A Study of Low-Mass X-Ray Binaries in the Low-Luminosity Regime

Abstract

A recent study of a small sample of X-ray binaries (XRBs) suggests a significant softening of spectra of neutron star (NS) binaries as compared to black hole (BH) binaries in the luminosity range 10${}^{34}$ - 10${}^{37}$ erg/s. This softening is quantified as an anticorrelation between the spectral index and the 0.5 - 10 keV X-ray luminosity. We extend the study to significantly lower luminosities (i.e., $\sim$ a few $\times$ $10^{30}$ erg/s) for a larger sample of XRBs. We find evidence for a significant anticorrelation between the spectral index and the luminosity for a group of NS binaries in the luminosity range 10${}^{32}$ to 10${}^{33}$ erg/s. Our analysis suggests a steep slope for the correlation i.e., -2.12 $\pm$ 0.63. In contrast, BH binaries do not exhibit the same behavior. We examine the possible dichotomy between NS and BH binaries in terms of a Comptonization model that assumes a feedback mechanism between an optically thin hot corona and an optically thick cool source of soft photons. We gauge the NS-BH dichotomy by comparing the extracted corona temperatures, Compton-y parameters and the Comptonization amplification factors: The mean temperature of the NS group is found to be significantly lower than the equivalent temperature for the BH group. The extracted Compton-y parameters and the amplification factors follow the theoretically predicted relation with the spectral index.

Accepted for publication in ApJ

References in corpus (8)

• X-ray Properties of Black-Hole Binaries
• Modelling the behaviour of accretion flows in X-ray binaries
• Cooling of the crust in the neutron star low-mass X-ray binary MXB 1659-29
• The Variable Quiescent X-ray Emission of the Neutron Star Transient XTE J1701-462
• A strongly heated neutron star in the transient Z source MAXI J0556-332
• Probing the Crust of the Neutron Star in EXO 0748-676
• A dichotomy between the hard state spectral properties of black hole and neutron star X-ray~binaries
• The thermal state of KS 1731-260 after 14.5 years in quiescence

Cited by in corpus (4)

• Chandra X-ray Observatory and Hubble Space Telescope observations of the intermediate-age cluster GLIMPSE-C01
• A model for the radio/X-ray correlation in three neutron star low-mass X-ray binaries 4U 1728-34, Aql X-1 and EXO 1745-248
• The radiative efficiency of neutron stars at low-level accretion
• The advection-dominated accretion flow for the anti-correlation between the X-ray photon index and the X-ray luminosity in neutron star low-mass X-ray binaries