http://arxiv.org/abs/1908.05461
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.
R. Mahmoud and C. Done
Fri, 16 Aug 19
28/54
Comments: 12 pages, 12 figures, Submitted to MNRAS
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