明亮X射线点燃事件 eRASSt J234402.9−352640 的发现；I. 潮汐瓦解事件还是活动星系核吸积的快速增强？
Discovery of the luminous X-ray ignition eRASSt J234402.9$-$352640; I. Tidal disruption event or a rapid increase in accretion in an active galactic nucleus?
AI总结：
本文研究新发现的X射线突亮天体eRASSt J234402.9−352640，结合多波段光变与光谱特征，判断其最可能是已关闭AGN中的TDE。
摘要：
2020年11月，在SRG/eROSITA的第二次全天巡天中发现了一个新的明亮天体 eRASSt J234402.9−352640。与六个月前得到的上限相比，该天体在0.2–2.0 keV能段的流量增亮了至少150倍，观测峰值达到1.76_{-0.24}^{+0.03}×10^-11 erg cm^-2 s^-1。这次X射线点燃与一个红移z=0.10的星系相关，使其峰值光度为log10(L_0.2–2keV/[erg s^-1])=44.7±0.1。在X射线流量上升前后，经宿主星系改正后，该星系核在光学测光中增亮约3个星等。我们展示了来自Swift、XMM-Newton和NICER的数据，这些数据揭示出非常软的光谱，以及多个时间尺度上强烈的0.2–2.0 keV流量变化。点燃事件后数周内取得的光学光谱显示出蓝色连续谱，伴有宽而不对称的Balmer发射线，以及高电离（[OIII] λλ4959,5007）和低电离（[NII] λ6585、[SII] λλ6716,6731）窄发射线。在光学光变曲线达到峰值之后，X射线、紫外和光学测光均显示快速衰减。X射线光变曲线显示33天内光度下降约0.45个数量级，紫外则下降约0.35个数量级。eRASSt J234402.9−352640还显示出中红外增亮，这可能由明亮点燃事件产生的尘埃回波驱动。我们在Fermi-LAT γ射线数据中未发现喷流式辐射的证据。该事件同时表现出潮汐瓦解事件（TDE）和活动星系核（AGN）的特征，因此其分类较为复杂。基于X射线光谱的软度、高电离光学发射线的存在以及可能的红外回波，我们认为发生在一个已关闭AGN中的TDE最符合观测结果。
英文摘要：
In November 2020, a new, bright object, eRASSt J234402.9$-$352640, was discovered in the second all-sky survey of SRG/eROSITA. The object brightened by a factor of at least 150 in 0.2--2.0 keV flux compared to an upper limit found six months previous, reaching an observed peak of $1.76_{-0.24}^{+0.03} \times 10^{-11}$ erg cm$^{-2}$ s$^{-1}$. The X-ray ignition is associated with a galaxy at $z=0.10$, making the peak luminosity log$_{10}(L_{\rm 0.2-2keV}/[\textrm{erg s}^{-1}])$=$44.7\pm0.1$. Around the time of the rise in X-ray flux, the nucleus of the galaxy brightened by approximately 3 mag. in optical photometry, after correcting for the host. We present data from Swift, XMM-Newton, and NICER, which reveal a very soft spectrum as well as strong 0.2--2.0 keV flux variability on multiple timescales. Optical spectra taken in the weeks after the ignition event show a blue continuum with broad, asymmetric Balmer emission lines, and high-ionisation ([OIII]$λλ$4959,5007) and low-ionisation ([NII]$λ$6585, [SII]$λλ$6716,6731) narrow emission lines. Following the peak in the optical light curve, the X-ray, UV, and optical photometry all show a rapid decline. The X-ray light curve shows a decrease in luminosity of $\sim$0.45 over 33 days and the UV shows a drop of $\sim$0.35. eRASSt J234402.9$-$352640 also shows a brightening in the mid-infrared, likely powered by a dust echo of the luminous ignition. We find no evidence in Fermi-LAT $γ$-ray data for jet-like emission. The event displays characteristics of a tidal disruption event (TDE) as well as of an active galactic nucleus (AGN), complicating its classification. Based on the softness of the X-ray spectrum, the presence of high-ionisation optical emission lines, and the likely infrared echo, we find that a TDE within a turned-off AGN best matches our observations.
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