A Candidate Tidal Disruption Event in a Quasar at z = 2.359 from Abundance Ratio Variability
A Candidate Tidal Disruption Event in a Quasar at z = 2.359 from Abundance Ratio Variability
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DOI：
10.3847/1538-4357/aabb04
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发表时间：
2018-03
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影响因子：
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通讯作者：
Xin Liu;A. Dittmann;Yue Shen;Linhua Jiang
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文献类型：
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作者：
Xin Liu;A. Dittmann;Yue Shen;Linhua Jiang
一小部分类星体在其光谱中显示出异常高的氮碳比（N/C）。这些“富氮”类星体是一个长期存在的谜团，因为它们的星际介质意味着恒星群体具有异常高的金属丰度。最近有人提出，富氮类星体可能是由超大质量黑洞对恒星的潮汐扰动事件（TDE）引起的。在超太阳质量恒星中，由于碳-氮-氧循环，氮的快速增加和碳的快速消耗，自然会产生高的N/C。然而，TDE假说预测N/C应该随时间而变化，这是迄今为止从未观察到的。在这里，我们报告了第一个富氮类星体的发现与快速N/C的变化，可能是由TDE造成的。两个间隔1.7年的光谱（静止坐标系）表明，Niii] λ1750/Ciii] λ1909强度比衰减了1.86% ± 14%（1σ）。光学（静止帧紫外）光变曲线和X射线观测定性与TDE假设一致，但时间基线福尔斯缺乏明确的证据。将单天体发现置于背景下，对1080个已知的富氮类星体的高质量档案光谱的统计分析显示，在>1年的时间尺度上（静止框架），N/C降低的证据（在5σ显著性水平）和恒定的电离水平（由C iii] λ1909/C iv λ1549强度比表示）。如果得到证实，我们的研究结果证明了通过丰度比变化识别类星体中TDE候选者的方法，为即将到来的大规模时域光谱调查打开了一个新的TDE观测窗口。
A small fraction of quasars show an unusually high nitrogen-to-carbon ratio (N/C) in their spectra. These “nitrogen-rich” (N-rich) quasars are a long-standing puzzle because their interstellar medium implies stellar populations with abnormally high metallicities. It has recently been proposed that N-rich quasars may result from tidal disruption events (TDEs) of stars by supermassive black holes. The rapid enhancement of nitrogen and the depletion of carbon due to the carbon–nitrogen–oxygen cycle in supersolar mass stars could naturally produce high N/C. However, the TDE hypothesis predicts that the N/C should change with time, which has never hitherto been observed. Here we report the discovery of the first N-rich quasar with rapid N/C variability that could be caused by a TDE. Two spectra separated by 1.7 years (rest-frame) show that the N iii] λ1750/C iii] λ1909 intensity ratio decayed by ∼86% ± 14% (1σ). Optical (rest-frame UV) light-curve and X-ray observations are qualitatively consistent with the TDE hypothesis; though, the time baseline falls short of a definitive proof. Putting the single-object discovery into context, statistical analyses of the ∼80 known N-rich quasars with high-quality archival spectra show evidence (at a 5σ significance level) of a decrease in N/C on timescales of >1 year (rest-frame) and a constant level of ionization (indicated by the C iii] λ1909/C iv λ1549 intensity ratio). If confirmed, our results demonstrate the method of identifying TDE candidates in quasars via abundance ratio variability, opening a new window of TDE observations at high redshift (z > 2) with upcoming large-scale time-domain spectroscopic surveys.