EP250108a/SN2025kg: A Magnetar-powered Gamma-Ray Burst Supernova Originating from a Close Helium-star Binary via Isolated Binary Evolution

TL;DR

This paper presents a magnetar-powered interpretation for SN 2025kg associated with EP250108a, invoking a jet–cocoon system that yields distinct cooling emissions from an outer cocoon (pre-SN bump) and an inner cocoon (prompt X-ray/early optical behavior) viewed off-axis. The main SN peak is explained by magnetar spin-down energy injection, with $P_i \sim 1.7$ ms and $B_p \sim 2\times10^{15}$ G, in ejecta of $M_{ej} \sim 2.5\,M_\odot$, while a low $^{56}$Ni fraction cannot fully account for the luminosity, indicating the magnetar's major role. A close helium-star binary origin is proposed, with a pre-SN orbital period of about $2$ days and a donor MS companion that can evaporate hydrogen-rich material, producing the late-time broad H$\alpha$ line; the high metallicity challenges quasi-chemically homogeneous evolution, favoring isolated binary evolution. The inferred weak wind ($A_{\star} \lesssim 10^{-2}$) supports a compact, low-wind helium-star progenitor, consistent with the estimated radius $r_{\star} \sim 5\,R_\odot$ and the observed spectroscopic features. Overall, the work links cocoon dynamics, magnetar energetics, and binary evolution to explain the unique observational signatures of EP250108a/SN 2025kg and highlights the role of isolated binary channels in forming rapidly rotating magnetars at solar metallicity.

Abstract

SN\,2025kg, linked to EP250108a, is among the brightest broad-lined Type Ic supernova (SN Ic-BL) known, showing unique helium absorptions, a late-time broad H$α$, and an early bump. In this {\em{Letter}}, we propose a jet-cocoon origin to explain EP250108a as off-axis cooling emission from a mildly relativistic inner cocoon viewed at $\sim45^\circ$ and the early bump of SN\,2025kg as the outer cocoon cooling emission, both constraining an energy of $\sim(1-2)\times10^{52}{\rm{erg}}$ and a progenitor radius of $\sim5\,R_\odot$. To explain SN\,2025kg's exceptionally luminous peak, potential energy injection into the $\sim2.5\,M_\odot$ ejecta from a magnetar with initial period $\sim1.7\,{\rm{ms}}$ and magnetic field $\sim2\times10^{15}{\rm{G}}$ may be required, implying a rapidly rotating $\sim4\,M_\odot$ progenitor. Thus, the progenitor may be a low-mass helium star with an extended helium envelope, supported by helium absorption lines and an inferred weak pre-SN wind. Hydrogen-rich material may reside in the inner ejecta layers, as suggested by the late-time broad H$α$, possibly originating from main-sequence companion material evaporated by the magnetar wind. Since the observed near-solar metallicity challenges the popular quasi-chemically homogeneous evolution channel, the rapidly rotating helium-star progenitor of EP250108a/SN\,2025kg might attain angular momentum by being tidally spun up by a main-sequence companion in a close binary formed through isolated binary evolution.

Figures (5)

• Figure 1: Multi-band lightcurve fits for the data of SN 2025kg Li2025. The solid lines represent the best fits for the multi-band lightcurves. The contributions from the outer cooling emission are marked by the dashed lines, while the dotted and dashed-dotted lines represent the SN emission input from the magnetar and $^{56}$Ni, respectively. The colors corresponding to different bands are shown in the top-right legend. Note that the subscript "M" denotes the Mephisto filter system, which differs from the Sloan $griz$ system.
• Figure 2: 1 keV (blue), $r$-band (green), and $9\,{\rm GHz}$ (gray) simulated afterglow lightcurves. The solid, dotted, and dashed lines represent $A_\star=10^{-1}$, $10^{-2}$, and $10^{-3}$, respectively. The data are collected from Li2025.
• Figure 3: Schematic illustration of the jet structure before the breakout and the resulting emission components after the breakout.
• Figure 4: Posterior distributions of fits to SN 2025kg. The medians and 1$\sigma$ ranges are labeled.
• Figure 5: Comparison of observed and simulated bolometric lightcurve (top panel), effective temperature (middle panel), and photospheric radius (bottom panel) for SN 2025kg. The purple points and blue solid lines represent the observed and simulated combined bolometric characteristics of SN 2025kg. In the top panel, the contributions from the outer cocoon cooling emission and SN emission are marked as orange and green dashed lines, respectively. In the SN emission, the inputs from the magnetar and $^{56}$Ni are represented by the green dotted and dashed-dotted lines.