Galactic nuclei showing recurrent phases of activity and quiescence have recently been discovered. Some have recurrence times as short as a few hours to a day and are known as quasi-periodic X-ray eruption (QPE) sources. Others have recurrence times as long as hundreds to a thousand days and are called repeating nuclear transients. Here we present a multiwavelength overview of Swift J023017.0+283603 (hereafter Swift J0230+28), a source from which repeating and quasi-periodic X-ray flares are emitted from the nucleus of a previously unremarkable galaxy at ∼165 Mpc. It has a recurrence time of approximately 22 days, an intermediary timescale between known repeating nuclear transients and QPE sources. The source also shows transient radio emission, likely associated with the X-ray emission. Such recurrent soft X-ray eruptions, with no accompanying ultraviolet or optical emission, are strikingly similar to QPE sources. However, in addition to having a recurrence time that is ∼25 times longer than the longest-known QPE source, Swift J0230+28’s eruptions exhibit somewhat distinct shapes and temperature evolution compared to the known QPE sources. Scenarios involving extreme mass ratio inspirals are favoured over disk instability models. The source reveals an unexplored timescale for repeating extragalactic transients and highlights the need for a wide-field, time-domain X-ray mission to explore the parameter space of recurring X-ray transients.

最近发现了显示活动和静止循环阶段的星系核。有些的复发时间短至几小时到一天，被称为准周期性 X 射线喷发 (QPE) 源。其他核瞬变的复发时间长达数百至一千天，称为重复核瞬变。在这里，我们介绍了 Swift J023017.0+283603（以下简称 Swift J0230+28）的多波长概述，该源从一个以前不起眼的星系的核心以∼ 165 Mpc发射重复的准周期 X 射线耀斑。它的复发时间约为 22 天，是已知重复核瞬变和 QPE 源之间的中间时间尺度。该源还显示出瞬态无线电发射，可能与 X 射线发射有关。这种反复出现的软 X 射线喷发不伴随紫外线或光学发射，与 QPE 源惊人地相似。然而，除了比已知最长的 QPE 源长约 25 倍的重现时间外，Swift J0230+28 的喷发还表现出与已知 QPE 源相比有些不同的形状和温度演变。涉及极端质量比螺旋的场景比盘不稳定性模型更受青睐。该消息来源揭示了重复河外瞬变的未探索的时间尺度，并强调需要进行宽视场、时域 X 射线任务来探索重复出现的 X 射线瞬变的参数空间。