AstroSat-CZTI searches for hard X-ray prompt emission from fast radio bursts

Abstract

Fast radio bursts (FRBs) are short-duration, highly-energetic radio transients with unclear origins and emission mechanisms, typically found at cosmological distances. Despite extensive searches, no credible prompt electromagnetic counterparts have been found for extragalactic FRBs. We presented the results from a comprehensive search for prompt X-ray counterparts using AstroSat-CZTI , which regularly detects other high-energy fast transients like gamma-ray bursts (GRBs). Our systematic search in CZTI data for hard X-ray transients temporally and spatially coincident with 572 FRBs yielded no credible counterparts. We estimated flux upper limits for these events and converted them to upper limits on X-ray-to-radio fluence ratios and found them to be distributed between \(10^{7}\) and \(10^{13}\) for all the three search timescales – 0.01, 0.1 and 1 s. Using redshifts derived from dispersion measures, we placed (\(L_\textrm{iso}\)), upper limits ranging from \(10^{49}\) to \(10^{55}\) \(\mathrm {ergs~s^{-1}}\)  on isotropic equivalent luminosities. We compared them with the isotropic luminosities of GRBs, to examine potential similarities between these transient classes. Finally, we explored the prospects for X-ray counterpart detections using other current and upcoming X-ray monitors, including Fermi-GBM, Swift-BAT, SVOM-ECLAIRs and Daksha, with next-generation FRB detection facilities, such as DSA-2000, CHORD and BURSTT. Our results highlight that highly sensitive X-ray monitors with large sky coverage, like Daksha, will provide the best opportunities to detect X-ray counterparts of bright FRBs.