The interband lags among the optical broad-band continua of active galactic nuclei (AGNs) have been intensively explored over the past decade. However, the nature of the lags remains under debate. Here, utilizing two distinct scenarios for AGN variability, i.e., the thermal fluctuation of accretion disk and the reprocessing of both the accretion disk and clouds in the broad line region, we show that, owing to the random nature of AGN variability, the interband lags of an individual AGN would vary from one campaign with a finite baseline to another. Specifically, the thermal fluctuation scenario implies larger variations in the lags than the reprocessing scenario. Moreover, the former predicts a positive correlation between the lag and variation amplitude, while the latter does not result in such a correlation. For both scenarios, averaging the lags of an individual AGN measured with repeated and nonoverlapping campaigns would give rise to a stable lag, which is larger for a longer baseline and gets to saturation for a sufficiently long baseline. However, obtaining the stable lag for an individual AGN is very time-consuming. Alternatively, it can be equivalently inferred by averaging the lags of a sample of AGNs with similar physical properties, and thus can be properly compared with predictions of AGN models. In addition, several new observational tests suggested by our simulations are discussed, as well as the role of the deep high-cadence surveys of the Wide Field Survey Telescope in enriching our knowledge of the lags.

中文翻译：

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活动星系核的带间滞后会随机变化吗？


在过去的十年中，人们深入探索了活动星系核 （AGN） 的光学宽带连续体中的带间滞后。然而，滞后的性质仍存在争议。在这里，利用 AGN 变率的两种不同情景，即吸积盘的热波动以及吸积盘和云在宽线区域中的再处理，我们表明，由于 AGN 变率的随机性，单个 AGN 的带间滞后会因具有有限基线的活动而异。具体来说，热波动情景意味着滞后的变化比再处理情景更大。此外，前者预测滞后和变化幅度之间存在正相关，而后者不会导致这种相关性。对于这两种情况，对使用重复和非重叠活动测量的单个 AGN 的滞后进行平均将产生稳定的滞后，对于较长的基线，该滞后会更大，而对于足够长的基线，该滞后会达到饱和。但是，获得单个 AGN 的稳定滞后非常耗时。或者，可以通过平均具有相似物理性质的 AGNs 样本的滞后来等效推断，因此可以与 AGN 模型的预测进行适当比较。此外，还讨论了我们的模拟提出的几项新的观测测试，以及宽视场巡天望远镜的深高节奏巡天在丰富我们对滞后的认识方面的作用。