Hydrodynamical modeling of SN 2025kg associated with the Fast X-ray Transient EP250108a

Abstract: Supernovae (SNe) associated with X-Ray Flashes (XRFs) are extremely rare. Therefore, the discovery of each new object in this class offers a unique opportunity to improve our understanding about 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 found that low ejecta masses (Mej ~ 2 Msun) and moderate explosion energies (E ~ 2e51 erg) are required to reproduce the data. Our models also show that a large amount of nickel (M_Ni = 0.85 Msun) 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, approximately, and a magnetic field of 28e14 G gives 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 Msun and an extension of 500 Rsun. A comparison and modeling of a select group of SNe--SN 2006aj, SN 2020bvc and SN 2023pel--is also presented. A remarkable similarity emerges between SN 2025kg and SN 2023pel. As SN 2023pel was recently proposed to be powered by a magnetar, this further supports the magnetar scenario for SN 2025kg.