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Bimodal Distribution of the Hyperluminous X-ray Sources

Singha A. Ch., Devi A. S.

Acta Astronomica

2019

Vol. 69, No. 4

339--360

163 X-ray point sources having luminosities in excess of 1038erg/s within the energy range 0.3-10.0 keV are identified in a sample of 50 galaxies observed by Chandra ACIS-S. 18 sources are found to have a total (time integrated) background flux greater than 2 cnts/arcsec2 and hence excluded from the analysis to avoid any possible background/foreground contamination. The spectra of the remaining 145 sources are fitted using two spectral models - an absorbed power law and an absorbed disk blackbody. We report the discovery of four hyperluminous X-ray source candidates (X-ray luminosity: LX>1041 erg/s) - Src1, Src2, Src3 and Src22. In this study, a total of 17 hyperluminous X-ray sources were identified. The spectra of 13 sources were found to be equally represented by both the models except for three sources preferring the power law model and one source preferring the disk blackbody model. It is found that the spectra of seven hyperluminous X-ray sources are in the soft state with their inner disk temperature (kTin) lying in the range of 0.19-0.56 keV (or power law photon index, Γ>3) and the remaining 10 hyperluminous X-ray sources are in hard state with kTin around 1.0-2.0 keV (or power law photon index, Γ≈1.6). This shows a clear bimodal distribution of the high luminosity X-ray sources and hence they can be grouped into two distinct spectral classes. From the disk blackbody model, the black hole masses of the soft hyperluminous X-ray sources are estimated (assuming the inner disk radius, Rin≈10GM/c2) to be within ≈103-104 M☉. Also considering an anisotropic emission, for maximum beaming (η=5) with no color correction, the estimated black hole masses lie within ≈102-103 M☉, which is in the intermediate mass black hole range.