Evolution of the hard X-ray photon index in black-hole X-ray binaries: hints for accretion physics. Issue 4 (1st July 2019)

• Record Type: Journal Article
• Authors: Liu, Hao; Dong, AiJun; Weng, ShanShan; Wu, Qingwen
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 487:Issue 4(2019)
• Page Start: 5335
• Page End: 5345
• Publication Date: 2019-07-01
• Subjects: accretion, accretion discs -- black hole physics -- stars: black holes -- X-rays: binaries; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz1622
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: Negative and positive correlations between the X-ray photon index and the Eddington-scaled X-ray luminosity were found in the decay phase of X-ray binary outbursts and a sample of active galactic nuclei in former works. We systematically investigate the evolution of the X-ray spectral index, along with the X-ray flux and Eddington ratio, in eight outbursts of four black-hole X-ray binaries, where all selected outbursts have observational data from the Rossi X-ray Timing Explorer in both rise and decay phases. In the initial rise phase, the X-ray spectral index is anticorrelated with the flux and the X-ray spectrum quickly softens when the X-ray flux is approaching the peak value. In the decay phase, the X-ray photon index and the flux follow two different positive correlations and they become anticorrelated again when the X-ray flux is below a critical value, where the anticorrelation part follows the same trend as found in the initial rise phase. Compared with other X-ray binaries, GRO J1655−40 has an evident lower critical Eddington ratio for the anticorrelation and positive transition, which suggests that its black-hole mass and distance are not well constrained, or its intrinsic physics is different.