X-ray Spectral Behavior in the Low/Hard States of Black Hole Candidates

Abstract

We carried out systematic spectral analysis for 10 black hole candidates (BHCs) in the low/hard states, using archival RXTE and Beppo-SAX data which cover a broad energy range of 2-250 keV. For in total 156 observations, the simple analytic model, a power law with an exponentiall cutoff modified with a smeared edge model, were adopted. Consequently, we found that the photon index was distributed over 1.2-1.8 and the cutoff energy was more than 40 keV. We also found that there is the different behaviour between the photon index/cutoff energy and the luminosity at lower and higher luminosities with its value of 2×1037 erg s-1. For example, a clear anti-correlation is found between the cutoff energy and the luminosity when its value is more than 2×1037 erg s-1. In addition, we performed the same analysis for 1 neutron star (NS) and 1 active galactic nuclei (AGN) for comparison. No clear differences between BHCs, NS and AGN were found, which might suggest that the accretion mechanisms in the low/hard states are independent of the mass of the central object.