Fast X-ray transient EP240315A from a Lyman-continuum-leaking galaxy at z ≈ 5

The nature of the minute-to-hour-long fast X-ray transients localized 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 1,600-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 and directly detect leaking ionizing Lyman continuum. The observed properties are consistent with EP240315a being a long-duration gamma-ray burst, and these observations suggest a possible interpretation in which a substantial fraction of the X-ray-transient population are lower-luminosity examples of similar events. If correct, then sensitive narrow-field searches could be a powerful complementary probe to traditional wide-field transient detection in the identification of samples of gamma-ray-burst-like events into the epoch of reionization.