Astronomy and Astrophysics – Astrophysics
Scientific paper
1997-10-21
Astronomy and Astrophysics
Astrophysics
Submitted to ApJL, 6 pages, 3 figures
Scientific paper
We here examine the conditions of the two-phase disk model under which magnetic flares arise above the cold accretion disk due to magnetic buoyancy and produce X-rays via Comptonization of the disk's soft radiation. We find that the disk's ability to produce strong magnetic flares is substantially diminished in its radiation dominated regions due to the diffusion of radiation into the magnetic flux tubes. Using a simplified, yet physically self-consistent, model that takes this effect into account, we show that the hard X-ray spectrum of some GBHCs can be explained as the X-ray emission by magnetic flares only when the disk's bolometric luminosity is a relatively small fraction ($\sim$ 5%) of the Eddington value, $L_{Edd}$. Further, we compute the hard ($20-200$ keV) and soft ($1-20$ keV) X-ray power as a function of the disk's luminosity, and find an excellent agreement with the available data for GBHC transient and persistent sources. We conclude that the observed high-energy spectrum of stellar-sized accretion disk systems can be explained by Comptonization of the disk's soft radiation by the hot gas trapped inside the magnetic flares when the luminosity falls in the range $\sim 10^{-3}-10^{-1}\times L_{Edd}$. For higher luminosities, another emission mechanism must be at work. For lower luminosities, the X-ray emissivity may still be dominated by magnetic flares, but this process is more likely to be thermal or non-thermal bremstrahlung, so that the X-ray spectrum below $\sim 10^{-3}L_{Edd}$ may be quite distinct from the typical hard spectrum for higher luminosities.
Melia Fulvio
Nayakshin Sergei
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