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arxiv: 2504.08889 · v4 · pith:7CVQRCDE · submitted 2025-04-11 · astro-ph.HE

EP 250108a/SN 2025kg: Observations of the most nearby Broad-Line Type Ic Supernova following an Einstein Probe Fast X-ray Transient

J. C. Rastinejad (Northwestern) , A. J. Levan , P. G. Jonker , C. D. Kilpatrick , C. L. Fryer , N. Sarin , B. P. Gompertz , C. Liu
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R. A. J. Eyles-Ferris W. Fong E. Burns J. H. Gillanders I. Mandel D. B. Malesani P. T. O'Brien N. R. Tanvir K. Ackley A. Aryan F. E. Bauer S. Bloemen T. de Boer C. R. Bom J. A. Chacon K. Chambers T.-W. Chen A. A. Chrimes J. N. D. van Dalen V. D'Elia M. De Pasquale M. D. Fulton P. J. Groot R. Gupta D. H. Hartmann A. P. C. van Hoof M. E. Huber L. Izzo W. Jacobson-Galan P. Jakobsson A. Kong T. Laskar T. B. Lowe E. A. Magnier E. Maiorano A. Martin-Carrillo L. Mas-Ribas D. Mata Sanchez M. Nicholl C. J. Nixon S. R. Oates G. Paek J. Palmerio D. Paris D. L. A. Pieterse G. Pugliese J. A. Quirola Vasquez J. van Roestel A. Rossi A. Rouco Escorial. R. Salvaterra B. Schneider S. J. Smartt K. Smith I. A. Smith S. Srivastav M. A. P. Torres C. Ventura P. Vreeswijk R. Wainscoat Y.-J. Yang S. Yang
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With a small sample of fast X-ray transients (FXTs) with multi-wavelength counterparts discovered to date, the progenitors of FXTs and their connections to gamma-ray bursts (GRBs) and supernovae (SNe) remain ambiguous. Here, we present photometric and spectroscopic observations of SN 2025kg, the supernova 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 also comparable to those of GRB-SNe, including SN 1998bw, and several past FXT SNe. We present JWST/NIRSpec spectroscopy taken around SN 2025kg's maximum light, and find weak absorption due to He I $\lambda 1.0830, \lambda 2.0581$ $\mu$m and a broad, unidentified feature at $\sim$ 4-4.5 $\mu$m. Further, we observe clear evidence for broadened H$\alpha$ in optical data at 42.5 days that is not detected at other epochs, indicating interaction with hydrogen-rich material. From its light curve, we derive a $^{56}$Ni mass of 0.2 - 0.6 $M_{\odot}$. Together with our companion paper (Eyles-Ferris et al. 2025), our broadband data of EP 250108a/SN 2025kg are consistent with a trapped or low energy ($\lesssim 10^{51}$ ergs) jet-driven explosion from a collapsar with a zero-age main sequence mass of 15-30 $M_{\odot}$. Finally, we show that the sample of EP FXT SNe support past rate 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 Ic-BL SNe.

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