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

摘要

The physical mechanisms responsible for production of the non-thermalemission in accreting black holes (BH) should be imprinted in the observationalapperances of the power law tails in the X-ray spectra from these objects.Different spectral states exhibited by galactic BH binaries allow examinationof the photon upscattering under different accretion regimes. We revisit thedata collected by Rossi X-ray Timing Explorer (RXTE) from the BH X-ray binaryXTE J1550-564 during two periods of X-ray activity in 1998 and 2000 focusing onthe 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 theintermediate state was accompanied by a gradual decrease in the cutoff energywhich then showed an abrupt reversal to a clear increasing trend as the sourceevolved to the very high and high-soft states. The 2000 outburst showed onlythe decreasing part of this pattern. Notably, the photon indexes correspondingto the cutoff increase for the 1998 event are much higher than the index valuesreached during the 2000 rise transition. We attribute this difference in thecutoff energy behavior to the different partial contributions of the thermaland non-thermal (bulk motion) Comptonization in photon upscattering. Namely,during the 1998 event the higher accretion rate presumably provided morecooling to the Comptonizing media and thus reducing the effectiveness of thethermal upscattering process. Under these conditions the bulk motion takes aleading role in boosting the input soft photons. Monte Carlo simulations of theComptonization in a bulk motion region near an accreting black hole by Laurent& Titarchuk 2010 strongly support this scenario.