A study of low-mass x-ray binaries in the low-luminosity regime

Sonbas, E. and Dhuga, K. S. and Göğüş, Ersin (2018) A study of low-mass x-ray binaries in the low-luminosity regime. Astrophysical Journal, 853 (2). ISSN 0004-637X (Print) 1538-4357 (Online)

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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 1034-1037 erg s-1. 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., ∼a few × 1030 erg s-1) 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 1032-1033 erg s-1. Our analysis suggests a steep slope for the correlation i.e., -2.12 ± 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.
Item Type: Article
Uncontrolled Keywords: methods: data analysis; stars: black holes; stars: neutron; X-rays: binaries Supporting material: machine-readable table
Divisions: Faculty of Engineering and Natural Sciences
Depositing User: Ersin Göğüş
Date Deposited: 22 May 2023 15:08
Last Modified: 22 May 2023 15:08
URI: https://research.sabanciuniv.edu/id/eprint/45647

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