The fast X-ray transient EP240315a: a z ~ 5 gamma-ray burst in a Lyman continuum leaking galaxy

TL;DR

This paper reports the discovery and analysis of a high-redshift, long-duration gamma-ray burst associated with a leaking Lyman continuum galaxy, demonstrating the potential of future X-ray observatories to probe the epoch of reionization.

Contribution

It provides multi-wavelength observations of a z~5 gamma-ray burst with Lyman continuum leakage, linking FXTs to lower-luminosity GRBs and highlighting their detectability at high redshifts.

Findings

The transient is at redshift z=4.859.

It shows a low neutral hydrogen column density.

The event is consistent with a long-duration gamma-ray burst.

Abstract

The nature of the minute-to-hour long Fast X-ray Transients (FXTs) localised by telescopes such as Chandra, Swift, and XMM-Newton remains mysterious, with numerous models suggested for the events. Here, we report multi-wavelength observations of EP240315a, a 1600 s long transient detected by the Einstein Probe, showing it to have a redshift of z=4.859. We measure a low column density of neutral hydrogen, indicating that the event is embedded in a low-density environment, further supported by direct detection of leaking ionising Lyman-continuum. The observed properties are consistent with EP240315a being a long-duration gamma-ray burst, and these observations support an interpretation in which a significant fraction of the FXT population are lower-luminosity examples of similar events. Such transients are detectable at high redshifts by the Einstein Probe and, in the (near) future, out to even larger distances by SVOM, THESEUS, and Athena, providing samples of events into the epoch of reionisation.