Investigating the off-axis GRB afterglow scenario for extragalactic fast X-ray transients

Abstract

Extragalactic fast X-ray transients (FXTs) are short-duration (sim ks) X-ray flashes of unknown origin, potentially arising from binary neutron star (BNS) mergers, tidal disruption events, or supernova shock breakouts. In the context of the BNS scenario, we investigate the possible link between FXTs and the afterglows of off-axis merger-induced gamma-ray bursts (GRBs). By modelling well-sampled broadband afterglows of 13 merger-induced GRBs, we make predictions for their X-ray light curve behaviour had they been observed off-axis, considering both a uniform jet with core angle $ C $ and a Gaussian-structured jet whose edge lies at an angle $ W C $. We compare their peak X-ray luminosity, duration, and temporal indices alpha (where $F alpha $) with those of the currently known extragalactic FXTs. Our analysis reveals that a slightly off-axis observing angle of $ obs C $ and a structured jet are required to explain the shallow ($| 0.3) temporal indices of the FXT light curves, which cannot be reproduced in the uniform-jet case at any viewing angle. In the case of a structured jet with truncation angle $ W C $, the distributions of the duration of the FXTs are consistent with those of the off-axis afterglows for the same range of observing angles, $ obs C $. While the distributions of the off-axis peak X-ray luminosity are consistent only for $ obs C $, focussing on individual events with different intrinsic luminosities reveals that the match of all three properties (peak X-ray luminosity, duration and temporal indices) of the FXTs at the same viewing angle is possible in the range $ obs C $. Despite the small sample of GRBs analysed, these results show that there is a region of the parameter space -- although quite limited -- where the observational properties of off-axis GRB afterglow can be consistent with those of the newly discovered FXTs. Future observations of FXTs discovered by the recently launched Einstein Probe mission and GRB population studies combined with more complex afterglow models will shed light on this possible GRB-FXT connection, and eventually unveil the progenitors of some FXTs.