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J. C. Ling (JPL/Caltech), Wm. A. Wheaton (IPAC/Caltech)
We report soft gamma-ray (30 keV to 1.7 MeV) spectral properties in the black hole GROJ0422+32, observed during its first known outburst in 1992, and similar to properties of Cygnus X-1 during its low-to-high state transitions in 1994 and 1996 (see Ling & Wheaton, 2003). In both cases, the high-intensity (``gamma2'') spectrum consists of two components: a Comptonized shape below 300 keV, plus a steep power-law tail above 300 keV. By contrast, the low-intensity spectrum (``gamma0'') has a power-law shape with an index of ~2. The two spectra cross at ~600 keV for GROJ0422+32, or about 1 MeV for Cyg X-1. We suggest a scenario for interpreting these common spectral features by including a separate non-thermal source region, possibly a jet, in the ADAF model of Esin et al (1998). During the high-intensity gamma2 state, the system consists of a hot inner corona, a cooler outer thin disk, and a region that produced the variable power-law gamma-ray emission. In this condition, the transition radius of the disk is ~100 Schwarzschild radii. Electrons in the hot corona up-scatter the low-energy photons, produced both within the corona as well as from the outer disk, to form the Comptonized component that dominates the spectrum in the 35-300 keV range. The same electrons also down-scatter high energy (>10 MeV) photons produced nonthermally in the ``jet'' region, forming the softer power-law component observed in the 0.3-1 MeV range. During the low-intensity gamma0 state, a large soft flux cools the inner corona, and moves the transition radius inward, close to the event horizon. Under this condition, the Comptonized component in the 35-300 keV range is suppressed, and the source spectrum is dominated by the unperturbed power-law emission produced in the non-thermal source region, with a characteristic index of ~2.
The work described in this paper was carried out by the Jet Propulsion Laboratory, under the contract with the National Aeronautics and Space Administration.
Ling & Wheaton, to be published in February 10, 2003 issue of ApJ
Esin et al., 1998, ApJ, 505, 854.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.