Thermal Reverberation in AGN

Several AGN have shown UV/optical variability lagging behind the X-ray emission by a few days. The simplest and most straightforward interpretation is that the variable X-ray flux from the corona illuminates the accretion disc below where it is partially reflected and observed as fast X-ray reverberation signal, and partially absorbed and thermalised in the disc, which produces a slow UV and optical reverberation signal. The compact corona first illuminates the hottest inner parts of the accretion disc and later on its colder farther out areas. Thus one expects to see the original X-ray fluctuations to be firstly followed by variations in the UV and then in the optical wavebands.

To study the thermal reverberation we have improved our GR-tracing reverberation KYNxilrev model to explore thermal disk reverberation from X-rays through to the optical.

The case of NGC 5548

NGC 5548 was the first source to be extensively monitored as part of the Space Telescope and Optical Reverberation Mapping (STORM) campaign using the Neil Gehrels Swift Observatory, the Hubble Space Telescope and a variety of ground-based observatories.

We applied our model to the NGC 5548 time-lag spectra and we show that it could explain well the data well .

Read more here: Kammoun et al. (2019c).

An in-depth look

We have then went on in exploring our model in more detail. So, we have studied the effect of various key parameters on the reverberation signal such as: the BH spin, the BH mass, the accretion rate, the height of the X-ray source, the inner/outer edges of the disc, the shape of the X-ray spectrum, and the X-ray luminosity. We also present an analytic prescription that can be used to fit time-lag spectra.

Read more here: Kammoun et al. (2021a).

Fitting the time lag

In Kammoun et al. (2021a) we derived an analytic expression to fit the time-lag spectra, taking into account all GR effects, and the effects of the X-ray spectrum and the corona properties.

We then used this analytic function to fit the lag-spectra for seven AGN intensively monitored in the X-ray, UV, and optical range using ground- based and space-based observatories. Using this model, we could derive limits on the accretion rate of the black hole, and on the height of the X-ray source. We show that modeling the continuum UV/optical time-lags can be used to estimate the black hole spin, when combined with results from multi-wavelength analysis of accretion rates of these sources.

Standard disc models fit well the time-lag spectra, contrary to previous models requiring larger discs. This is due to the fact that we consider GR-ray-tracing to estimate the disc response, leading to larger amplitude but consistent slope when compared to previous models.

Read more here: Kammoun et al. (2021b).

Fitting the time lag

In Dovciak et al. (2022) we present KYNSED, a new SED model that describes the broadband X-ray/UV/optical spectra of AGN within the context of thermal reverberation. The model assumes that all the accretion power of the disc within a given Rtrans is transferred to the corona. The corona by itself will illuminate back the disc, resulting in a thermalized emission. After few iterations, we reach an equilibrium that defines the average state of the source. The assumed geometry is shown in the figure above.

We applied this model to the average SED of NGC 5548, obtained throughout the STORM campaign and we were able to explain its Optical-to-X-ray SED within this context.

Read more details here: Dovciak et al. (2022)