Cygnus X-1 and the gamma-ray transients GROJ0422+32 and GROJ1719-24 displayed
similar spectral properties when they underwent transitions between the high
and low gamma-ray (30 keV to few MeV) intensity states. When these sources were
in the high gamma-ray intensity state (gamma-2, for Cygnus X-1), their spectra
featured two components: a Comptonized shape below 200-300 keV with a soft
power-law tail (photon index >3) that extended to ~1 MeV or beyond. When the
sources were in the low-intensity state (gamma-0, for Cygnus X-1), the
Comptonized spectral shape below 200 keV typically vanished and the entire
spectrum from 30 keV to ~1 MeV can be characterized by a single power law with
a relatively harder photon index ~2-2.7. Consequently the high- and
low-intensity gamma-ray spectra intersect, generally in the ~400 keV - 1 MeV
range, in contrast to the spectral pivoting seen previously at lower (~10 keV)
energies. The presence of the power-law component in both the high- and
low-intensity gamma-ray spectra strongly suggests that the non-thermal process
is likely to be at work in both the high and the low-intensity situations. We
have suggested a possible scenario (Ling & Wheaton, 2003), by combining the
ADAF model of Esin et al. (1998) with a separate jet region that produces the
non-thermal gamma-ray emission, and which explains the state transitions. Such
a scenario will be discussed in the context of the observational evidence,
summarized above, from the database produced by EBOP, JPL's BATSE earth
occultation analysis system.Comment: 6 pages, 3 figures, accepted for publication in Proceedings of 2004
Microquasar Conference, Beijing, China, Chinese Journal of Astronomy and
Astrophysics, minor corrections per refere