Abstract
Several recent papers have shown that an accretion disk corona model, where a slab corona sandwiches the cold accretion disk, is problematic for hard-state spectra of galactic black hole candidates (GBHCs) because the model spectra are never hard enough to match the observations. However, it has recently been pointed out that because of a thermal ionization instability, a hot forms on the top of the illuminated disk. Through numerical simulations in the slab corona geometry, we show that the completely ionized skin leads to a reduction in the reflected thermal blackbody component that amounts to a decrease in the Compton cooling rate of the corona and thus allows the X-ray spectra to be harder. While this brings the model closer to observations, in order for the predicted spectrum to be as hard as the observed spectra of Cyg X-1, the Thomson optical depth of the transition layer must be greater than 10, which is inconsistent with the ionization physics and observations. Therefore, the model with a planar corona covering the whole accretion disk is still strongly ruled out by the observations of GBHCs. Finally, we discuss accretion disks with magnetic flares (i.e., patchy corona) and show that the ionized skin resolves many of the arguments made in the literature against this model, although more quantitative future work is needed to test the model thoroughly.
Highlights
The X-ray spectra of Seyfert galaxies and galactic black hole candidates (GBHCs) indicate that the reÑection and reprocessing of incident X-rays into lower frequency radiation is a ubiquitous and important process
It was originally thought that for Seyfert galaxies, the X-ray energy spectral index hovers near a ““ canonical value ÏÏ (! D 1.95 ; Pounds et al 1990 ; Nandra & Pounds 1994 ; Zdziarski et al 1996) after the reÑection component has been subtracted out of the observed spectrum. The universality of this X-ray spectral index may be attributed to the fact that the reprocessing of X-rays within the cold accretion disk leads to an electron cooling rate that is roughly proportional to the heating rate inside the active regions (Haardt & Maraschi 1991, 1993 ; Haardt, Maraschi, & Ghisellini 1994 ; Svensson 1996)
We argue for the existence of a transition layer in the vicinity of an active coronal region
Summary
The X-ray spectra of Seyfert galaxies and galactic black hole candidates (GBHCs) indicate that the reÑection and reprocessing of incident X-rays into lower frequency radiation is a ubiquitous and important process. This argument does not apply for a patchy corona geometry, since most of the photons reprocessed in the disk can escape to the observer without reentering the X-ray source, decreasing the cooling rate and increasing the cuto† energy in the spectrum (e.g., Stern et al 1995 ; Poutanen & Svensson 1996) In the latter case, Gierlin ski et al (1997) showed that because most of the X-ray photons illuminating the disk are thermalized (e.g., Magdziarz & Zdziarski 1995), one expects to see in the spectra a roughly equal amount of energy in the thermalized component, which is not the case for Cyg X-1 in the hard state (e.g., Zhang et al 1997). We caution that the ““ Ðnal ÏÏ answer on whether the model works for Cyg X-1 will be available only when more detailed full disk models are developed and tested against the data
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