Prompt X-ray emission from Fast Radio Bursts -- Upper limits with AstroSat

TL;DR

This study searched for prompt X-ray emissions from 41 FRBs using AstroSat's CZTI but found none, setting upper limits on X-ray to radio fluence ratios to constrain theoretical models.

Contribution

First systematic search for prompt hard X-ray emission from FRBs using AstroSat's CZTI, establishing upper limits and aiding model constraints.

Findings

No X-ray counterparts detected in 20-200 keV range.

Upper limits on X-ray to radio fluence ratio: η ≤ 10^{8-10}.

Results constrain theoretical models of FRB emission mechanisms.

Abstract

Fast Radio Bursts (FRBs) are short lived ($\sim$ msec), energetic transients (having a peak flux density of $\sim$ 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 (CZTI) on AstroSat which continuously monitors $\sim70\%$ 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 $\eta$: the ratio X-ray to radio fluence of FRBs. We find $\eta \leq 10^{8-10}$ 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.