SN 2025fhm: A central-engine powered Ic-BL supernova associated with X-ray transient EP250304a
Abstract
We present X-ray, optical, and radio follow-up observations of EP250304a, an extragalactic fast X-ray transient (EFXT) discovered by the Einstein Probe. Its X-ray light curve exhibits two broad pulses with comparable peak fluxes within the first ∼1~ks, a feature rarely seen among low-luminosity gamma-ray bursts or EFXTs. Optical follow-up observations were carried out using the Korea Microlensing Telescope Network, the Thai Robotic Telescope, the Las Cumbres Observatory 1~m global network, the Gemini Multi-Object Spectrograph on Gemini south telescope, and the Global Supernova Network. The fast-cooling phase (within 3 days) of optical data can be well fitted by a shocked cocoon model. However, during the supernova phase (SN 2025fhm, from 3 to 88 days), the late-time light curve cannot be explained solely by radioactive 56Ni decay, as demonstrated by a grid of simulations using the one-dimensional Lagrangian radiation hydrodynamics code SNEC, which reveals a significant energy excess at late epochs. To account for this excess, a central engine like a rapidly spinning, highly magnetized neutron star is needed to provide additional energy injection. This model yields a best-fit spin period of ∼12.60~ms and magnetic field strength of ∼ 3.52×1015 G, and it successfully explains both the late-time bolometric light curve and the early X-ray pulse structures. Our results indicate that EP250304a/SN 2025fhm is likely powered by a central magnetar rather than by radioactive decay alone, offering new insights into the energy budget and physical origin of EFXTs and their associated supernovae.
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BibTeX
@misc{song20262025fhm,
title = {{SN 2025fhm: A central-engine powered Ic-BL supernova associated with X-ray transient EP250304a}},
author = {Cui-Ying Song and Nan Jiang and Xiaofeng Wang and Yi-Han Iris Yin and Lingzhi Wang and Wenxiong Li and Shengyu Yan and Dae-Sik Moon and Tao An and Aleksandar Cikota and Samaporn Tinyanont and Liang-Duan Liu and Cui-Yuan Dai and Christopher D. Matzner and Bin-Bin Zhang and Lixin Yu and Qinyu Wu and Hong Soo Park and Sang Chul Kim and Youngdae Lee and Yu-Hao Zhang and Haowei Peng and Franz E. Bauer and Joseph R. Farah and Moira Andrews and Kathryn Wynn and Yuan Qi Ni and D. Andrew Howell and Curtis McCully and Ning-Chen Sun and Danfeng Xiang and Yuan Liu and Wenxin Wang and Yijia Zhang and Wei Chen},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.08356},
eprint = {2609.08356},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.08356},
}
RIS
TY - UNPB
TI - SN 2025fhm: A central-engine powered Ic-BL supernova associated with X-ray transient EP250304a
AU - Cui-Ying Song
AU - Nan Jiang
AU - Xiaofeng Wang
AU - Yi-Han Iris Yin
AU - Lingzhi Wang
AU - Wenxiong Li
AU - Shengyu Yan
AU - Dae-Sik Moon
AU - Tao An
AU - Aleksandar Cikota
AU - Samaporn Tinyanont
AU - Liang-Duan Liu
AU - Cui-Yuan Dai
AU - Christopher D. Matzner
AU - Bin-Bin Zhang
AU - Lixin Yu
AU - Qinyu Wu
AU - Hong Soo Park
AU - Sang Chul Kim
AU - Youngdae Lee
AU - Yu-Hao Zhang
AU - Haowei Peng
AU - Franz E. Bauer
AU - Joseph R. Farah
AU - Moira Andrews
AU - Kathryn Wynn
AU - Yuan Qi Ni
AU - D. Andrew Howell
AU - Curtis McCully
AU - Ning-Chen Sun
AU - Danfeng Xiang
AU - Yuan Liu
AU - Wenxin Wang
AU - Yijia Zhang
AU - Wei Chen
PY - 2026
DA - 2026/09/08
PB - arXiv
DO - 10.48550/arxiv.2609.08356
UR - https://arxiv.org/abs/2609.08356
AB - We present X-ray, optical, and radio follow-up observations of EP250304a, an extragalactic fast X-ray transient (EFXT) discovered by the Einstein Probe. Its X-ray light curve exhibits two broad pulses with comparable peak fluxes within the first ∼1~ks, a feature rarely seen among low-luminosity gamma-ray bursts or EFXTs. Optical follow-up observations were carried out using the Korea Microlensing Telescope Network, the Thai Robotic Telescope, the Las Cumbres Observatory 1~m global network, the Gemini Multi-Object Spectrograph on Gemini south telescope, and the Global Supernova Network. The fast-cooling phase (within 3 days) of optical data can be well fitted by a shocked cocoon model. However, during the supernova phase (SN 2025fhm, from 3 to 88 days), the late-time light curve cannot be explained solely by radioactive ^56Ni decay, as demonstrated by a grid of simulations using the one-dimensional Lagrangian radiation hydrodynamics code SNEC, which reveals a significant energy excess at late epochs. To account for this excess, a central engine like a rapidly spinning, highly magnetized neutron star is needed to provide additional energy injection. This model yields a best-fit spin period of ∼12.60~ms and magnetic field strength of ∼ 3.52×10^15 G, and it successfully explains both the late-time bolometric light curve and the early X-ray pulse structures. Our results indicate that EP250304a/SN 2025fhm is likely powered by a central magnetar rather than by radioactive decay alone, offering new insights into the energy budget and physical origin of EFXTs and their associated supernovae.
ER -