Discovery of the optical and radio counterpart to the fast X-ray transient EP240315a

TL;DR

This paper reports the first detection of optical and radio counterparts to a distant fast X-ray transient, demonstrating the potential of the Einstein Probe for discovering high-redshift, high-energy transients in real time.

Contribution

It presents the first multi-wavelength counterpart identification for an FXT detected by the Einstein Probe, linking X-ray, optical, and radio observations at high redshift.

Findings

Optical transient with redshift z=4.859 identified within FXT localization.

Radio counterpart detected at 3.0 GHz with MeerKAT.

Likely origin from a gamma-ray burst or tidal disruption event.

Abstract

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.