Prompt X-Ray Emission from Fast Radio Bursts—Upper Limits with AstroSat

Abstract

Fast radio bursts (FRBs) are short-lived (∼ms), energetic transients (having a peak flux density of ∼Jy) with no known prompt emission in other energy bands. We present results of a search for prompt X-ray emissions from 41 FRBs using the Cadmium Zinc Telluride Imager on AstroSat , which continuously monitors ∼70% of the sky. Our searches on various timescales in the 20–200 keV range, did not yield any counterparts in this hard X-ray band. We calculate upper limits on hard X-ray flux, in the same energy range and convert them to upper bounds for η : the ratio of X-ray to radio fluence of FRBs. We find η  ≤ 10 8–10 for hard X-ray emission. Our results will help constrain the theoretical models of FRBs as the models become more quantitative and nearer, brighter FRBs are discovered.