Title: The day-long, repeating GRB 250702BDE / EP250702a: A unique extragalactic transient Show Chinese title

Title: 全天重复的伽马射线暴 250702BDE / EP250702a：一个独特的河外瞬变源

Abstract: Gamma-ray bursts (GRBs) are singular outbursts of high-energy radiation with durations typically lasting from milliseconds to minutes and, in extreme cases, a few hours. They are attributed to the catastrophic outcomes of stellar-scale events and, as such, are not expected to recur. Here, we present observations of an exceptional GRB\,250702BDE which triggered the {\em Fermi} gamma-ray burst monitor on three occasions over several hours, and which was detected in soft X-rays by the \textit{Einstein Probe} a day before the $\gamma$-ray triggers (EP250702a). We present the discovery of an extremely red infrared counterpart of the event with the VLT, as well as radio observations from MeerKAT. Hubble Space Telescope observations pinpoint the source to a non-nuclear location in a host galaxy with complex morphology, implying GRB 250702BDE is an extragalactic event. The multi-wavelength counterpart is well described with standard afterglow models at a relatively low redshift $z \sim 0.2$, but the prompt emission does not readily fit within the expectations for either collapsar or merger-driven GRBs. Indeed, a striking feature of the multiple prompt outbursts is that the third occurs at an integer multiple of the interval between the first two. Although not conclusive, this could be indicative of periodicity in the progenitor system. We discuss several possible scenarios to explain the exceptional properties of the burst, which suggest that either a very unusual collapsar or the tidal disruption of a white dwarf by an intermediate-mass black hole are plausible explanations for this unprecedented GRB. Show Chinese abstract

Abstract: 伽马射线暴（GRBs）是高能辐射的特殊爆发，持续时间通常从毫秒到分钟，在极端情况下可达几小时。 它们归因于恒星尺度事件的灾难性结果，因此预计不会重复发生。 在此，我们介绍了对一个异常的GRB 250702BDE的观测，该事件在数小时内触发了{\em 费米}伽马射线暴监测器三次，并在$\gamma$射线触发（EP250702a）前一天被\textit{爱因斯坦探针}检测到软X射线。 我们使用VLT发现了该事件极其红色的红外对应体，以及来自MeerKAT的射电观测。 哈勃空间望远镜的观测将源定位到宿主星系中一个非核位置，该星系具有复杂的形态，表明GRB 250702BDE是一个河外事件。 多波长对应体在相对较低的红移$z \sim 0.2$下可以用标准余辉模型很好地描述，但瞬时辐射并不容易符合塌缩星或合并驱动的GRBs的预期。 事实上，多次瞬时爆发的一个显著特点是第三次爆发发生在前两次间隔的整数倍。 尽管尚不明确，这可能表明原始系统中存在周期性。 我们讨论了几种可能的情景来解释该爆发的异常特性，这些情景表明，要么是一个非常不寻常的塌缩星，要么是中等质量黑洞对白矮星的潮汐撕裂，可能是这一前所未有的GRB的合理解释。