NGC 5548 was recently monitored intensively from NIR to X-rays as part of the STORM campaign. Its disc emission was found to lag behind the observed X-rays, while the measured time lag was increasing with wavelength. These results are consistent with the assumption that short term variability in AGN emission is driven by the X-ray illumination of the accretion disc. In this work, we studied the power spectrum of UV/optical and X-ray emission of NGC 5548, using the data of the STORM campaign as well as previous Swift data, in order to investigate the relation between the UV/optical and X-ray variability and to examine its consistency with the above picture. We demonstrate that even the power spectrum results are compatible with a standard disc being illuminated by X-rays, with low accretion rates, but the details are not entirely consistent with the results from the modelling of the "{\tau} vs {\lambda}" relation. The differences indicate that the inner disc might be covered by a "warm corona" which does not allow the detection of UV/optical emission from the inner disc. Finally, we found strong evidence that the UV emission of NGC 5548 is not stationary.

中文翻译：

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NGC 5548 的多波长功率谱分析

作为 STORM 活动的一部分，NGC 5548 最近受到了从 NIR 到 X 射线的密集监测。发现它的圆盘发射滞后于观察到的 X 射线，而测量的时间滞后随着波长的增加而增加。这些结果与 AGN 发射的短期变化是由吸积盘的 X 射线照射驱动的假设一致。在这项工作中，我们研究了 NGC 5548 的紫外/光学和 X 射线发射功率谱，利用 STORM 活动的数据以及以前的 Swift 数据，以研究紫外/光学和 X-射线可变性并检查其与上图的一致性。我们证明，即使功率谱结果也与被 X 射线照射的标准圆盘兼容，吸积率低，但细节与“{\tau} vs {\lambda}”关系建模的结果并不完全一致。差异表明内盘可能被“暖电晕”覆盖，这不允许检测来自内盘的紫外线/光发射。最后，我们发现了强有力的证据表明 NGC 5548 的紫外线辐射不是静止的。