Bright in the Black: Searching for Electromagnetic Counterparts to Gravitational-Wave Candidates in LIGO-Virgo-KAGRA Observation Runs with AstroSat-CZTI
GW150914 marked the start of the gravitational wave (GW) era with the direct detection of binary black hole (BBH) merger by the LIGO-Virgo GW detectors. The event was temporally coincident with a weak signal detected by Fermi-GBM, which hinted towards the possibility of electromagnetic emission associated with the compact object coalescence. The detection of a short Gamma-Ray Burst (GRB) associated with GW1708017, along with several multi-wavelength detections, truly established that compact object mergers are indeed multi-messenger events. The Cadmium Zinc Telluride Imager onboard AstroSat can search for X-ray counterparts of the GW events and has detected over 600 GRBs since launch. Here we present results from our searches for counterparts coincident with GW triggers from the first three LIGO-Virgo-KAGRA (LVK) GW Transient Catalogs. For 72 out of 90 GW events for which AstroSat-CZTI data was available, we undertook a systematic search for temporally coincident transients and we detected no X-ray counterparts. We evaluate the upper limits on the maximum possible flux from the source in a 100 s window centered around each trigger, consistent with the GW localization of the event. Thanks to the high sensitivity of CZTI, these upper limits are highly competitive with those from other spacecraft. We use these upper limits to constrain the theoretical models that predict high-energy counterparts to the BBH mergers. We also discuss the probability of non-detections of BBH mergers at different luminosities and the implications of such non-detections from the ongoing fourth observing run of the LVK detectors.
Evolution of the Inner Accretion Flow in Swift J1727.8$-$1613 across Intermediate States: Insights from Broadband Spectral and Timing Analysis
Inter-University Centre for Astronomy and Astrophysics（天文与天体物理大学间中心）；Institute of Astronomy, National Tsing Hua University（国立清华大学天文研究所）；Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences（波兰科学院尼古拉·哥白尼天文学中心）；Department of Pure and Applied Physics, Guru Ghasidas Vishwavidyalaya (A Central University)（古鲁·加西达斯维什瓦维迪亚亚拉（一所中央大学）纯与应用物理系）
