AT 2020afjz（TSS2020a）：TESS发现的首个河外快速暂现源
AT 2020afjz (TSS2020a): The First Fast Extragalactic Transient Discovered by TESS
AI总结：
本研究发现了TESS首个小时级河外快速暂现源AT 2020afjz，经建模其或为轴上“脏火球”或轴外孤儿余烬，展现了TESS发现此类暂现源的潜力。
AI中文摘要：
我们报告AT 2020afjz（TSS2020a）的发现：这是首个在光学波段发现的时标为小时级的河外暂现源，其从爆发起始到衰减的完整演化过程在时间上被完整解析，也是首个由TESS发现的此类暂现源。AT 2020afjz是在TESSELLATE巡天框架下开展的HiLaTS预研项目中被识别的，其显著性大于10σ，该项目通过TESSELLATE pipeline盲搜TESS数据中的暂现现象。通过与 legacy成像数据交叉匹配，我们将其与DES J042144.37$-$383311.3相关联，该天体是一个相互作用星系对的成员，其测光红移为$z_{\rm phot}=0.67^{+0.07}_{-0.10}$。AT 2020afjz的持续时间和亮度与伽马射线暴（GRB）余烬类似，但它的上升时标约为1小时，在半峰值亮度以上仅持续2.4小时；使用VegasAfterglow对TESS光变曲线建模发现，它最适合被归类为轴上“脏火球”或轴外孤儿余烬。这两种罕见分类均依赖于伽马射线能量的非探测，但由于费米伽马射线暴监测器（Fermi-GBM）在爆发时处于地球掩蔽状态，无法明确确认AT 2020afjz的伽马射线宁静特性。无论如何，AT 2020afjz证明了TESS发现河外快速暂现源的能力，并预示着将有新的暂现源群有待通过TESSELLATE项目发现。
英文摘要：
We report the discovery of AT 2020afjz (TSS2020a): the first hour-scale extragalactic transient discovered in optical wavelengths whose complete evolution -- from explosion onset to decay -- is temporally resolved, and the first such transient discovered by TESS. AT 2020afjz was identified as a $>10σ$ detection in the pilot HiLaTS program run within the TESSELLATE Sky Survey, which blindly searches for transient phenomena in TESS data with the TESSELLATE pipeline. Through cross-matching with legacy imaging, we associate it with DES J042144.37$-$383311.3, a member of an interacting galaxy pair at $z_{\rm phot}=0.67^{+0.07}_{-0.10}$. While AT 2020afjz is similar in duration and brightness to GRB afterglows, it exhibits a slow rise time of $\sim1$ hr and lasts for only 2.4 hr above the half-max brightness; modeling the TESS light curve with VegasAfterglow finds that it is best described as either an on-axis "dirty-fireball" or off-axis orphan afterglow. Each of these rare classifications hinge upon a non-detection at gamma-ray energies, but as Fermi-GBM was Earth-occulted at the time of explosion, AT 2020afjz's gamma-quiet nature cannot be definitively confirmed. Regardless, AT 2020afjz demonstrates TESS's power to discover fast extragalactic transients, and heralds a new population awaiting discovery with TESSELLATE.
