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

¹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
² DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327/328, DK-2800 Kongens Lyngby, Denmark
³ INAF–Astronomical Observatory of Brera, via E. Bianchi 46, I-23807 Merate, Italy
⁴ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁵ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁶ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
⁷ Millennium Institute of Astrophysics, Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
⁸ Observatorio Astronómico de Quito, Escuela Politécnica Nacional, 170136 Quito, Ecuador
⁹ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
¹⁰ Hessian Research Cluster ELEMENTS, Giersch Science Center, Max-von-Laue-Straße 12, Goethe University Frankfurt, Campus Riedberg, 60438 Frankfurt am Main, Germany
¹¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía, 18008 Granada, Spain

Received: 25 March 2024
Accepted: 27 June 2024

Abstract

Context. Extragalactic fast X-ray transients (FXTs) are short-duration (∼ks) X-ray flashes of unknown origin, potentially arising from binary neutron star (BNS) mergers, tidal disruption events, or supernova shock breakouts.

Aims. 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).

Methods. 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 θ_C and a Gaussian-structured jet whose edge lies at an angle θ_W = 2θ_C. We compare their peak X-ray luminosity, duration, and temporal indices α (where F ∝ t^α) with those of the currently known extragalactic FXTs.

Results. Our analysis reveals that a slightly off-axis observing angle of θ_obs ≈ (2.2 − 3)θ_C and a structured jet are required to explain the shallow (|α|≲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 θ_W = 2θ_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, θ_obs ≈ (2.2 − 3)θ_C. While the distributions of the off-axis peak X-ray luminosity are consistent only for θ_obs = 2.2θ_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 θ_obs ∼ (2.2 − 2.6)θ_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.