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 a binary black hole (BBH) merger by the LIGO-Virgo GW detectors. The event was tentatively associated with a signal detected by the Fermi Gamma-ray Burst Monitor (GBM), which hinted toward electromagnetic emission associated with compact object coalescence. The detection of a short gamma-ray burst (GRB) associated with GW170817, along with several multiwavelength detections, truly established that compact object mergers are indeed multimessenger events. The CadmiumZincTelluride Imager (CZTI) on board AstroSat can search for X-ray counterparts of the GW events and has detected over 650 GRBs in the 8.5 yr since it was launched. We present results from our searches for counterparts coincident with GW triggers from the first three LIGO-Virgo-KAGRA (LVK) GW Transient Catalogs. For 71 out of 90 GW events for which AstroSat-CZTI data was available, we undertook a systematic search for temporally coincident transients in the 20–200 keV band and 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 sensitivity of CZTI, these upper limits are competitive with those from other spacecraft. We use these upper limits to constrain theoretical models that predict high-energy counterparts to BBH mergers. We also discuss the probability of nondetections of BBH mergers at different luminosities and the implications of such nondetections from the ongoing fourth observing run of the LVK detectors.