发表机构
Tsinghua University; University of Toronto; Department of Physics, University of Hong Kong; Hong Kong Institute for Astronomy and Astrophysics, University of Hong Kong; Hainan Tropical Ocean University; Chinese Academy of Sciences South America Center for Astronomy (CASSACA), National Astronomical Observatories, CAS; National Astronomical Observatories, Chinese Academy of Sciences（清华大学; 多伦多大学; 香港大学物理系; 香港大学天文与天体物理研究所; 海南热带海洋学院; 中国科学院国家天文台南美天文中心; 中国科学院国家天文台）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究通过多波段观测和模拟，发现EP250304a/SN 2025fhm的晚期光变曲线需中心磁星引擎解释，表明其能量来源为磁星而非仅放射性衰变。
AI 中文摘要
我们展示了EP250304a的X射线、光学和射电后续观测，EP250304a是由爱因斯坦探针发现的一个河外快速X射线瞬变源（EFXT）。其X射线光变曲线在最初约1千秒内表现出两个宽脉冲，峰值流量相当，这一特征在低光度伽马射线暴或EFXT中很少见。光学后续观测使用了韩国微引力透镜望远镜网络、泰国机器人望远镜、拉斯昆布雷斯天文台1米全球网络、双子座南望远镜上的双子座多目标光谱仪以及全球超新星网络。光学数据的快速冷却阶段（3天内）可以很好地由激波茧模型拟合。然而，在超新星阶段（SN 2025fhm，从3天到88天），晚期光变曲线不能仅由放射性$^{56}$Ni衰变解释，正如使用一维拉格朗日辐射流体动力学代码SNEC进行的网格模拟所证明的那样，该模拟揭示了晚期历元存在显著的能量过剩。为了解释这一过剩，需要像快速旋转、高度磁化的中子星这样的中心引擎来提供额外的能量注入。该模型得出的最佳拟合自转周期约为12.60毫秒，磁场强度约为$3.52\times10^{15}$高斯，并且成功解释了晚期热光度光变曲线和早期X射线脉冲结构。我们的结果表明，EP250304a/SN 2025fhm很可能由中心磁星而非仅由放射性衰变提供能量，这为EFXT及其相关超新星的能量预算和物理起源提供了新的见解。
英文摘要
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 $\sim$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 $^{56}$Ni 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 $\sim$12.60~ms and magnetic field strength of $\sim 3.52\times10^{15} \rm G$, 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.
