  Fast X-ray Transients (FXTs) are extragalactic bursts of soft X-rays first
identified >10 years ago. Since then, nearly 40 events have been discovered,
although almost all of these have been recovered from archival Chandra and
XMM-Newton data. To date, optical sky surveys and follow-up searches have not
revealed any multi-wavelength counterparts. The Einstein Probe, launched in
January 2024, has started surveying the sky in the soft X-ray regime (0.5-4
keV) and will rapidly increase the sample of FXTs discovered in real time.
Here, we report the first discovery of both an optical and radio counterpart to
a distant FXT, the fourth source publicly released by the Einstein Probe. We
discovered a fast-fading optical transient within the 3 arcmin localisation
radius of EP240315a with the all-sky optical survey ATLAS, and our follow-up
Gemini spectrum provides a redshift, z=4.859+/-0.002. Furthermore, we uncovered
a radio counterpart in the S-band (3.0 GHz) with the MeerKAT radio
interferometer. The optical (rest-frame UV) and radio luminosities indicate the
FXT most likely originates from either a long gamma-ray burst or a relativistic
tidal disruption event. This may be a fortuitous early mission detection by the
Einstein Probe or may signpost a mode of discovery for high-redshift,
high-energy transients through soft X-ray surveys, combined with locating
multi-wavelength counterparts.
