A Sample Bias in Quasar Variability Studies
A Sample Bias in Quasar Variability Studies
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DOI：
10.3847/2041-8213/ac1e2e
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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;C. J. Burke
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文献类型：
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作者：
Yue Shen;C. J. Burke
当后来观测到一个通量有限的类星体样本时，由于样本选择时产生的选择偏差，暗淡的类星体会比明亮的类星体多。类星体是不断变化的，暗淡的类星体比明亮的类星体多。在选择时，即使是对称变异性也会导致瞬时通量高于通量极限的类星体多于低于通量极限的类星体，从而导致通量随时间变化的不对称性。同样的偏置会导致样品的系综结构函数（SF）的不对称，使得基于通量增加的对的SF略小于基于通量减少的对的SF。我们使用基于阻尼随机游走处方的模拟时间对称类星体光曲线来说明这种偏差的影响。这种偏差的程度取决于样本、幅度变化的阈值和光曲线的覆盖范围，但一般行为是一致的。特别是，与最近的观测研究相匹配的模拟与十年的光曲线在几个百分点的水平上产生了SF测量的不对称性，与观测值相似。这些结果为最近一些类星体变异性研究中报道的时间不对称性提供了一个警告。
When a flux-limited quasar sample is observed at later times, there will be more dimmed quasars than brightened ones, due to a selection bias induced at the time of sample selection. Quasars are continuously varying and there are more fainter quasars than brighter ones. At the time of selection, even symmetrical variability will result in more quasars with their instantaneous fluxes scattered above the flux limit than those scattered below, leading to an asymmetry in flux changes over time. The same bias would lead to an asymmetry in the ensemble structure function (SF) of the sample such that the SF based on pairs with increasing fluxes will be slightly smaller than that based on pairs with decreasing fluxes. We use simulated time-symmetric quasar light curves based on the damped random walk prescription to illustrate the effects of this bias. The level of this bias depends on the sample, the threshold of magnitude changes, and the coverage of light curves, but the general behaviors are consistent. In particular, the simulations matched to recent observational studies with decade-long light curves produce an asymmetry in the SF measurements at the few percent level, similar to the observed values. These results provide a cautionary note on the reported time asymmetry in some recent quasar variability studies.