EUCLIA. II. On the puzzling large UV to X-ray lags in Seyfert galaxies. (arXiv:2002.11116v1 [astro-ph.HE])

EUCLIA. II. On the puzzling large UV to X-ray lags in Seyfert galaxies. (arXiv:2002.11116v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Cai_Z/0/1/0/all/0/1">Zhen-Yi Cai</a> (USTC), <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_J/0/1/0/all/0/1">Jun-Xian Wang</a> (USTC), <a href="http://arxiv.org/find/astro-ph/1/au:+Sun_M/0/1/0/all/0/1">Mouyuan Sun</a> (XMU)

Recent intense X-ray and UV monitoring campaigns with Swift have detected
clear UV lags behind X-ray in several local active galactic nuclei (AGNs). The
UV to X-ray lags are often larger (by a factor up to ~ 20) than expected if the
UV variation is simply due to the X-ray reprocessing. We previously developed a
model in which the UV/optical variations are attributed to disk turbulences,
and the effect of large-scale turbulence is considered. Our model, which
overcomes many severe challenges to the reprocessing scheme, can well explain
the observed variations in NGC 5548, particularly the correlations and lags
among the UV/optical bands. In this work, assuming the corona heating is
associated with turbulences in the inner accretion disk, we extend our study to
model the correlations and lags between the X-ray and UV/optical bands. We find
that our model, without the need of light echoing, can well reproduce the
observed UV to X-ray lags and the optical to UV lags simultaneously in four
local Seyfert galaxies, including NGC 4151, NGC 4395, NGC 4593, and NGC 5548.
In our scenario, relatively larger UV to X-ray lag is expected for AGN with
smaller innermost disk radius and thus more compact corona. Interestingly, for
these Seyfert galaxies studied in this work, sources with relatively larger UV
to X-ray lags do have broader Fe Ka lines, indicative of relativistic
broadening due to more compact corona and smaller innermost disk radius. If
confirmed with more X-ray and UV monitoring campaigns, this interesting
discovery would provide a new probe to the inner disk/corona.

Recent intense X-ray and UV monitoring campaigns with Swift have detected
clear UV lags behind X-ray in several local active galactic nuclei (AGNs). The
UV to X-ray lags are often larger (by a factor up to ~ 20) than expected if the
UV variation is simply due to the X-ray reprocessing. We previously developed a
model in which the UV/optical variations are attributed to disk turbulences,
and the effect of large-scale turbulence is considered. Our model, which
overcomes many severe challenges to the reprocessing scheme, can well explain
the observed variations in NGC 5548, particularly the correlations and lags
among the UV/optical bands. In this work, assuming the corona heating is
associated with turbulences in the inner accretion disk, we extend our study to
model the correlations and lags between the X-ray and UV/optical bands. We find
that our model, without the need of light echoing, can well reproduce the
observed UV to X-ray lags and the optical to UV lags simultaneously in four
local Seyfert galaxies, including NGC 4151, NGC 4395, NGC 4593, and NGC 5548.
In our scenario, relatively larger UV to X-ray lag is expected for AGN with
smaller innermost disk radius and thus more compact corona. Interestingly, for
these Seyfert galaxies studied in this work, sources with relatively larger UV
to X-ray lags do have broader Fe Ka lines, indicative of relativistic
broadening due to more compact corona and smaller innermost disk radius. If
confirmed with more X-ray and UV monitoring campaigns, this interesting
discovery would provide a new probe to the inner disk/corona.

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