THE EMISSION FROM AN INNER DISK AND A CORONA IN THE LOW AND INTERMEDIATE SPECTRAL STATES OF BLACK HOLE X-RAY BINARIES
Recent observations reveal that a cool disk may survive in the innermost stable circular orbit (ISCO) for some black hole X-ray binaries (BHXRBs) in the canonical low/hard state. The spectrum is characterized by a power law with a photon index {Gamma} {approx} 1.5-2.1 in the range of 2-10 keV and a weak disk component with a temperature of {approx}0.2 keV. In this work, we revisit the formation of such a cool, optically thick, geometrically thin disk in the innermost region of BHXRBs at the low/hard state within the context of disk accretion fed by condensation of the hot corona. By taking into account the cooling process associated with both Compton and conductive processes in a corona and the irradiation of the hot corona to the disk, we calculate the structure of the corona. For viscosity parameter {alpha} = 0.2, it is found that the inner disk can exist for an accretion rate ranging from M-dot{approx}0.006 to 0.03 M-dot{sub Edd}, over which the electron temperatures of the corona are in the range of 1-5 Multiplication-Sign 10{sup 9} K producing the hard X-ray emission. We calculate the emergent spectra of the inner disk and corona for different mass accretion rates. The effect of viscosity parameter {alpha} and albedo a (a is defined as the energy ratio of reflected radiation from the surface of the thin disk to incident radiation upon it from the corona) on the emergent spectra is also presented. Our model is used to explain the recent observations of GX 339-4 and Cyg X-1, in which the thin disk may exist at the ISCO region in the low/hard state at a luminosity around a few percent of L{sub Edd}. It is found that the observed maximal effective temperature of the thermal component and the hard X-ray photon index {Gamma} can be matched well by our model.
- OSTI ID:
- 22004199
- Journal Information:
- Astrophysical Journal, Vol. 744, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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