EP250108a/SN 2025kg: A Jet-driven Stellar Explosion Interacting with Circumstellar Material
Gokul P. Srinivasaragavan, Hamid Hamidani, Genevieve Schroeder, Nikhil Sarin, Anna Y. Q. Ho, Anthony L. Piro, S. B. Cenko, Shreya Anand, J. Sollerman, D. A. Perley, Keiichi Maeda, Brendan O’Connor, H. Kuncarayakti, M. Coleman Miller, Tomás Ahumada, Jada L. Vail, Paul C. Duffell, Ranadeep G. Dastidar, Igor Andreoni, Aleksandra Bochenek, S. Brennan, Jonathan Carney, Ping Chen, James Freeburn, A. Gal‐Yam, W. V. Jacobson-Galán, M. M. Kasliwal, Jiaxuan Li, Maggie L. Li, Niharika Sravan, Daniel E. Warshofsky
Abstract
Abstract We present optical, radio, and X-ray observations of EP250108a/SN 2025kg, a broad-line Type Ic supernova (SN Ic-BL) accompanying an Einstein Probe (EP) fast X-ray transient at z = 0.176. EP250108a/SN 2025kg possesses a double-peaked optical light curve, and its spectrum transitions from a blue underlying continuum to a typical SN Ic-BL spectrum over time. We fit a radioactive decay model to the second peak of the optical light curve and find SN parameters that are consistent with the SN Ic-BL population, while its X-ray and radio properties are consistent with those of low-luminosity GRB (LLGRB) 060218/SN 2006aj. We explore three scenarios to understand the system’s multiwavelength emission: (a) SN ejecta interacting with an extended circumstellar medium (CSM), (b) the shocked cocoon of a collapsar-driven jet choked in its stellar envelope, and (c) the shocked cocoon of a collapsar-driven jet choked in an extended CSM. Models (b) and (c) can explain the optical light curve and are also consistent with the radio and X-ray observations. We favor model (c) because it can self-consistently explain both the X-ray prompt emission and first optical peak, but we do not rule out model (b). From the properties of the first peak in model (c), we find evidence that EP250108a/SN 2025kg interacts with an extended CSM and infer an envelope mass M e ∼ 0.1 M ⊙ and radius R e ∼ 4 × 10 13 cm. EP250108a/SN 2025kg’s multiwavelength properties make it a close analog to LLGRB 060218/SN 2006aj and highlight the power of early follow-up observations in mapping the environments of massive stars prior to core collapse.