the relative role of HXR in atmospheric escape. We find that HXR accounts for only a few percent of the total radiative energy budget during both quiescent and flaring states. While a high-energy spectral tail is detected in the second flare, time-resolved spectroscopy reveals a dominant chromospheric-evaporation signature, indicating that the flare energetics are primarily thermal. △ Less

Submitted 28 October, 2025; v1 submitted 20 August, 2025; originally announced August 2025.

Comments: 20 pages, 9 figures, accepted by ApJ

Multiwavelength Observations of the Apparently Non-repeating FRB 20250316A

Authors: Ye Li, Hui Sun, Lei Qian, Dong-Yue Li, Yan-Long Hua, Li-Ping Xin, Cheng-Kui Li, Yi-Han Wang, Jia-Rui Niu, Tian-Rui Sun, Zhu-Heng Yao, Jin-Jun Geng, Chi-Chuan Jin, Nanda Rea, Yuan Liu, Zhi-Chen Pan, Tao An, Vadim Burwitz, Zhi-Ming Cai, Jin-Huang Cao, Yong Chen, Hua-Qing Cheng, Wei-Wei Cui, Hua Feng, Peter Friedrich , et al. (50 additional authors not shown)

Abstract: The physical origin of fast radio bursts (FRBs) remains uncertain. Although multiwavelength observations have been widely conducted, only Galactic FRB~20200428D is associated with an X-ray burst from the magnetar SGR J1935+2154. Here, we present multiwavelength follow-up observations of the nearby bright FRB~20250316A, including the Five-hundred-meter Aperture Spherical radio Telescope (FAST), Ein… ▽ More The physical origin of fast radio bursts (FRBs) remains uncertain. Although multiwavelength observations have been widely conducted, only Galactic FRB~20200428D is associated with an X-ray burst from the magnetar SGR J1935+2154. Here, we present multiwavelength follow-up observations of the nearby bright FRB~20250316A, including the Five-hundred-meter Aperture Spherical radio Telescope (FAST), Einstein Probe (EP) X-ray mission, Chandra X-ray Observatory, Wide Field Survey Telescope (WFST) and Space Variable Object Monitor/Visible Telescope (SVOM/VT). The 13.08-hour FAST follow-up campaign without pulse detection requires an energy distribution flatter than those of well-known repeating FRBs, suggesting that this burst is likely a one-off event. A prompt EP follow-up and multi-epoch observational campaign totaling $>$ 100 ks led to the detection of an X-ray source within the angular resolution of its Follow-up X-ray Telescope (FXT, $10^{\prime\prime}$). A subsequent Chandra observation revealed this source to be offset by $7^{\prime\prime}$ from the FRB position, and established a 0.5-10 keV flux upper limit of $7.6\times 10^{-15}$ $\rm erg\,cm^{-2}\,s^{-1}$ at the FRB position, corresponding to $\sim 10^{39}$ $\rm erg\,s^{-1}$ at the 40 Mpc distance of the host galaxy NGC~4141. These results set one of the most stringent limits on X-ray emission from a non-repeating FRB, disfavoring ultra-luminous X-ray sources (ULXs) as counterparts of apparently one-off FRBs and offering critical insights into afterglow models. Our study suggests that an arcsecond localization of both the FRB and its potential X-ray counterpart is essential for exploring the X-ray counterpart of an FRB. △ Less

Submitted 18 November, 2025; v1 submitted 19 August, 2025; originally announced August 2025.

Comments: Accepted for publication in The Astrophysical Journal

Finetuning Stellar Spectra Foundation Models with LoRA

Authors: Xiaosheng Zhao, Yuan-Sen Ting, Alexander S. Szalay, Yang Huang

Abstract: Foundation models are beginning to impact stellar spectroscopy, where spectra encode rich physical information in a structured, language-like form. A key challenge is adapting these models across heterogeneous surveys with differing resolution and coverage. We apply Low-Rank Adaptation (LoRA) to fine-tune SpecCLIP--a contrastively pre-trained model on LAMOST and Gaia XP spectra--for downstream tas… ▽ More Foundation models are beginning to impact stellar spectroscopy, where spectra encode rich physical information in a structured, language-like form. A key challenge is adapting these models across heterogeneous surveys with differing resolution and coverage. We apply Low-Rank Adaptation (LoRA) to fine-tune SpecCLIP--a contrastively pre-trained model on LAMOST and Gaia XP spectra--for downstream tasks on DESI Early Data Release (EDR) spectra. We show that LoRA enables few-shot learning on DESI, with performance varying by fine-tuned module and benefiting from Gaia XP knowledge embedded in the pre-trained model. Our results demonstrate that LoRA provides a lightweight and effective strategy for extending spectral foundation models to new instruments and survey domains. △ Less

Submitted 28 July, 2025; originally announced July 2025.

Comments: 7 pages, 2 figures. Accepted to the Machine Learning for Astrophysics (ML4Astro) Colocated Workshop at ICML 2025. Presented as a spotlight talk

Imaging-spectroscopy diagnosis of the giant sloshing spiral in the Virgo cluster with the Einstein Probe Follow-up X-ray Telescope

Authors: X. Zheng, S. Jia, C. Li, Y. Chen, H. Yu, H. Feng, D. Xu, A. Liu, L. Song, C. Liu, F. Lu, S. Zhang, W. Yuan, J. Sanders, J. Wang, T. Chen, C. Cui, W. Cui, W. Feng, N. Gao, J. Guan, D. Han, D. Hou, H. Hu, M. Huang , et al. (26 additional authors not shown)

Abstract: We performed deep X-ray observations of the Virgo cluster using the Einstein Probe Follow-up X-ray Telescope (EP-FXT) with a total exposure of 295 ks. Leveraging the large field of view (FoV) and low particle background of EP-FXT, the image reveals a giant spiral feature connecting the cold fronts in the northwest and southeast, forming a coherent structure consistent with earlier results from XMM… ▽ More We performed deep X-ray observations of the Virgo cluster using the Einstein Probe Follow-up X-ray Telescope (EP-FXT) with a total exposure of 295 ks. Leveraging the large field of view (FoV) and low particle background of EP-FXT, the image reveals a giant spiral feature connecting the cold fronts in the northwest and southeast, forming a coherent structure consistent with earlier results from XMM-Newton and Suzaku. We also present two-dimensional maps of temperature, metallicity, and entropy across the Virgo Cluster, covering a FoV of approximately 28.5 arcmin. These maps clearly show a spiral structure with high density, low temperature, high metallicity, and low entropy. The results support a scenario where the spiral morphology arises from gas sloshing driven by a minor merger. Additionally, EP-FXT temperature measurements agree well with XMM-Newton data within uncertainties. △ Less

Submitted 23 July, 2025; v1 submitted 10 July, 2025; originally announced July 2025.

Comments: 10 pages, 16 figures. Accepted for publication in A&A

Journal ref: A&A 700, A248 (2025)

SpecCLIP: Aligning and Translating Spectroscopic Measurements for Stars

Authors: Xiaosheng Zhao, Yang Huang, Guirong Xue, Xiao Kong, Jifeng Liu, Xiaoyu Tang, Timothy C. Beers, Yuan-Sen Ting, A-Li Luo

Abstract: In recent years, large language models (LLMs) have transformed natural language understanding through vast datasets and large-scale parameterization. Inspired by this success, we present SpecCLIP, a foundation model framework that extends LLM-inspired methodologies to stellar spectral analysis. Stellar spectra, akin to structured language, encode rich physical and chemical information about stars.… ▽ More In recent years, large language models (LLMs) have transformed natural language understanding through vast datasets and large-scale parameterization. Inspired by this success, we present SpecCLIP, a foundation model framework that extends LLM-inspired methodologies to stellar spectral analysis. Stellar spectra, akin to structured language, encode rich physical and chemical information about stars. By training foundation models on large-scale spectral datasets, our goal is to learn robust and informative embeddings that support diverse downstream applications. As a proof of concept, SpecCLIP involves pre-training on two spectral types--LAMOST low-resolution and Gaia XP--followed by contrastive alignment using the CLIP (Contrastive Language-Image Pre-training) framework, adapted to associate spectra from different instruments. This alignment is complemented by auxiliary decoders that preserve spectrum-specific information and enable translation (prediction) between spectral types, with the former achieved by maximizing mutual information between embeddings and input spectra. The result is a cross-spectrum framework enabling intrinsic calibration and flexible applications across instruments. We demonstrate that fine-tuning these models on moderate-sized labeled datasets improves adaptability to tasks such as stellar-parameter estimation and chemical-abundance determination. SpecCLIP also enhances the accuracy and precision of parameter estimates benchmarked against external survey data. Additionally, its similarity search and cross-spectrum prediction capabilities offer potential for anomaly detection. Our results suggest that contrastively trained foundation models enriched with spectrum-aware decoders can advance precision stellar spectroscopy. Our code SpecCLIP is publicly available at https://github.com/Xiaosheng-Zhao/SpecCLIP △ Less

Submitted 19 December, 2025; v1 submitted 2 July, 2025; originally announced July 2025.

Comments: 29 pages, 8 figures, 6 tables. Accepted for publication in ApJ. Comments welcome

In-flight Characteristics and Modelling of the Instrumental Background of EP/FXT

Authors: Juan Zhang, Yong Chen, Shumei Jia, Haisheng Zhao, WeiWei Cui, Tianxiang Chen, Juan Wang, Hao Wang, Jin Wang, Chengkui Li, Xiaofan Zhao, Ju Guan, Dawei Han, Jingjing Xu, Liming Song, Hua Feng, Shuangnan Zhang, Weimin Yuan

Abstract: The in-flight instrumental background of the Follow-up X-ray Telescope (FXT) onboard Einstein Probe (EP) mission is analysed in this work by utilizing observations collected during Performance Verification phase and subsequent dedicated filter wheel closed observations. The instrumental backgrounds of the two FXT modules are consistent with each other, with an average rate of… ▽ More The in-flight instrumental background of the Follow-up X-ray Telescope (FXT) onboard Einstein Probe (EP) mission is analysed in this work by utilizing observations collected during Performance Verification phase and subsequent dedicated filter wheel closed observations. The instrumental backgrounds of the two FXT modules are consistent with each other, with an average rate of $\sim 4\times10^{-2}$\,counts/s/keV in the 0.5--10\,keV band for each module. The background is nearly uniformly distributed across the detector area, with a minor increase ($<8\%$) observed along rows. The spatial distribution shows significant modulation by the geomagnetic field. The spectral shapes remain unchanged in 0.5--10\,keV at different rates. The long-term temporal variation indicates a periodic change associated with the orbital precession ($\sim 57$ days). The innovative design of FXT full-frame readout mode enables simultaneous recording of events in both the imaging area (IMG) and the frame store area (FSA) of the pnCCD. FSA event rates show a strong linear correlation with the IMG, based on which the IMG instrumental background modelling is established. △ Less

Submitted 18 July, 2025; v1 submitted 1 July, 2025; originally announced July 2025.

Comments: 13 pages, 10 figures

Prospects for Time-Domain and Multi-Messenger Science with eXTP

Authors: Shu-Xu Yi, Wen Zhao, Ren-Xin Xu, Xue-Feng Wu, Giulia Stratta, Simone Dall'Osso, Yan-Jun Xu, Andrea Santangelo, Silvia Zane, Shuang-Nan Zhang, Hua Feng, Huan Yang, Junjie Mao, Junqiang Ge, Lijing Shao, Mi-Xiang Lan, He Gao, Lin Lin, Ning Jiang, Qingwen Wu, Tong Liu, Yun-Wei Yu, Xiang-Yu Wang, Jin Zhang, Dafne Guetta , et al. (53 additional authors not shown)

Abstract: In this new era of time-domain and multi-messenger astronomy, various new transients and new phenomena are constantly being discovered thanks to the rapid advances in observations, which provide the excellent opportunity to study the physics in the extreme environments. The enhanced X-ray Timing and Polarimetry mission (eXTP), planned to be launched in 2030, has several key advantages, including a… ▽ More In this new era of time-domain and multi-messenger astronomy, various new transients and new phenomena are constantly being discovered thanks to the rapid advances in observations, which provide the excellent opportunity to study the physics in the extreme environments. The enhanced X-ray Timing and Polarimetry mission (eXTP), planned to be launched in 2030, has several key advantages, including advanced polarimetry, high sensitivity & large effective area, and wide energy range coverage, which make it a groundbreaking project in high-energy astrophysics. In this article, we briefly introduce the potential time-domain and multi-messenger targets for eXTP, including gravitational-wave (GW) counterparts, gamma-ray bursts (GRBs), magnetars and fast radio bursts (FRBs), tidal disruption events (TDEs), supernovae, high energy neutrinos and TeV active galactic nucleus (AGNs), and so on. We discuss the advantages of future eXTP observations for detecting these sources, their detection capabilities, the abilities to distinguish theoretical models, and their applications in gravity and cosmology. △ Less

Submitted 8 September, 2025; v1 submitted 9 June, 2025; originally announced June 2025.

Comments: accepted for publication in the SCIENCE CHINA Physics, Mechanics & Astronomy

The enhanced X-ray Timing and Polarimetry mission -- eXTP for launch in 2030

Authors: Shuang-Nan Zhang, Andrea Santangelo, Yupeng Xu, Hua Feng, Fangjun Lu, Yong Chen, Mingyu Ge, Kirpal Nandra, Xin Wu, Marco Feroci, Margarita Hernanz, Congzhan Liu, Huilin He, Yusa Wang, Weichun Jiang, Weiwei Cui, Yanji Yang, Juan Wang, Wei Li, Xiaohua Liu, Bin Meng, Xiangyang Wen, Aimei Zhang, Jia Ma, Maoshun Li , et al. (136 additional authors not shown)

Abstract: In this paper we present the current status of the enhanced X-ray Timing and Polarimetry mission, which has been fully approved for launch in 2030. eXTP is a space science mission designed to study fundamental physics under extreme conditions of matter density, gravity, and magnetism. The mission aims at determining the equation of state of matter at supra-nuclear density, measuring the effects of… ▽ More In this paper we present the current status of the enhanced X-ray Timing and Polarimetry mission, which has been fully approved for launch in 2030. eXTP is a space science mission designed to study fundamental physics under extreme conditions of matter density, gravity, and magnetism. The mission aims at determining the equation of state of matter at supra-nuclear density, measuring the effects of quantum electro-dynamics, and understanding the dynamics of matter in strong-field gravity. In addition to investigating fundamental physics, the eXTP mission is poised to become a leading observatory for time-domain and multi-messenger astronomy in the 2030's, as well as providing observations of unprecedented quality on a variety of galactic and extragalactic objects. After briefly introducing the history and a summary of the scientific objectives of the eXTP mission, this paper presents a comprehensive overview of: 1) the cutting-edge technology, technical specifications, and anticipated performance of the mission's scientific instruments; 2) the full mission profile, encompassing spacecraft design, operational capabilities, and ground segment infrastructure. △ Less

Submitted 8 September, 2025; v1 submitted 9 June, 2025; originally announced June 2025.

Comments: accepted for publication in the SCIENCE CHINA Physics, Mechanics & Astronomy

Multi-Instrument Search for Gamma-Ray Counterpart of X-ray Transients detected by EP/WXT

Authors: Yan-Qiu Zhang, Wang-Chen Xue, Jin-Peng Zhang, Ce Cai, Shao-Lin Xiong, Cheng-Kui Li, Yuan Liu, Chen-Wei Wang, Hao-Xuan Guo, Shuo Xiao, Wen-Jun Tan, Chao Zheng, Jia-Cong Liu, Sheng-Lun Xie, Peng Zhang, Wen-Long Zhang, Yue Wang, Zheng-Hang Yu, Yang-Zhao Ren, Ping Wang, Yue Huang, Xiao-Bo Li, Xiao-Yun Zhao, Shi-Jie Zheng, Zhen Zhang , et al. (3 additional authors not shown)

Abstract: As a soft X-ray imager with unprecedentedly large field of view, EP/WXT has detected many (fast) X-ray transients, whose nature is very intriguing. Whether there is gamma-ray counterpart for the X-ray transient provides important implications for its origin. Some of them have been reported to be associated with GRB, however, a systematic study on the gamma-ray emission of these X-ray transients is… ▽ More As a soft X-ray imager with unprecedentedly large field of view, EP/WXT has detected many (fast) X-ray transients, whose nature is very intriguing. Whether there is gamma-ray counterpart for the X-ray transient provides important implications for its origin. Some of them have been reported to be associated with GRB, however, a systematic study on the gamma-ray emission of these X-ray transients is lacking. In this work, we implemented a comprehensive targeted search for gamma-ray counterparts to 63 X-ray transients reported by EP/WXT during its first year of operation, using the dedicated multiple-instrument search pipeline, ETJASMIN, with GECAM-B, GECAM-C, Fermi/GBM, and \textit{Insight}-HXMT data. We find that 14 out of 63 (22\%) EP/WXT X-ray transients have gamma-ray counterparts. For other transients, ETJASMIN pipeline provided upper limit of gamma-ray emission, which is more stringent than that given by individual instrument. Moreover, we investigated the properties of the X-ray transients and their gamma-ray counterparts, and explored the relation between the x-ray transient and gamma-ray counterpart. △ Less

Submitted 6 June, 2025; originally announced June 2025.

Comments: Accepted by ApJL

A systematic search for AGN obscuration variability in the Chandra archive

Authors: Isaiah S. Cox, Núria Torres-Albà, Stefano Marchesi, Vittoria E. Gianolli, Xiurui Zhao, Marco Ajello, Indrani Pal, Ross Silver

Abstract: The nature of the obscuring material in active galactic nuclei (AGN) can be studied by measuring changes in the line-of-sight column density, $N_{\rm H,los}$, over time. This can be accomplished by monitoring AGN over long periods of time and at all timescales. However, this can only be done for a few selected objects as it is resource intensive. Therefore, the best option currently is to focus on… ▽ More The nature of the obscuring material in active galactic nuclei (AGN) can be studied by measuring changes in the line-of-sight column density, $N_{\rm H,los}$, over time. This can be accomplished by monitoring AGN over long periods of time and at all timescales. However, this can only be done for a few selected objects as it is resource intensive. Therefore, the best option currently is to focus on population statistics based on the available archival data. In this work, we study 79 Seyfert 1 and Seyfert 2 galaxies from the Milliquas catalog to estimate a lower limit on the fraction of sources in the local $(z<0.1)$ universe that display spectral variability among observations. We find that 43 sources $(54\pm11\%)$, show indications of $N_{\rm H,los}$ variability at 90% confidence level. Interestingly, we also find that the variable fraction is similar for both Seyfert 1 $(f_{\rm Sy1}\sim61^{+13}_{-15}\%)$ and Seyfert 2 $(f_{\rm Sy2}\sim47\pm15\%)$ galaxies. The slightly higher $f_{\rm Sy1}$ fraction could be due to either a physical difference in the obscurers or the higher data quality in the Sy1 population. We also search for potential dependencies on the timescale between variable and non-variable observation pairs within a given source. In agreement with previous studies, we find evidence that more variability occurs on longer timescales than on shorter timescales. We present the 43 variable sources as a promising sample for future $N_{\rm H}$ variability studies. △ Less

Submitted 20 November, 2025; v1 submitted 4 June, 2025; originally announced June 2025.

Comments: 17 pages, 7 figures, 1 table, accepted to ApJ

All-sky search for individual Primordial Black Hole bursts with LHAASO

Authors: Zhen Cao, F. Aharonian, Y. X. Bai, Y. W. Bao, D. Bastieri, X. J. Bi, Y. J. Bi, W. Bian, A. V. Bukevich, C. M. Cai, W. Y. Cao, Zhe Cao, J. Chang, J. F. Chang, A. M. Chen, E. S. Chen, G. H. Chen, H. X. Chen, Liang Chen, Long Chen, M. J. Chen, M. L. Chen, Q. H. Chen, S. Chen, S. H. Chen , et al. (293 additional authors not shown)

Abstract: Primordial Black Holes~(PBHs) are hypothetical black holes with a wide range of masses that formed in the early universe. As a result, they may play an important cosmological role and provide a unique probe of the early universe. A PBH with an initial mass of approximately $10^{15}$~g is expected to explode today in a final burst of Hawking radiation. In this work, we conduct an all-sky search for… ▽ More Primordial Black Holes~(PBHs) are hypothetical black holes with a wide range of masses that formed in the early universe. As a result, they may play an important cosmological role and provide a unique probe of the early universe. A PBH with an initial mass of approximately $10^{15}$~g is expected to explode today in a final burst of Hawking radiation. In this work, we conduct an all-sky search for individual PBH burst events using the data collected from March 2021 to July 2024 by the Water Cherenkov Detector Array of the Large High Altitude Air Shower Observatory (LHAASO). Three PBH burst durations, 10~s, 20~s, and 100~s, are searched, with no significant PBH bursts observed. The upper limit on the local PBH burst rate density is set to be as low as 181~pc$^{-3}$~yr$^{-1}$ at 99$\%$ confidence level, representing the most stringent limit achieved to date. △ Less

Submitted 2 November, 2025; v1 submitted 30 May, 2025; originally announced May 2025.

Comments: 8 pages, 2 figures

A Be star-black hole binary with a wide orbit from LAMOST time-domain survey

Authors: Qian-Yu An, Yang Huang, Wei-Min Gu, Yong Shao, Zhi-Xiang Zhang, Tuan Yi, B. D. Lailey, T. A. A. Sigut, Kyle Akira Rocha, Meng Sun, Seth Gossage, Shi-Jie Gao, Shan-Shan Weng, Song Wang, Bowen Zhang, Xinlin Zhao, Senyu Qi, Shilong Liao, Jianghui Ji, Junfeng Wang, Jianfeng Wu, Mouyuan Sun, Xiang-Dong Li, Jifeng Liu

Abstract: Binary systems consisting of an early type star and a black hole (BH) are crucial for understanding various astrophysical phenomena, particularly the origins of detected gravitational wave sources. Be binary systems are expected to represent a key evolutionary stage in hosting BHs. However, while hundreds of Be X-ray binaries are known, the only confirmed BH candidate in a Be binary remains highly… ▽ More Binary systems consisting of an early type star and a black hole (BH) are crucial for understanding various astrophysical phenomena, particularly the origins of detected gravitational wave sources. Be binary systems are expected to represent a key evolutionary stage in hosting BHs. However, while hundreds of Be X-ray binaries are known, the only confirmed BH candidate in a Be binary remains highly controversial. We report the discovery of ALS 8814, a Be star-BH binary with a moderately eccentric ($e = 0.23$) and wide orbit ($P = 176.6$ days), revealed by the radial velocity (RV) measurement of the visible Be star. Our analysis, combining flux-calibrated spectra in the Balmer discontinuity region and spectral template matching, yields a mass of $11.2^{+1.4}_{-1.2}$ $M_\odot$ for the Be star. The minimum mass of the unseen companion, assuming an edge-on inclination ($i = 90^{\circ}$), is $9.8\pm 0.7\,M_\odot$. We rule out the presence of non-degenerate companions in ALS 8814, indicating that it can only be a BH. This discovery represents a robust case of a Be-BH binary, identified purely through precise RV measurements from a single set of lines. The extremely low peculiar velocity of ALS 8814 suggests that the BH is formed via a direct core-collapse with a negligible natal kick, implying an almost perfect alignment between the Be star's spin and the orbital plane. In this context, the binary's inclination angle is estimated to be 22$^{\circ}$-49$^{\circ}$ by analyzing the shallow double-peaked profile of the H$α$ emission line. This inclination range corresponds to a BH mass estimate between $15\,M_\odot$ and $58\,M_\odot$. As the only unambiguous Be-BH binary system known to date, ALS 8814 provides valuable constraints on the BH formation in a binary system with a high-mass companion. △ Less

Submitted 29 May, 2025; originally announced May 2025.

Comments: 76 pages, 29 figures, to be submitted

First Identification and Precise Spectral Measurement of the Proton Component in the Cosmic-Ray `Knee'

Authors: The LHAASO Collaboration, Zhen Cao, F. Aharonian, Y. X. Bai, Y. W. Bao, D. Bastieri, X. J. Bi, Y. J. Bi, W. Bian, A. V. Bukevich, C. M. Cai, W. Y. Cao, Zhe Cao, J. Chang, J. F. Chang, A. M. Chen, E. S. Chen, G. H. Chen, H. X. Chen, Liang Chen, Long Chen, M. J. Chen, M. L. Chen, Q. H. Chen, S. Chen , et al. (292 additional authors not shown)

Abstract: We report the first high-purity identification of cosmic-ray (CR) protons and a precise measurement of their energy spectrum from 0.15 to 12 PeV using the Large High Altitude Air Shower Observatory (LHAASO). Abundant event statistics, combined with the simultaneous detection of electrons/photons, muons, and Cherenkov light in air showers, enable spectroscopic measurements with statistical and syst… ▽ More We report the first high-purity identification of cosmic-ray (CR) protons and a precise measurement of their energy spectrum from 0.15 to 12 PeV using the Large High Altitude Air Shower Observatory (LHAASO). Abundant event statistics, combined with the simultaneous detection of electrons/photons, muons, and Cherenkov light in air showers, enable spectroscopic measurements with statistical and systematic accuracy comparable to satellite data at lower energies. The proton spectrum shows significant hardening relative to low-energy extrapolations, culminating at 3 PeV, followed by sharp softening. This distinct spectral structure - closely aligned with the knee in the all-particle spectrum - points to the emergence of a new CR component at PeV energies, likely linked to the dozens of PeVatrons recently discovered by LHAASO, and offers crucial clues to the origin of Galactic cosmic rays. △ Less

Submitted 20 May, 2025; originally announced May 2025.

GECAM Discovery of Peculiar Oscillating Particle Precipitation Events

Authors: Chenwei Wang, Shaolin Xiong, Yi Zhao, Wei Xu, Gaopeng Lu, Xuzhi Zhou, Xiaocheng Guo, Wenya Li, Xiaochao Yang, Qinghe Zhang, Xinqiao Li, Zhenxia Zhang, Zhenghua An, Ce Cai, Peiyi Feng, Yue Huang, Min Gao, Ke Gong, Dongya Guo, Haoxuan Guo, Bing Li, Xiaobo Li, Yaqing Liu, Jiacong Liu, Xiaojing Liu , et al. (30 additional authors not shown)

Abstract: Charged particle precipitation typically manifests as a gradual increase and decrease of flux observed by space detectors. Cases with rapidly flux variation are very rare. Periodic events are even more extraordinary. These oscillating particle precipitation (OPP) events are usually attributed to the bounce motion of electrons, which are induced by lightning. Owing to the observation limitations, t… ▽ More Charged particle precipitation typically manifests as a gradual increase and decrease of flux observed by space detectors. Cases with rapidly flux variation are very rare. Periodic events are even more extraordinary. These oscillating particle precipitation (OPP) events are usually attributed to the bounce motion of electrons, which are induced by lightning. Owing to the observation limitations, there has been debate regarding whether these oscillations originate from temporal flux evolution or spatial structure evolution. Here we report three peculiar charged particle precipitation events detected by GECAM during a geomagnetic storm on March 21, 2024, with two exhibiting significant periodicity. These events were observed around the same region during three consecutive orbits. Through comprehensive temporal and spectral analyses, we revealed that one of the OPP events exhibited a transition in spectral lag of mini-pulses, shifting from "softer-earlier" to "softer-later" while showing no significant time evolution in overall frequency characteristics. And there is no association found between these two OPP events and lightning activity. Several possible scenarios are discussed to explain these charged particles with a life time of more than 3.5 hours, but the nature of these three events remains an enigma. We suggest that these GECAM-detected OPP events may represent a new type of particle precipitation event or a peculiar Lightning-induced Electron Precipitations (LEPs). △ Less

Submitted 9 May, 2025; originally announced May 2025.

Pitch Angle Measurement Method based on Detector Counts Distribution. -I. Basic conception

Authors: Chenwei Wang, Shaolin Xiong, Hongbo Xue, Yiteng Zhang, Shanzhi Ye, Wei Xu, Jinpeng Zhang, Zhenghua An, Ce Cai, Peiyi Feng, Ke Gong, Haoxuan Guo, Yue Huang, Xinqiao Li, Jiacong Liu, Xiaojing Liu, Xiang Ma, Liming Song, Wenjun Tan, Jin Wang, Ping Wang, Yue Wang, Xiangyang Wen, Shuo Xiao, Shenlun Xie , et al. (14 additional authors not shown)

Abstract: As an X-ray and gamma-ray all-sky monitor aiming for high energy astrophysical transients, Gravitational-wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) has also made a series of observational discoveries on burst events of gamma-rays and particles in the low Earth orbit. Pitch angle is one of the key parameters of charged particles traveling around geomagnetic field. However,… ▽ More As an X-ray and gamma-ray all-sky monitor aiming for high energy astrophysical transients, Gravitational-wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) has also made a series of observational discoveries on burst events of gamma-rays and particles in the low Earth orbit. Pitch angle is one of the key parameters of charged particles traveling around geomagnetic field. However, the usage of the GECAM-style instruments to measure the pitch angle of charged particles is still lacking. Here we propose a novel method for GECAM and similar instruments to measure the pitch angle of charged particles based on detector counts distribution. The basic conception of this method and simulation studies are described. With this method, the pitch angle of a peculiar electron precipitation event detected by GECAM-C is derived to be about 90$^\circ$, demonstrating the feasibility of our method. We note that the application of this method on GECAM-style instruments may open a new window for studying space particle events, such as Terrestrial Electron Beams (TEBs) and Lightning-induced Electron Precipitations (LEPs). △ Less

Submitted 9 May, 2025; originally announced May 2025.

Measurement of separate electron and positron spectra from 10 GeV to 20GeV with the geomagnetic field on DAMPE

Authors: DAMPE Collaboration, F. Alemanno, Q. An, P. Azzarello, F. C. T. Barbato, P. Bernardini, X. J. Bi, H. Boutin, I. Cagnoli, M. S. Cai, E. Casilli, E. Catanzani, J. Chang, D. Y. Chen, J. L. Chen, Z. F. Chen, Z. X. Chen, P. Coppin, M. Y. Cui, T. S. Cui, Y. X. Cui, I. DeMitri, F. dePalma, A. DiGiovanni, T. K. Dong , et al. (127 additional authors not shown)

Abstract: The cosmic-ray (CR) electrons and positrons in space are of great significance for studying the origin and propagation of cosmic-rays. The satellite-borne experiment DArk Matter Particle Explorer (DAMPE) has been used to measure the separate electron and positron spectra, as well as the positron fraction. In this work, the Earth's magnetic field is used to distinguish CR electrons and positrons, a… ▽ More The cosmic-ray (CR) electrons and positrons in space are of great significance for studying the origin and propagation of cosmic-rays. The satellite-borne experiment DArk Matter Particle Explorer (DAMPE) has been used to measure the separate electron and positron spectra, as well as the positron fraction. In this work, the Earth's magnetic field is used to distinguish CR electrons and positrons, as the DAMPE detector does not carry an onboard magnet. The energy range for the measurements is from 10 to 20 GeV, being currently limited at high energy by the zenith pointing orientation of DAMPE. The results are consistent with previous measurements based on the magnetic spectrometer by AMS-02 and PAMELA, while the results of Fermi-LAT seem then to be systematically shifted to larger values. △ Less

Submitted 21 August, 2025; v1 submitted 9 May, 2025; originally announced May 2025.

Comments: Accepted for publication in Chinese Physics C

Synchrotron Circular Polarization in Gamma-Ray Burst Prompt Optical Emission: Relativistic Thermal Electron Contribution

Authors: Kangfa Cheng, Jirong Mao, Xiaohong Zhao, Hongbang Liu, Zhegeng Chen, Gaojin Yu, Zhifu Chen

Abstract: Synchrotron circular polarization of a non-thermal power-law electron distribution in gamma-ray bursts (GRBs) has been studied. However, some numerical simulations have shown that the resulting distribution of electrons is a combination of a thermal component and a non-thermal power-law component. In this paper, we investigate synchrotron circular polarization using such a hybrid energy distributi… ▽ More Synchrotron circular polarization of a non-thermal power-law electron distribution in gamma-ray bursts (GRBs) has been studied. However, some numerical simulations have shown that the resulting distribution of electrons is a combination of a thermal component and a non-thermal power-law component. In this paper, we investigate synchrotron circular polarization using such a hybrid energy distribution of relativistic thermal and nonthermal electrons within a globally toroidal magnetic field in GRB prompt optical emission. Our results show that compared to the solely nonthermal electron model, the synchrotron circular polarization degree (PD) in the hybrid electron model can vary widely in the optical band, depending on different parameters. The lower the electron temperature, the higher the circular PD. The time-averaged circular PD in the hybrid electron model can be higher than $\sim 1\%$ when the electron temperature is as low as $\sim 10^{10}$ K, while in the solely nonthermal electron model is usually lower than $\sim 1\%$. We further calculate the radiative transfer of the circular and linear polarization in the optical band. Our results show that both of the circular and linear PDs decrease with the increase of optical depth, but the linear PDs decline faster than the circular PDs. To further examine the physical mechanisms of both radiation and particle acceleration, we expect that instruments will be capable of measuring the circular polarization of GRB prompt optical emission in the future. △ Less

Submitted 28 May, 2025; v1 submitted 28 April, 2025; originally announced April 2025.

Comments: 12 pages, 5 figures. Accepted for publication in ApJ. arXiv admin note: text overlap with arXiv:2410.00471

Hemispheric Distribution of Solar Active Regions During Solar Cycles 23-25

Authors: Yuxia Liu, Tingting Xu, Miao Wan, Linhua Deng, Xinhua Zhao, Shiyang Qi, Nanbin Xiang, Weihong Zhou

Abstract: Solar active regions (ARs) are crucial for understanding the long-term evolution of solar activities and predicting eruptive phenomena, including solar flares and coronal mass ejections. However, the cycle-dependent properties in the north-south asymmetry of ARs have not been fully understood. In this study, we investigate the hemispheric distribution of ARs from Carrington Rotation 1909 to 2278 (… ▽ More Solar active regions (ARs) are crucial for understanding the long-term evolution of solar activities and predicting eruptive phenomena, including solar flares and coronal mass ejections. However, the cycle-dependent properties in the north-south asymmetry of ARs have not been fully understood. In this study, we investigate the hemispheric distribution of ARs from Carrington Rotation 1909 to 2278 (between 1996 May and 2023 November) by using three parameters that describe the magnetic field distribution of ARs: number, area, and flux. The main findings are as follows: (1) The three AR parameters show significant hemispheric asymmetry in cycles 23-25. The strong correlation between AR area and flux indicates that they can better reflect the intrinsic properties of solar magnetic field. (2) The correlation between sunspot activity and AR parameters varies in the two hemispheres across the different cycles. The AR parameters provide additional information for the variations in sunspot activity, which can better predict the intensity and cyclical changes of solar activity. (3) The variation in the fitting slope sign of the asymmetry index for AR parameters reflects periodic changes in hemispheric ARs, providing valuable insights into the activity of other stars. (4) Both the dominant hemisphere and the cumulative trend of AR parameters display a cycle-dependent behavior. Moreover, the trend variations of AR area and flux are similar, reflecting the long-term evolutionary characteristics of solar magnetic field. Our analysis results are relevant for understanding the hemispheric coupling of solar magnetic activity and its cyclic evolutionary patterns. △ Less

Submitted 24 April, 2025; originally announced April 2025.

Molecular Clouds at the Edge of the Galaxy I. Variation of CO J=2-1/1-0 Line Ratio

Authors: C. S. Luo, X. D. Tang, C. Henkel, K. M. Menten, Y. Sun, Y. Gong, X. W. Zheng, D. L. Li, Y. X. He, X. Lu, Y. P. Ao, X. P. Chen, T. Liu, K. Wang, J. W. Wu, J. Esimbek, J. J. Zhou, J. J. Qiu, X. Zhao, J. S. Li, Q. Zhao, L. D. Liu

Abstract: The Galactic edge at Galactocentric distances of 14\,--\,22\,kpc provides an ideal laboratory to study molecular clouds in an environment that is different from the solar neighborhood, due to its lower gas density, lower metallicity, and little or no perturbation from the spiral arms. Observations of CO\,($J$\,=\,2--1) spectral lines were carried out towards 72 molecular clouds located at the Gala… ▽ More The Galactic edge at Galactocentric distances of 14\,--\,22\,kpc provides an ideal laboratory to study molecular clouds in an environment that is different from the solar neighborhood, due to its lower gas density, lower metallicity, and little or no perturbation from the spiral arms. Observations of CO\,($J$\,=\,2--1) spectral lines were carried out towards 72 molecular clouds located at the Galactic edge using the IRAM\,30\,m telescope. Combined with CO\,($J$\,=\,1--0) data from the MWISP project, we investigate the variations of $R_{21}$ across these Galactic edge clouds, with $R_{21}$ representing CO(2-1)/CO(1-0) integrated intensity ratios. These are found to range from 0.3 to 3.0 with a mean of 1.0\,$\pm$\,0.1 in the Galactic edge clouds. The proportions of very low ratio gas (VLRG; $R_{21}$\,<\,0.4), low ratio gas (LRG; 0.4\,$\le$\,$R_{21}$\,<\,0.7), high ratio gas (HRG; 0.7\,$\le$\,$R_{21}$\,<\,1.0), and very high ratio gas (VHRG; $R_{21}$\,$\ge$\,1.0) are 6.9\%, 29.2\%, 26.4\%, and 37.5\%, respectively, indicating a significant presence of high $R_{21}$ ratio molecular gas within these regions. In our Galaxy, the $R_{21}$ ratio exhibits a gradient of initial radial decline followed by a high dispersion with increasing Galacticentric distance and a prevalence for high ratio gas. There is no apparent systematic variation within the Galactocentric distance range of 14 to 22\,kpc. A substantial proportion of HRG and VHRG is found to be associated with compact clouds and regions displaying star-forming activity, suggesting that the high $R_{21}$ ratios may stem from dense gas concentrations and recent episodes of star formation. △ Less

Submitted 18 April, 2025; originally announced April 2025.

Journal ref: A&A 698, A54 (2025)

Contemporaneous X-ray and Optical Polarization of EHSP Blazar H 1426+428

Authors: Anuvab Banerjee, Akash Garg, Divya Rawat, Svetlana Jorstad, Alan P. Marscher, Ivan Agudo, Jorge Otero-Santos, Daniel Morcuende, Juan Escudero Pedrosa, Alberto Dominguez, Ayan Bhattacharjee, Isaiah Cox, Indrani Pal, Xiurui Zhao, Andrealuna Pizzetti, Stefano Marchesi, Nuria Torres-Alba, Kouser Imam, Ross Silver, Marco Ajello

Abstract: We present the first contemporaneous X-ray and optical polarimetric measurement of the extremely high synchrotron peaked (EHSP) blazar H 1426+428. The X-ray polarimetric observations were undertaken using the Imaging X-ray Polarimetry Explorer (\textit{IXPE}) on 2024 May 27, and 2024 July 5. The \textit{IXPE} pointings were accompanied by contemporaneous optical observations of the Observatorio de… ▽ More We present the first contemporaneous X-ray and optical polarimetric measurement of the extremely high synchrotron peaked (EHSP) blazar H 1426+428. The X-ray polarimetric observations were undertaken using the Imaging X-ray Polarimetry Explorer (\textit{IXPE}) on 2024 May 27, and 2024 July 5. The \textit{IXPE} pointings were accompanied by contemporaneous optical observations of the Observatorio de Sierra Nevada, Calar Alto Observatory and the Perkins Telescope Observatory. While we observed the X-ray degree of polarization to be $>20\%$, the polarization in the optical band was found to be only $1-3\%$. This trend has been observed in several HSP blazars with available optical and X-ray polarimetric data and is typically explained in terms of energy stratification downstream of a shock. However, we observed a significant difference between the optical and X-ray polarization angles, a feature that has been observed in certain HSP blazars, such as Mrk 421, but remains a relatively rare or underreported phenomenon. We discuss possible scenarios for these findings within the framework of a partially turbulent jet model. △ Less

Submitted 16 April, 2025; originally announced April 2025.

Exploring the origin of multi-periodic pulsations during a white-light flare

Authors: Dong Li, Ding Yuan, Jingye Yan, Xinhua Zhao, Zhao Wu, Jincheng Wang, Zhenyong Hou, Chuan Li, Haisheng Zhao, Libo Fu, Lin Wu, Li Deng

Abstract: We explored the quasi-periodic pulsations (QPPs) at multiple periods during an X4.0 flare on 2024 May 10 (SOL2024-05-10T06:27), which occurred in the complex active region of NOAA 13664. The flare radiation reveals five prominent periods in multiple wavelengths. A 8-min QPP is simultaneously detected in wavelengths of HXR, radio, UV/EUV, Lya, and white light, which may be associated with nontherma… ▽ More We explored the quasi-periodic pulsations (QPPs) at multiple periods during an X4.0 flare on 2024 May 10 (SOL2024-05-10T06:27), which occurred in the complex active region of NOAA 13664. The flare radiation reveals five prominent periods in multiple wavelengths. A 8-min QPP is simultaneously detected in wavelengths of HXR, radio, UV/EUV, Lya, and white light, which may be associated with nonthermal electrons periodically accelerated by intermittent magnetic reconnection that is modulated by the slow wave. A quasi-period at 14 minutes is observed in the SXR and high-temperature EUV wavebands, and it may be caused by repeatedly heated plasmas in hot flare loops. A quasiperiod at about 18 minutes is only observed by STIX, with reconstructed SXR images suggesting that the 18-min period pulsations should be considered as different flares. Meanwhile, a 3-min QPP is simultaneously detected in wavelengths of HXR, radio, and UV/ EUV, which is directly modulated by the slow magnetoacoustic wave leaking from sunspot umbrae. At last, a 2-min QPP is simultaneously detected in HXR and radio emissions during the pre-flare phase, which is possibly generated by a quasi-periodic regime of magnetic reconnection that is triggered by the kink wave. △ Less

Submitted 11 April, 2025; originally announced April 2025.

Comments: 32 pages, 11 figures, accepted to Journal of Geophysical Research-Space Physics

The Mini-SiTian Array: first-two-year operation

Authors: Min He, Hong Wu, Liang Ge, Jian-feng Tian, Zheng Wang, Hai-yang Mu, Yu Zhang, Yang Huang, Jie Zheng, Zhou Fan, Zheng-yang Li, Hong-hui Gu, Heng-geng Han, Kai Xiao, Zhi-rui Li, Jun-jie Jin, Bei-chuan Wang, Jun Ma, Jin-hang Zou, Ying Wu, Jiu-peng Guo, Li-guo Fang, Zhi-gang Hou, Bo-wen Zhang, Yun-fei Xu , et al. (48 additional authors not shown)

Abstract: The SiTian project, designed to utilize 60 telescopes distributed across multiple sites in China, is a next-generation time-domain survey initiative. As a pathfinder for the SiTian project, the Mini-SiTian (MST) has been proposed and implemented to test the SiTian's brain and data pipeline, and to evaluate the feasibility of its technology and science cases. Mounted at the Xinglong Observatory, th… ▽ More The SiTian project, designed to utilize 60 telescopes distributed across multiple sites in China, is a next-generation time-domain survey initiative. As a pathfinder for the SiTian project, the Mini-SiTian (MST) has been proposed and implemented to test the SiTian's brain and data pipeline, and to evaluate the feasibility of its technology and science cases. Mounted at the Xinglong Observatory, the MST project comprises three 30 cm telescopes and has been operated since Nov. 2022. Each telescope of the MST possesses a large field of view, covering $2.29^{\circ}$ $\times$ $1.53^{\circ}$ FOV, and is equipped with $g'$, $r'$ and $i'$ filters, respectively. Acting as the pioneer of the forthcoming SiTian project, the MST is dedicated to the discovery of variable stars, transients, and outburst events, and has already obtained some interesting scientific results. In this paper, we will summarize the first-two-year operation of the MST project. △ Less

Submitted 2 April, 2025; originally announced April 2025.

Comments: 10 pages, 11 figures, Accepted for publication in a special issue of Research in Astronomy and Astrophysics on the Mini-SiTian Array

Multi-scale Energy Release Events in the Quiet Sun: A Possible Source for Coronal Heating

Authors: Rui Wang, Yiming Jiao, Xiaowei Zhao, Chong Huang

Abstract: The coronal heating problem remains one of the most challenging questions in solar physics. The energy driving coronal heating is widely understood to be associated with convective motions below the photosphere. Recent high-resolution observations reveal that photospheric magnetic fields in the quiet Sun undergo complex and rapid evolution. These photospheric dynamics are expected to be reflected… ▽ More The coronal heating problem remains one of the most challenging questions in solar physics. The energy driving coronal heating is widely understood to be associated with convective motions below the photosphere. Recent high-resolution observations reveal that photospheric magnetic fields in the quiet Sun undergo complex and rapid evolution. These photospheric dynamics are expected to be reflected in the coronal magnetic field. Motivated by these insights, our research aims to explore the relationship between magnetic energy and coronal heating. By combining observations from Solar Orbiter and SDO with a magnetic field extrapolation technique, we estimate the magnetic free energy of multi-scale energy release events in the quiet Sun. Interestingly, our results reveal a strong correlation between the evolution of free energy and the integrated intensity of extreme ultraviolet emission at 171 Å~in these events. We quantitatively assess the potential energy flux budget of these events to evaluate their contribution to coronal heating. Our study implies a link between photospheric magnetic field evolution and coronal temperature variations, paving the way for further research into similar phenomena. △ Less

Submitted 20 March, 2025; originally announced March 2025.

Comments: 11 pages, 4 figures, published in Front. Astron. Space Sci

Searching for accreting compact binary systems from spectroscopy and photometry: Application to LAMOST spectra

Authors: Xinlin Zhao, Song Wang, Jifeng Liu

Abstract: Compact objects undergoing mass transfer exhibit significant (and double-peaked) $H_α$ emission lines. Recently, new methods have been developed to identify black hole X-ray binaries (BHXBs) and calculate their systematic parameters using $H_α$ line parameters, such as the full-width at half maximum (FWHM), equivalent width (EW), and separation of double peaks. In addition, the FWHM-EW plane from… ▽ More Compact objects undergoing mass transfer exhibit significant (and double-peaked) $H_α$ emission lines. Recently, new methods have been developed to identify black hole X-ray binaries (BHXBs) and calculate their systematic parameters using $H_α$ line parameters, such as the full-width at half maximum (FWHM), equivalent width (EW), and separation of double peaks. In addition, the FWHM-EW plane from spectroscopy and the $H_α$ color-color diagram from photometry can be used for rapid stellar classification. We measure the $H_α$ and $H_β$ profiles (e.g., FWHM and EW) using the LAMOST DR9 low- and medium-resolution spectra, and calculate the systematic parameters (e.g., velocity semi-amplitude of the donor star, mass ratio, inclination angle, and mass of the accretor). A new correlation between FWHM and $K_{\rm 2}$, $K_{\rm 2} = 0.205(18)\ \rm{FWHM}$, is obtained for cataclysmic variables (CVs) in our sample. Both the FWHM-EW plane and the $H_α$ color-color diagram can distinguish CVs with FWHM $\gtrsim$ 1000 km/s from Be stars and young stellar objects (YSOs) to some extent. To improve classification accuracy and enhance the selection of compact objects, we propose a new set of idealized filters with effective widths of 30 Å, 130 Å, and 400 Å\ for the narrow $H_α$ filter, broad $H_α$ filter, and $r$-band filter, respectively. △ Less

Submitted 16 March, 2025; originally announced March 2025.

Comments: 18pages, 15 figures, accepted for publication in APJ

The GECAM Ground Search System for Gamma-ray Transients

Authors: Ce Cai, Yan-Qiu Zhang, Shao-Lin Xiong, Ping Wang, Jian-Hui Li, Xiao-Bo Li, Cheng-Kui Li, Yue Huang, Shi-Jie Zheng, Li-Ming Song, Shuo Xiao, Qi-Bin Yi, Yi Zhao, Sheng-Lun Xie, Rui Qiao, Yan-Qi Du, Zhi-Wei Guo, Wang-Chen Xue, Chao Zheng, Jia-Cong Liu, Chen-Wei Wang, Wen-Jun Tan, Yue Wang, Jin-Peng Zhang, Chao-Yang Li , et al. (13 additional authors not shown)

Abstract: In the era of time-domain, multi-messenger astronomy, the detection of transient events on the high-energy electromagnetic sky has become more important than ever. The Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is a dedicated mission to monitor gamma-ray transients, launched in December, 2020. A real-time on-board trigger and location software, using the tra… ▽ More In the era of time-domain, multi-messenger astronomy, the detection of transient events on the high-energy electromagnetic sky has become more important than ever. The Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is a dedicated mission to monitor gamma-ray transients, launched in December, 2020. A real-time on-board trigger and location software, using the traditional signal-to-noise ratio (SNR) method for blind search, is constrained to relatively bright signals due to the limitations in on-board computing resources and the need for real-time search. In this work, we developed a ground-based pipeline for GECAM to search for various transients, especially for weak bursts missed by on-board software. This pipeline includes both automatic and manual mode, offering options for blind search and targeted search. The targeted search is specifically designed to search for interesting weak bursts, such as gravitational wave-associated gamma-ray bursts (GRBs). From the ground search of the data in the first year, GECAM has been triggered by 54 GRBs and other transients, including soft gamma-ray repeaters, X-ray binaries, solar flares, terrestrial gamma-ray flashes. We report the properties of each type of triggers,such as trigger time and light curves. With this search pipeline and assuming a soft Band spectrum, the GRB detection sensitivity of GECAM is increased to about 1.1E-08 erg cm-2 s-1 (10 keV - 1000 keV, burst duration of 20 s). These results demonstrate that the GECAM ground search system (both blind search and targeted search) is a versatile pipeline to recover true astrophysical signals which were too weak to be found in the on-board search. △ Less

Submitted 4 March, 2025; originally announced March 2025.

Comments: Accepted by SCIENCE CHINA Physics, Mechanics & Astronomy (SCPMA)

Journal ref: The GECAM ground search system for gamma-ray transients. Sci. China-Phys. Mech. Astron. Volume 68, article number 239511, (2025)