A Transition Disk Model for Cygnus X-1

Details A Transition Disk Model for Cygnus X-1 A Transition Disk Model for Cygnus X-1

Aug 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...467..405m&link_type=abstract

Astrophysical Journal v.467, p.405

Astronomy and Astrophysics

Astronomy

14

Accretion, Accretion Disks, Radiation Mechanisms: Nonthermal, Stars: Individual Constellation Name: Cygnus X-1, Stars: Magnetic Fields

Scientific paper

The standard cold accretion disk theory for black hole systems such as Cygnus X-1 is not self- consistent when the effective optical depth is less than unity. In this paper, we develop a scheme for solving the disk structure that is valid at all optical depths. We show that the disk typically consists of three regions: (a) an outer cold disk, (b) a middle transition zone, and (c) an inner two-temperature disk. The emergent spectrum from the transition region is due primarily to a sum of Wien peaks instead of the soft-photon Comptonization within a geometrically thick, hot inner region as proposed in earlier models, and it matches well with the observed X-ray power-law continuum from these sources. In the picture developed here, we include the possibility of the presence of a magnetic field having an equipartition pressure with the plasma. We suggest that variations in this magnetic field and the corresponding viscosity parameter α can account for the two long-term X-ray states of Cygnus X-1. The transition disk region is subject to both thermal and secular instabilities, but it appears that these do not transform the disk into a two-temperature configuration (Taam & Lin 1984). Instead, we speculate that these instabilities give rise to structural fluctuations which we observe as quasi-periodic oscillations.

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