http://arxiv.org/abs/1711.00870
The reprocessing scenario is widely adopted in literature to explain the observed tight inter-band correlation and short lags in the UV/optical variations of active galactic nuclei. In this work we look into the color variability of the famous Seyfert galaxy NGC 5548 with high quality \Swift{} multi-band UV/optical light curves. We find its color variation is clearly timescale-dependent, in a way that the bluer-when-brighter trend is more prominent at shorter timescales. This is similar to that previously detected in quasar samples, but for the first time in an individual AGN. We show that while a reprocessing model with strict assumptions on the driving source and the disk size can apparently match the observed light curves and inter-band lags, it fails to reproduce the observed timescale dependency in the color variation. Such discrepancy raises a severe challenge to, and can hardly be reconciled under the widely accepted reprocessing diagram. It also demonstrates that the timescale dependency of the color variation is uniquely powerful in probing the physics behind AGN UV/optical variations.
F. Zhu, J. Wang, Z. Cai, et. al.
Mon, 6 Nov 17
16/53
Comments: 11 pages, 4 figures, submitted to apj with referee’s comment incorporated
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