2003MNRAS.342.1083G

We fit the broad-band X-ray/γ-ray spectrum (0.8-1000 keV) of the accreting black hole XTE J1550-564 in the very high state. The quasi-simultaneous data from ASCA, RXTE and OSSE (on CGRO) show that the disc is strongly Comptonized, with a high-energy tail extending out to several hundred keV. However, inverse Compton scattering by a purely thermal or purely power-law electron distribution cannot explain the observed spectrum. Instead, the data require a hybrid distribution, with both thermal and non-thermal electrons scattering the disc photons. This is very similar to the electron distribution inferred for other high and very high state black hole binaries, showing that it is a generic feature of high mass accretion rate black holes.