KeywordsPhysics, Astronomy and Astrophysics.
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PublisherThe University of Arizona.
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AbstractThe hard X-ray spectrum from black hole candidates, such as 1E1740.7-2942 and Cygnus X-1, has been attributed to an inner hot (Tₑ ≈ 10⁹ K) two-temperature disk which Comptonizes externally produced soft photons from the outer disk. We developed a natural extension of this model, wherein the innermost region of the two-temperature disk, is much hotter (Tₑ ≈ 5 x 10⁹ K) since it is shielded from the external photons and is forced to cool via bremsstrahlung self-comptonization. The emission from this region can account for the long term γ-ray variability in Cygnus X-1. The e⁺e⁻ pairs produced above the hot plasma give rise to the annihilation line observed in 1E1740.7-2942 and the residual pairs form the extended radio jets observed in this source. These early successes called for more detailed modeling of the hot disk. The effects of e⁺e⁻ pairs produced inside the disk were investigated using a better technique for the Comptonization process than what had been reported previously in the literature. This has important quantitative (but no qualitative implications) on the model. Another crucial effect is that of the proton thermal energy being advected to smaller radii. This makes the disk sensitive to the outer boundary conditions (i.e the structure of the transition zone between the outer cold disk and the inner hot region). To determine the physics of the transition region we have developed a scheme for the radiative cooling which is valid at all optical depths. The application of this scheme revealed that the transition region is an extended one. The disk has a hot two-temperature configuration even without the assumption that instabilities in the disk drive the cold disk to this stage (which has been doubted in the past). Moreover, the spectrum from the transition zone matches well with the observed X-ray spectrum of Cygnus X-1 and variations in the magnetic field can account for the two X-ray states. We conclude from this new self-consistent model that the soft X-ray photons observed arise from the cold disk, the X-ray continuum is produced in the transition zone, while the γ-rays and associated phenomena (like the e⁺e⁻ line and the radio jets) are due to the inner hot disk.
Degree ProgramGraduate College