EP 250108a/SN 2025kg: a magnetar-powered gamma-ray burst supernova originating from a close helium-star binary via isolated binary evolution

SN 2025kg, linked to EP 250108a, is among the brightest broad-lined Type Ic supernova (SN) known, showing unique helium absorptions, a late-time broad H \, α, and an early bump. In this Letter, we propose a jet–cocoon origin to explain EP 250108a as off-axis cooling emission from a mildly relativistic inner cocoon viewed at ~ 45^∘ and the early bump of SN 2025kg as the outer cocoon cooling emission, both constraining an energy of ~ (1\!-\!2) × 10⁵² \, erg and a progenitor radius of ~ 5\, R_. To explain SN 2025kg’s exceptionally luminous peak, potential energy injection into the ~ 2.5\, M_ ejecta from a magnetar with initial period ~ 1.7\, ms and magnetic field ~ 2× 10¹⁵ \, G may be required, implying a rapidly rotating ~ 4\, M_ 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 EP 250108a/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.