Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999bha..proge..52z&link_type=abstract
Presented at the KITP Program: Black Hole Astrophysics, Jun 30, 1999, Kavli Institute for Theoretical Physics, University of Cal
Astronomy and Astrophysics
Astrophysics
Scientific paper
We present X-ray/gamma-ray spectra of Cyg X-1 observed during the transitionfrom the hard to the soft state and in the soft state by ASCA, RXTE and OSSE in1996 May and June. The spectra consist of a dominant soft component below ~2keV and a power-law-like continuum extending to at least ~800 keV. We interpretthem as emission from an optically-thick, cold accretion disc and from anoptically-thin, non-thermal corona above the disc. A fraction f ~ 0.6 of totalavailable power is dissipated in the corona. We model the soft component bymulti-colour blackbody disc emission taking into account the torque-free inner-boundary condition. If the disc extends down to the minimum stable orbit, theASCA/RXTE data yield the most probable black hole mass of about 10 solar massesand an accretion rate about 0.5 L_E/c^2, locating Cyg X-1 in the soft state inthe upper part of the stable, gas-pressure dominated, accretion-disc solutionbranch. The spectrum of the corona is well modelled by repeated Comptonscattering of seed photons from the disc off electrons with a hybrid,thermal/non-thermal distribution. The electron distribution can becharacterized by a Maxwellian with an equilibrium temperature of kT ~ 30--50keV and a Thomson optical depth of ~0.3 and a quasi-power-law tail. Thecompactness of the corona is between 2 and 7, and a presence of a significantpopulation of electron-positron pairs is ruled out. We find strong signaturesof Compton reflection from a cold and ionized medium, presumably an accretiondisc, with an apparent reflector solid angle ~0.5--0.7. The reflected continuumis accompanied by a broad iron K-alpha line.
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