Unveiling the nature of the Einstein Probe transient EP241021a

Quirola-Vásquez, J, Jonker, PG, Levan, AJ, Malesani, DB, Bauer, FE, Sarin, N, Lamb, GP ORCID: 0000-0001-5169-4143, Martin-Carrillo, A, Sánchez-Sierras, J, Fraser, M, Izzo, L, Ravasio, ME, Mata Sánchez, D, Torres, MAP, Van Dalen, JND, Van Hoof, APC, Chacón, JA, Littlefair, S, Dhillon, VS, Cotter, L et al (2025) Unveiling the nature of the Einstein Probe transient EP241021a. Monthly Notices of the Royal Astronomical Society, 545 (4). ISSN 0035-8711

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Abstract

We present a multiwavelength analysis of the fast X-ray transient EP241021a, discovered by the Wide-field X-ray Telescope aboard the Einstein Probe satellite on 2024 October 21. The event was not detected in gamma-rays. Follow-up observations from 1.5 to 100 d post-trigger were obtained across X-ray, ultraviolet, 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 d 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 an SN contribution at late times, suggesting a collapsar origin for EP241021a. The likely SN in EP241021a appears to require an additional energy source beyond Ni decay. These results support the view that some fast X-ray transients detected by the Einstein Probe arise from massive stellar explosions.

Item Type:	Article
Uncontrolled Keywords:	X-rays: bursts; gamma-ray bursts; transients: supernovae; 5101 Astronomical Sciences; 51 Physical Sciences; 7 Affordable and Clean Energy; 5101 Astronomical Sciences; 51 Physical Sciences; 7 Affordable and Clean Energy; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	Oxford University Press
Date of acceptance:	15 November 2025
Date of first compliant Open Access:	10 September 2026
Date Deposited:	10 Sep 2026 13:42
Last Modified:	10 Sep 2026 13:42
DOI or ID number:	10.1093/mnras/staf2064
URI:	https://researchonline.ljmu.ac.uk/id/eprint/29380

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