1994A&A...287..105D

We present results of Mir-Kvant-HEXE observations in the energy range 20-200 keV of the three X-ray transients and black hole candidates GS 2000+25, GRO J0422+32, and GS 2023+338, observed between 1988 and 1992. The spectra of GS 2023+338 and GRO J0422+32 can be interpreted as due to thermal Compton-upscattering of low-energy photons to hard X-ray energies in a hot plasma with electron temperatures of typically 35 to 40 keV, or as due to Compton reflection of hard photons off cool matter with covering factors f_c ranging from 0.4 to 0.8. A combination of Compton-upscattering and Compton reflection is also possible. The spectrum of GS 2000+25 is softer and can be described by a power law with a photon index α of ≃2.0 (E > 20 keV).