A global spectral study of black hole X-ray binaries

Abstract

(Abridged) We report on a consistent and comprehensive spectral analysis of\nthe X-ray emission of 25 Black Hole X-ray Binaries. All publicly available\nobservations of the black hole binaries in the RXTE archive were analysed.\nThree different types of model were fitted to investigate the spectral changes\noccurring during an outburst. For the population, as well as each binary and\neach outburst from each binary, we construct two diagnostic diagrams. The\nHardness Intensity/Luminosity Diagram (HID/HLD) is most useful when studying a\nsingle binary. However, to compare between different binary systems, the Disc\nFraction Luminosity diagram (DFLD) is more useful. We discuss the limitations\nof both diagnostic diagrams for the study of the X-ray binary outbursts, and we\nclearly illustrate how the two diagrams map onto each other for real outburst\ndata. We extract the peak luminosities in a single outburst, as well as the\nluminosities at the transitions away from- and returning to the powerlaw\ndominated state for each outburst. The distribution of the luminosities at the\ntransition from the powerlaw to the disc dominated state peaks at around\n0.3L_Edd, the same as the peak of the distribution of the peak luminosities in\nan outburst. Using the disc fraction to calculate the transition luminosities\nshows that the distributions of the luminosities for the transitions away from-\nand return to the powerlaw dominated state are both broad and appear to\noverlap. Finally we compare the measured X-ray luminosities with a small number\nof contemporaneous radio measurements. Overall this is the most comprehensive\nand uniform global study of black hole X-ray binaries to date.\n