EP250827b/SN 2025wkm: An X-Ray Flash-supernova Powered by a Central Engine and Circumstellar Interaction

Srinivasaragavan, GP, Li, D, Hall, XJ, Gottlieb, O, Schroeder, G, Liu, H, O’Connor, B, Jin, C, Kasliwal, M, Ahumada, T, Wu, Q, Fryer, CL, Niblett, AE, Xu, D, Ravasio, ME, Daja, G, Li, W, Anand, S, Ho, AYQ, Sun, H et al (2026) EP250827b/SN 2025wkm: An X-Ray Flash-supernova Powered by a Central Engine and Circumstellar Interaction. Astrophysical Journal Letters, 1006 (2). ISSN 2041-8205

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Abstract

We present the discovery of EP250827b/SN 2025wkm, an X-ray flash (XRF) discovered by the Einstein Probe (EP), accompanied by a broad-line Type Ic supernova (SN Ic-BL) at z = 0.1194. EP250827b possesses a prompt X-ray luminosity of ∼1045 erg s−1, lasts over 1000 s, and has a peak energy Ep < 1.5 keV at 90% confidence. SN 2025wkm possesses a double-peaked optical light curve (LC), though its bolometric luminosity plateaus after its initial peak for ∼20 days, consistent with a central engine injecting additional energy into the explosion. Its spectrum transitions from a blue to red continuum with clear blueshifted broad absorption features consistent with a SN Ic-BL classification. We do not detect any transient radio emission and rule out the existence of an on-axis, energetic jet ≳1050 erg assuming a typical LGRB circumburst constant density (n ≈ 10−3–10−1 cm−3) and microphysical parameters (epsilone = 0.1 and epsilonB = 0.01). In the model we invoke, the collapse gives rise to a long-lived magnetar, potentially surrounded by an accretion disk. Magnetically driven winds from the magnetar and the disk mix together and break out with a velocity ∼0.35c and interact with an extended circumstellar medium with radius ∼1013 cm, generating X-ray breakout emission through nonthermal free–free processes. The disk outflows and magnetar winds power blackbody photospheric emission as they cool adiabatically and thermalize, producing the first SN peak. The spin-down luminosity of the magnetar and radioactive decay of 56Ni power the late-time emission. We end by discussing the landscape of XRF-SNe within the context of EP’s recent discoveries.

Item Type:	Article
Uncontrolled Keywords:	5101 Astronomical Sciences; 51 Physical Sciences; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5109 Space sciences
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	IOP Publishing
Date of acceptance:	5 May 2026
Date of first compliant Open Access:	22 September 2026
Date Deposited:	22 Sep 2026 15:02
Last Modified:	22 Sep 2026 15:02
DOI or ID number:	10.3847/2041-8213/ae7a68
URI:	https://researchonline.ljmu.ac.uk/id/eprint/29512

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