Discarding the disc in a changing state AGN: the UV/X-ray relation in NGC 4151. (arXiv:1908.05461v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Mahmoud_R/0/1/0/all/0/1">Ra'ad D. Mahmoud</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Done_C/0/1/0/all/0/1">Chris Done</a>
Recent monitoring campaigns designed to map the accretion regime in AGN show
major discrepancies with models where the optical/ultraviolet (UV) is produced
by X-ray-illuminated, optically thick material in a disc geometry within a few
hundred gravitational radii. However, these campaigns only monitored X-rays up
to $sim3-5$ keV, whereas the bolometric luminosity for most of these AGN peaks
above $50$ keV. Here we use data from the recent multi-wavelength campaign by
Edelson et al. (2017) on NGC 4151 – the only AGN bright enough to be monitored
at higher energies with the Swift BAT. We develop a spectral-timing model with
a hot corona, warm Comptonization, and outer standard disc. This fits the
time-averaged spectrum well, but completely fails to match the UV variability
predicted from the observed hard X-ray light-curve. However, it reveals that
NGC 4151 had a bolometric luminosity around $0.015$ of the Eddington luminosity
at the time of this campaign, close to the luminosity at which AGN show a
`changing state’ transition, where the broad optical lines disappear. Stellar
mass black holes show a spectral state transition at a similarly low Eddington
fraction, which is generally interpreted as the inner disc being replaced by a
hot flow out to a few hundred gravitational radii. We find that the UV
light-curve can instead be matched by reprocessing of the X-ray flux on size
scales of the broad line region ($2-20$~light-days) and rule out there being
optically thick material inwards of this, as expected if the thin disc is
replaced by the hot flow below the inner radius of the BLR.
Recent monitoring campaigns designed to map the accretion regime in AGN show
major discrepancies with models where the optical/ultraviolet (UV) is produced
by X-ray-illuminated, optically thick material in a disc geometry within a few
hundred gravitational radii. However, these campaigns only monitored X-rays up
to $sim3-5$ keV, whereas the bolometric luminosity for most of these AGN peaks
above $50$ keV. Here we use data from the recent multi-wavelength campaign by
Edelson et al. (2017) on NGC 4151 – the only AGN bright enough to be monitored
at higher energies with the Swift BAT. We develop a spectral-timing model with
a hot corona, warm Comptonization, and outer standard disc. This fits the
time-averaged spectrum well, but completely fails to match the UV variability
predicted from the observed hard X-ray light-curve. However, it reveals that
NGC 4151 had a bolometric luminosity around $0.015$ of the Eddington luminosity
at the time of this campaign, close to the luminosity at which AGN show a
`changing state’ transition, where the broad optical lines disappear. Stellar
mass black holes show a spectral state transition at a similarly low Eddington
fraction, which is generally interpreted as the inner disc being replaced by a
hot flow out to a few hundred gravitational radii. We find that the UV
light-curve can instead be matched by reprocessing of the X-ray flux on size
scales of the broad line region ($2-20$~light-days) and rule out there being
optically thick material inwards of this, as expected if the thin disc is
replaced by the hot flow below the inner radius of the BLR.
http://arxiv.org/icons/sfx.gif
