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Low-state disks and low-beta disksStellar black hole candidates (BHCs) exhibit bimodal spectral states. We calculate nonthermal disk spectra, demonstrating that a large photon index (alpha (sub x) approximately 2-3) observed in the soft (high) state is due to a copious soft photon supply, whereas soft photon starvation leads to a smaller index (alpha (sub x) approximately 1.5-2) in the hard (low) state. Thus, the absence of the soft component flux in the low state cannot be due to obscuration. A possible disk configuration during the low state is discussed. We proposed that a low-state disk may be a low-beta disk in which magnetic pressure may exceed gas pressure becuase of the suppression of field escape by a strong shear. As a result, disk material will take the form of blobs constricted by mainly toroidal magnetic fields. Fields are dissipated mainly by occasional reconnection events with a huge energy release. This will account for large-amplitude, aperiodic X-ray variations (flickering) and high-energy radiation with small alpha(sub x) from hard state BHCs and possibly from active galactic nuclei. Further, we propose a hysteretic relation between the mass-flow rate and plasma-beta, a ratio of gas pressure to magnetic pressure, for the spectral evolution of transient BHCs. The disk is in the low-beta state in quiescence and early rise. The low-beta disk is optically thin and affected by advection. A hard-to-soft transition occurs before the peak luminosity, since there is no advection-dominated branch at higher luminosities. An optically thick, high-beta disk appears at small radii. In the decay phase of the light curve, the standard-type disk becomes effectively optically thin, when a soft-hard transition is triggered. High-beta plasmas in the main body shrink to form minute blobs, and low-beta coronal plasma fills interblob space.
Document ID
19950056281
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Mineshige, Shin
(Kyoto University Sakyoku, Kyoto, Japan)
Kusnose, Masaaki
(University of Texas at Austin, Austin, Texas United States)
Matsumoto, Ryoji
(Chiba University Inage-ku, Chiba, Japan)
Date Acquired
August 16, 2013
Publication Date
May 20, 1995
Publication Information
Publication: Astrophysical Journal, Part 2 - Letters
Volume: 445
Issue: 1
ISSN: 0004-637X
Subject Category
Astronomy
Accession Number
95A87880
Funding Number(s)
CONTRACT_GRANT: MOESC-06233101
CONTRACT_GRANT: NAGW-2957
CONTRACT_GRANT: MOESC-05836017
Distribution Limits
Public
Copyright
Other

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