Letter to the Editor

Hydrodynamical modeling of SN 2025kg associated with the fast X-ray transient EP250108a

¹ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque S/N B1900FWA, La Plata, Argentina
² Instituto de Astrofísica de La Plata, CONICET-UNLP, Paseo del Bosque S/N B1900FWA, La Plata, Argentina
³ Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan

^⋆ Corresponding author: michelleroman2305@gmail.com

Received: 26 July 2025
Accepted: 6 October 2025

Abstract

Supernovae (SNe) associated with X-Ray flashes (XRFs) are extremely rare and the discovery of each new object in this class offers a unique opportunity to improve our understanding of their origins and potential connection with other high-energy phenomena. SN 2025kg is one of the most recent events discovered in this category and exhibits a double-peaked light curve, with an initial cooling phase followed by the main peak. Here, we investigate the possible mechanisms powering its bolometric light curve and expansion velocities, using numerical calculations to simulate the explosion. We find that low ejecta masses (M_ej ∼ 2 M_⊙) and moderate explosion energies (E ∼2 × 10⁵¹ erg) are required to reproduce the data. Our models also show that a large amount of nickel (MNi ∼ 0.85 M_⊙) is needed to achieve the high luminosity of SN 2025kg, which makes this scenario difficult to sustain. As an alternative, we explore a model in which a millisecond magnetar serves as the primary energy source. A magnetar with a spin period of ∼3 ms and a magnetic field of 28 × 10¹⁴ G provides an adequate match to the data. To account for the early cooling phase, we assume the presence of a dense circumstellar material surrounding the progenitor, with a mass of 0.27 M_⊙ and an extension of 500 R_⊙. A comparison and modeling of a selected group of SNe (SN 2006aj, SN 2020bvc, and SN 2023pel) is also presented in this work. We note a remarkable similarity emerging between SN 2025kg and SN 2023pel. Since SN 2023pel was recently proposed to be powered by a magnetar, this further supports the magnetar scenario for SN 2025kg.