paper

Unveiling the nature of the Einstein Probe transient EP 241021a

arXiv:2511.13314 · doi:10.1093/mnras/staf2064

Abstract

We present a multi-wavelength analysis of the fast X-ray transient EP 241021a, discovered by the Wide-field X-ray Telescope aboard the \emph{Einstein Probe} satellite on 2024 October 21. The event was not detected in gamma-rays. Follow-up observations from 1.5 to 100 days post-trigger were obtained across X-ray, UV, optical, near-infrared, and radio bands with ground- and space-based facilities. The redshift is constrained to from prominent optical spectral features. The optical light curve shows complex evolution: an initial decay, followed by a rapid re-brightening peaking at day 7.7 with decay, and a third phase peaking near day 19 with decay. The spectral energy distribution (SED) and its temporal evolution are consistent with a mix of non-thermal and thermal components. Early optical-to-X-ray spectral indices agree with optically thin synchrotron emission, while steepening of the optical SED after 20 days indicates either a shift in emission mechanism or the emergence of an additional component. Although broad-lined absorption features are absent, comparisons with type Ic-BL supernovae suggest a SN contribution at late times, suggesting a collapsar origin for EP 241021a. The likely SN in EP 241021a appears to require an additional energy source beyond Ni decay. These results support the view that some fast X-ray transients detected by the \emph{Einstein Probe} arise from massive stellar explosions.

arXiv comment21 pages, 15 Figures (+3 in an Appendix), accepted for publication in MNRAS

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