Extreme Variability and Episodic Lifetime of Quasars
Extreme Variability and Episodic Lifetime of Quasars
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DOI：
10.3847/1538-4357/ac1ce4
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发表时间：
2021-08
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影响因子：
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通讯作者：
Yue Shen
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文献类型：
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作者：
Yue Shen
我们使用最近关闭的类星体的两个统计数据（即，由大的因子变暗）：（1）在延长的时间段内光度测量地观察到的统计样本中关闭的类星体的分数（例如，Δt = 20 yr）和（2）显示“孤儿”宽Mg ii发射的大质量星系的比例，被认为是最近关闭的类星体的短暂回声。这两个统计数据将幕式类星体的平均寿命限制在数百到数千年。更长（或更短）的事件寿命是强烈不赞成这些意见。这一平均幕寿命与类星体标准吸积盘模型中的下落时间尺度（粘性时间）大致一致，表明类星体的幕是由吸积盘物理学而不是更大尺度上的气体供应所控制的。与类星体聚集和大质量黑洞人口统计限制的106 - 108年的累积寿命相比，我们的结果表明，在超大质量黑洞的组装历史中，有103 - 105个类星体吸积事件。这样的短事件应该聚集在± 104年的间隔内，以解释类星体中电离窄线区域的大小。我们的统计参数还表明，在多年的观测窗口中，总是会有一小部分极端变异的类星体陷入“状态转换”，尽管它们的寿命要长得多。在非稳定吸积期间，这些转变可能以相当突然的方式发生。
We constrain the average episodic quasar lifetime (as in steady-state accretion) using two statistics of quasars that are recently turned off (i.e., dimmed by a large factor): (1) the fraction of turned-off quasars in a statistical sample photometrically observed over an extended period (e.g., Δt = 20 yr) and (2) the fraction of massive galaxies that show “orphan” broad Mg ii emission, argued to be short-lived echoes of recently turned-off quasars. The two statistics constrain the average episodic quasar lifetime to be hundreds to thousands of years. Much longer (or shorter) episodic lifetimes are strongly disfavored by these observations. This average episodic lifetime is broadly consistent with the infall timescale (viscous time) in the standard accretion-disk model for quasars, suggesting that quasar episodes are governed by accretion-disk physics rather than by the gas supply on much larger scales. Compared with the cumulative quasar lifetime of ∼106−108 yr constrained from quasar clustering and massive black hole demographics, our results suggest that there are ∼103−105 episodes of quasar accretion during the assembly history of the supermassive black hole. Such short episodes should be clustered over intervals of ∼104 yr to account for the sizes of ionized narrow-line regions in quasars. Our statistical argument also dictates that there will always be a small fraction of extreme variability quasars caught in “state transitions” over multiyear observing windows, despite the much longer episodic lifetime. These transitions could occur in a rather abrupt fashion during non-steady accretion.