On the Geometry of the X-Ray–emitting Region in Seyfert Galaxies

Abstract

For the first time, detailed radiative transfer calculations of Comptonized X-ray and γ-ray radiation in a hot pair plasma above a cold accretion disk are performed using two independent codes and methods. The simulations include both energy and pair balance as well as reprocessing of the X-and γ-rays by the cold disk. We study both plane-parallel coronae as well as active dissipation regions having shapes of hemispheres and pill boxes located on the disk surface. It is shown, contrary to earlier claims, that plane-parallel coronae in pair balance have difficulties in selfconsistently reproducing the ranges of 2-20 keV spectral slopes, high energy cutoffs, and compactnesses inferred from observations of type 1 Seyfert galaxies. Instead, the observations are consistent with the X-rays coming from a number of individual active regions located on the surface of the disk.A number of effects such as anisotropic Compton scattering, the reflection hump, feedback to the soft photon source by reprocessing, and an active region in pair equilibrium all conspire to produce the observed ranges of X-ray slopes, high energy cutoffs, and compactnesses. The spread in spectral X-ray slopes can be due to a spread in the properties of the active regions such as their compactnesses and their elevations above the disk surface. Simplified models invoking isotropic Comptonization in spherical clouds are no longer sufficient when interpreting the data.

On the Lamppost Model of Accreting Black Holes

“…This yields ℓ  7×10 4 . For kT e ; 130 keV, the curves in Figure 1 of Stern et al (1995) yield ℓ  (1-3)×10 2 , while Figure 3(a) of Zdziarski (1985) yields ℓ  50. Thus, the model is out of pair equilibrium in the allowed parameter range.…”

“…Still, the lack of a visible cutoff constrains kT e /(1+z)  150 keV (assuming the observer's frame temperature is at least twice the maximum observed energy), and thus kT e  1 MeV. At this temperature, the results of Stern et al (1995) require ℓ  0.1-1.…”

“…Calculations of pair equilibrium using spectra from thermal Comptonization were performed by Zdziarski (1985) and Stern et al (1995). Their constraints on pair balance at kT e = 0.1 and 1 MeV were ℓ  10 2 -10 3 and 0.1-1, respectively.…”

Fluorescent iron lines as a probe of astrophysical black hole systems

“…The remaining three show "photon starved geometries" wherein the corona is less effectively cooled by soft photons from the disk. The middle two geometries are often referred to as "sphere+disk geometries", while the bottom geometry is often referred to as a "patchy corona" or "pill box" model [140].…”

“…Second, 100% of the soft, seed photons from the disk pass through the corona. Thus, the corona is relatively easily Compton cooled and it is difficult for it to obtain very high temperatures [140,144]. Even if the optical depth of such a corona is made small, pair production within the corona will ensue until the rate of heating is balanced by Compton cooling.…”