标题： AT 2019avd：核瞬变体多样群体中的新成员 显示英文标题

标题： AT 2019avd: A novel addition to the diverse population of nuclear transients

摘要： 我们报告了SRG/eROSITA、ZTF、ASAS-SN、Las Cumbres、NEOWISE-R和Swift XRT/UVOT对独特持续事件AT 2019avd的观测，该事件位于一个之前不活跃的星系的核心，距离为$z=0.029$。 eROSITA在2020年4月28日首次进行全天调查时观测到了AT 2019avd，当时它被检测为一个超软X射线源（$kT\sim 85$eV），在$0.2-2$keV波段的亮度是之前3$\sigma$上限通量值的$\gtrsim 90$倍（在此位置没有档案X射线检测）。 ZTF在eROSITA检测前的$\sim 450$天内的光学光变曲线呈双峰，eROSITA的检测与第二峰的上升阶段一致。 后续的光学光谱显示出现了Bowen荧光特征和高电离的冕线([\ion{Fe}{X}] 6375 {\AA }, [\ion{Fe}{XIV}] 5303 {\AA })，同时持续存在宽的巴尔末发射线(FWHM$\sim 1400$ km s$^{-1}$)。尽管X射线特性使AT 2019avd成为一个有希望的潮汐撕裂事件(TDE)候选者，但其光学特性对于光学选择的TDE来说是不寻常的。我们讨论了可能的其他起源，这些起源可以解释AT 2019avd的观测特性，例如一个恒星双星TDE候选者，或者涉及超大质量黑洞双星的TDE。 显示英文摘要

摘要： We report on SRG/eROSITA, ZTF, ASAS-SN, Las Cumbres, NEOWISE-R, and Swift XRT/UVOT observations of the unique ongoing event AT 2019avd, located in the nucleus of a previously inactive galaxy at $z=0.029$. eROSITA first observed AT 2019avd on 2020-04-28 during its first all sky survey, when it was detected as an ultra-soft X-ray source ($kT\sim 85$ eV) that was $\gtrsim 90$ times brighter in the $0.2-2$ keV band than a previous 3$\sigma$ upper flux detection limit (with no archival X-ray detection at this position). The ZTF optical light curve in the $\sim 450$ days preceding the eROSITA detection is double peaked, and the eROSITA detection coincides with the rise of the second peak. Follow-up optical spectroscopy shows the emergence of a Bowen fluorescence feature and high-ionisation coronal lines ([\ion{Fe}{X}] 6375 {\AA}, [\ion{Fe}{XIV}] 5303 {\AA}), along with persistent broad Balmer emission lines (FWHM$\sim 1400$ km s$^{-1}$). Whilst the X-ray properties make AT 2019avd a promising tidal disruption event (TDE) candidate, the optical properties are atypical for optically selected TDEs. We discuss potential alternative origins that could explain the observed properties of AT 2019avd, such as a stellar binary TDE candidate, or a TDE involving a super massive black hole binary.