Discovery of the Optical and Radio Counterpart to the Fast X-Ray Transient EP 240315a

• Published in: Astrophysical journal. Letters Vol. 969; no. 1; p. L14
• Main Authors: Gillanders, J. H., Rhodes, L., Srivastav, S., Carotenuto, F., Bright, J., Huber, M. E., Stevance, H. F., Smartt, S. J., Chambers, K. C., Chen, T.-W., Fender, R., Andersson, A., Cooper, A. J., Jonker, P. G., Cowie, F. J., de Boer, T., Erasmus, N., Fulton, M. D., Gao, H., Herman, J., Lin, C.-C., Lowe, T., Magnier, E. A., Miao, H.-Y., Minguez, P., Moore, T., Ngeow, C.-C., Nicholl, M., Pan, Y.-C., Pignata, G., Rest, A., Sheng, X., Smith, I. A., Smith, K. W., Tonry, J. L., Wainscoat, R. J., Weston, J., Yang, S., Young, D. R.
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01.07.2024; IOP Publishing
• Subjects: Gamma ray bursts; Gamma rays; High energy astrophysics; Optical identification; Radio interferometers; Radio interferometry; Red shift; Relativistic jets; Sky surveys (astronomy); Soft x rays; Transient sources; Transients (astronomy); X-ray transient sources; X-rays; XMM (spacecraft)
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ad55cd

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.