Evolution of the X-ray spectrum in the flare model  of Active Galactic Nuclei

Abstract

Abstract. Nayakshin & Kazanas (2002) have considered the time-dependent illumination of an accretion disc in Active Galactic Nuclei, in the lamppost model, where it is assumed that an X-ray source illuminates the whole inner-disc region in a relatively steady way. We extend their study to the flare model, which postulates the release of a large X-ray flux above a small region of the accretion disc. A fundamental difference to the lamppost model is that the region of the disc below the flare is not illuminated before the onset of the flare. After the onset, the temperature and the ionization state of the irradiated skin respond immediately to the increase of the continuum, but pressure equilibrium is achieved later. A few typical test models show that the reflected spectrum that follows immediately the increase in continuum flux should always display the characteristics of a highly illuminated but dense gas, i.e. very intense X-ray emission lines and ionization edges in the soft X-ray range. The behaviour of the iron line is however different in the case of a "moderate" and a "strong" flare: for a moderate flare, the spectrum displays a neutral component of the Fe Kα line at 6.4 keV, gradually leading to more highly ionized lines. For a strong flare, the lines are already emitted by FeXXV (around 6.7 keV) after the onset, and are very intense, with an equivalent width of several hundreds eV. A strong flare is also characterized by a steep soft X-ray spectrum. The variation timescale in the flare model is likely smaller than in the lamppost model, due to the smaller dimension of the emission region, so the timescale for pressure equilibrium is long compared to the duration of a flare. It is therefore highly probable that several flares contribute at the same time to the luminosity. We find that the observed correlations between R, Γ, and the X-ray flux are well accounted for by a combination of flares having not achieved pressure equilibrium, also strongly suggesting that the observed spectrum is always dominated by regions in non-pressure equilibrium, typical of the onset of the flares. Finally, a flare being confined to a small region of the disc, the spectral lines should be narrow (except for a weak Compton broadening) and Doppler shifted, as stressed by . All these features should constitute specific variable signatures of the flare model, distinguishing it from the lamppost model. It is however difficult, on the basis of the present observations and models, to conclude in favor of one of the hypothese.

The structure and radiation spectra of illuminated accretion disks in AGN

“…A grid of spot spectra should be computed (parameter space of the model is rather rich, and so the task is numerically very time consuming and we postpone it for future work); -hydrostatic equilibrium was assumed to compute the irradiated disk structure. However, timescales of the flares and of restoring the hydrostatic equilibrium are roughly comparable (see the discussion by Nayakshin &Kazanas 2002 andCollin et al 2003). Therefore, neither the assumption of hydrostatic equilibrium nor the assumption of an unperturbed disk are satisfactory.…”

The role of absorption and reflection in the soft X-ray excess of Active Galactic Nuclei

“…shorter due to the high gas and radiation density in these central regions (for a discussion, see for instance Collin et al 2003). So if any perturbation of the pressure equilibrium is produced, another equilibrium state will be established in less than a day for r 10 ∼ M 7 ∼ 1.…”

Thermal instability in X-ray photoionized media in active galactic nuclei

“…Dumont et al 2002;Różańska et al 2002;Collin et al 2003), is based on subsequent (instead of simultaneous) determination of the density and the temperature. As explained in Paper I, this solution is smooth (without a sharp discontinuity), but it does not accurately satisfy the condition of the constant pressure.…”