EP 250108a/SN 2025kg: Observations of the Most Nearby Broad-line Type Ic Supernova Following an Einstein Probe Fast X-Ray Transient
Jillian Rastinejad, A. J. Levan, P. G. Jonker, C. D. Kilpatrick, Chris L. Fryer, Nikhil Sarin, B. P. Gompertz, Chang Liu, R. A. J. Eyles-Ferris, Wen‐fai Fong, Eric Burns, J. H. Gillanders, Ilya Mandel, D. Malesani, P. T. O’Brien, N. R. Tanvir, K. Ackley, Amar Aryan, F. E. Bauer, S. Bloemen, Thomas de Boer, Clécio R. Bom, J. Chacon, K. C. Chambers, T. W. Chen, A A Chrimes, Joyce N. D. van Dalen, V. D’Elia, M. de Pasquale, M. Fulton, P. Groot, Rahul Gupta, D. H. Hartmann, Agnes P. C. van Hoof, M. E. Huber, L. Izzo, W. V. Jacobson-Galán, P. Jakobsson, A. K. H. Kong, T. Laskar, T. Lowe, E. A. Magnier, E. Maiorano, A. Martín-Carrillo, Lluís Mas-Ribas, D. Mata Sánchez, M. Nicholl, C. J. Nixon, S. R. Oates, Gregory S. H. Paek, J. T. Palmerio, D. Paris, D. L. A. Pieterse, G. Pugliese, J. Quirola-Vásquez, Jan van Roestel, A. Rossi, Alicia Rouco Escorial, R. Salvaterra, Benjamin Schneider, S. J. Smartt, K. Smith, I. A. Smith, Shubham Srivastav, M. A. P. Torres, C Vidal Ventura, P. M. Vreeswijk, R. J. Wainscoat, Yi-Jung Yang, S. Yang
Abstract
Abstract With a small sample of fast X-ray transients (FXTs) with multiwavelength counterparts discovered to date, their progenitors and connections to γ -ray bursts (GRBs) and supernovae (SNe) remain ambiguous. Here, we present photometric and spectroscopic observations of SN 2025kg, the SN counterpart to the FXT EP 250108a. At z = 0.17641, this is the closest known SN discovered following an Einstein Probe (EP) FXT. We show that SN 2025kg’s optical spectra reveal the hallmark features of a broad-lined Type Ic SN. Its light-curve evolution and expansion velocities are comparable to those of GRB-SNe, including SN 1998bw, and two past FXT-SNe. We present JWST/NIRSpec spectroscopy taken around SN 2025kg’s maximum light, and find weak absorption due to He I 1.0830 μ m and 2.0581 μ m and a broad, unidentified emission feature at ∼4–4.5 μ m. Further, we observe broadened H α in optical data at 42.5 days that is not detected at other epochs, indicating interaction with H-rich material. From its light curve, we derive a 56 Ni mass of 0.2–0.6 M ⊙ . Together with our companion Letter, our broadband data are consistent with a trapped or low-energy (≲10 51 erg) jet-driven explosion from a collapsar with a zero-age main-sequence mass of 15–30 M ⊙ . Finally, we show that the sample of EP FXT-SNe supports past estimates that low-luminosity jets seen through FXTs are more common than successful (GRB) jets, and that similar FXT-like signatures are likely present in at least a few percent of the brightest Type Ic-BL SNe.