Implication of the observable spectral cutoff energy evolution in XTE J1550-564

Abstract: The physical mechanisms responsible for production of the non-thermal emission in accreting black holes (BH) should be imprinted in the observational apperances of the power law tails in the X-ray spectra from these objects. Different spectral states exhibited by galactic BH binaries allow examination of the photon upscattering under different accretion regimes. We revisit the data collected by Rossi X-ray Timing Explorer (RXTE) from the BH X-ray binary XTE J1550-564 during two periods of X-ray activity in 1998 and 2000 focusing on the behavior of the high energy cutoff of the power law part of the spectrum. For the 1998 outburst the transition from the low-hard state to the intermediate state was accompanied by a gradual decrease in the cutoff energy which then showed an abrupt reversal to a clear increasing trend as the source evolved to the very high and high-soft states. The 2000 outburst showed only the decreasing part of this pattern. Notably, the photon indexes corresponding to the cutoff increase for the 1998 event are much higher than the index values reached during the 2000 rise transition. We attribute this difference in the cutoff energy behavior to the different partial contributions of the thermal and non-thermal (bulk motion) Comptonization in photon upscattering. Namely, during the 1998 event the higher accretion rate presumably provided more cooling to the Comptonizing media and thus reducing the effectiveness of the thermal upscattering process. Under these conditions the bulk motion takes a leading role in boosting the input soft photons. Monte Carlo simulations of the Comptonization in a bulk motion region near an accreting black hole by Laurent & Titarchuk 2010 strongly support this scenario.