Discovery of the Optical and Radio Counterpart to the Fast X-Ray Transient EP 240315a
The Astrophysical Journal Letters American Astronomical Society 969:1 (2024) L14
Authors:
JH Gillanders, L Rhodes, S Srivastav, F Carotenuto, J Bright, ME Huber, HF Stevance, SJ Smartt, KC Chambers, T-W Chen, R Fender, A Andersson, AJ Cooper, PG Jonker, FJ Cowie, T de Boer, N Erasmus, MD Fulton, H Gao, J Herman, C-C Lin, T Lowe, EA Magnier, H-Y Miao
Abstract:
Fast X-ray Transients (FXTs) are extragalactic bursts of soft X-rays first identified ≳10 yr 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 multiwavelength counterparts. The Einstein Probe, launched in 2024 January, 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′ localization radius of EP 240315a 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 that 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 multiwavelength counterparts.
