A hard look at thermal reverberation and optical/UV lags in NGC 5548

Abstract

The UV/optical variations in many AGN are very well correlated, showing delays which increase with increasing wavelength. It is thought that this is due to thermal reprocessing of the X-ray emission by the accretion disk. In this scenario, the variable X-ray flux from the corona illuminates the accretion disk where it is partially reflected, and partially absorbed and thermalized in the disk producing a UV/optical reverberation signal. This will lead to a time lag increasing with wavelength. However, although the shape of the observed time-lags as a function of wavelength is consistent with the model predictions, their amplitude suggested a disk which is significantly hotter than expected. In this work, we estimate the response functions and the corresponding time lags assuming a standard Novikov-Thorne accretion disk illuminated by a point-like X-ray source. We take into account all relativistic effects in the light propagation from the X-ray source to the disk then to the observer. We also compute the disk reflection, accounting for its ionization profile. Our results show that thermal reverberation effects are stronger in sources with large X-ray source height and low accretion rate. We also found that the time lags increase with height and accretion rate. We apply our model to NGC 5548 and we show that the observed lags in this source can be explained by the model, for a source height of $\sim 60{\mathrm{r}}_{\mathrm{g}}$ and an accretion rate of a few percent of the Eddington limit for a maximally-spinning black hole.

Accepted for publication in ApJL

References in corpus (8)

• The AGN Black Hole Mass Database
• Space Telescope and Optical Reverberation Mapping Project. II. Swift and HST Reverberation Mapping of the Accretion Disk of NGC 5548
• Mean Spectral Energy Distributions and Bolometric Corrections for Luminous Quasars
• Swift monitoring of NGC 5548: X-ray reprocessing and short term UV/optical variability
• On the Bardeen-Petterson Effect in black hole accretion discs
• Space Telescope and Optical Reverberation Mapping Project VI: reverberating Disk Models for NGC 5548
• Theoretical modelling of the AGN iron-line vs continuum time-lags in the lamp-post geometry
• Steep X-ray reflection emissivity profiles in AGN as the result of radially-structured disc ionisation

Cited by in corpus (4)

• Supermassive black holes with high accretion rates in active galactic nuclei. XI. Accretion disk reverberation mapping of Mrk 142
• Evidence for variability timescale dependent UV/X-ray delay in Seyfert 1 AGN NGC 7469
• Multiwavelength power-spectrum analysis of NGC 5548
• The nature of the broadband X-ray variability in the dwarf Seyfert galaxy NGC 4395