[Paper Review] X-rays and gamma-rays from accretion flows onto black holes in Seyferts and X-ray binaries

A. A. Zdziarski, W. N. Johnson+3 more|Discovery Research Portal (University of Dundee)|Dec 10, 1996

Astrophysical Phenomena and Observations3 citations

TL;DR

This paper investigates X-ray and gamma-ray emission from accretion flows around black holes in Seyfert galaxies and Galactic black-hole candidates, finding that broad-band spectra are best explained by Compton upscattering of soft photons in a thermal plasma with temperature ~100 keV and optical depth ~1. The spectra exhibit a high-energy cutoff near 200 keV, consistent with e+e− pair production regulating the plasma temperature, and reflection components indicate cold matter covering ~0.4–1 times 2π steradians, with weak non-thermal emission (<15%) in NGC 4151.

ABSTRACT

We review observations and theoretical models of X-ray/gamma-ray spectra of radio-quiet Seyfert galaxies and of Galactic black-hole candidates (in the hard spectral state). The observed spectra share all their basic components: an underlying power law, a Compton-reflection component with an Fe K-alpha line, low-energy absorption by intervening cold matter, and a high-energy cutoff above ~ 200 keV. The X-ray energy spectral index, alpha, is typically in the range ~ 0.8-1 in Seyfert spectra from Ginga, EXOSAT and OSSE. The hard-state spectra of black-hole candidates Cyg X-1 and GX 339-4 from simultaneous Ginga/OSSE observations have alpha ~ 0.6-0.8. The Compton-reflection component corresponds to cold matter (e.g., inner or outer parts of an accretion disk) covering a solid angle of ~ (0.4-1) times 2 pi as seen from the X-ray source. The spectra are cut off in soft gamma-rays above ~ 200 keV. The broad-band spectra of both Seyferts and black-hole sources are well fitted by Compton upscattering of soft photons in thermal plasmas. Our fits yield the thermal plasma temperature of ~ 100 keV and the Thomson optical depth of tau 1. A fraction of the luminosity emitted nonthermally appears to be small and it can be constrained to &lt; 15% in the Seyfert galaxy NGC 4151. The spectra are cut off before 511 keV, which is strongly suggestive of a thermostatic role of e+e- pair production in constraining the temperature and optical depth of the sources. The source geometry is compatible with a patchy corona above a cold disk in Seyferts, but not in Cyg X-1. In the latter, the relative weakness of reflection is compatible with reflection of emission of a hot inner disk from outer disk regions.

Motivation & Objective

• To understand the origin of X-ray and gamma-ray emission in radio-quiet Seyfert galaxies and black-hole candidates in the hard state.
• To determine the physical conditions in the X-ray emitting plasma, such as temperature and optical depth, via spectral fitting.
• To assess the role of Compton upscattering and reflection in shaping observed spectra.
• To constrain non-thermal emission and investigate the high-energy cutoff mechanism.
• To compare source geometries between Seyferts and X-ray binaries, particularly regarding corona and disk reflection.

Proposed method

• Fits observed broad-band X-ray and gamma-ray spectra from Ginga, EXOSAT, and OSSE observations using a Comptonization model for thermal plasma.
• Applies a thermal plasma model with a Maxwellian electron distribution to simulate Compton upscattering of soft seed photons.
• Incorporates a Compton reflection component from cold matter, including an Fe K-alpha line, with solid angle coverage of ~0.4–1 times 2π steradians.
• Uses the high-energy cutoff near 200 keV as a diagnostic for e+e− pair production feedback on plasma temperature and optical depth.
• Compares spectral fits to data from Seyfert galaxies (e.g., NGC 4151) and black-hole binaries (Cyg X-1, GX 339-4) to infer source geometry.
• Applies constraints on non-thermal luminosity by comparing model fluxes to observed fluxes in the 200–511 keV range.

Experimental results

Research questions

• RQ1 What physical mechanism produces the observed X-ray and gamma-ray spectra in Seyfert galaxies and black-hole candidates?
• RQ2 How do the spectral components—power law, reflection, and high-energy cutoff—constrain the plasma temperature and optical depth?
• RQ3 What is the role of e+e− pair production in regulating the maximum temperature and spectral cutoff in these sources?
• RQ4 Why is the reflection component weaker in Cyg X-1 compared to Seyfert galaxies, and what does this imply about the geometry?
• RQ5 To what extent is non-thermal emission present in these sources, particularly in NGC 4151?

Key findings

• The X-ray spectra of Seyfert galaxies and black-hole candidates in the hard state are well fitted by Compton upscattering in a thermal plasma with temperature ~100 keV and optical depth ~1.
• The observed energy spectral index α is ~0.8–1.0 in Seyferts and ~0.6–0.8 in black-hole binaries, consistent with thermal Comptonization.
• The high-energy cutoff occurs above ~200 keV and is consistent with the onset of e+e− pair production, which acts as a thermostatic regulator.
• The reflection component arises from cold matter covering ~0.4–1 times 2π steradians, indicating a patchy corona or disk geometry.
• Non-thermal emission is constrained to <15% of the total luminosity in NGC 4151, indicating a dominant thermal origin.
• The geometry in Seyferts is compatible with a patchy corona above a cold disk, but in Cyg X-1, the weak reflection suggests emission from the hot inner disk is reflected off outer disk regions.