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Spectral and Temporal Analysis of Ultraluminous X-ray Sources in NGC 2276

Singha A. Ch., Devi A. S.

Acta Astronomica

2021

Vol. 71, No. 4

261--279

We present the results of spectral and temporal analysis of Ultraluminous X-ray sources (ULXs) present in the relatively nearby spiral galaxy NGC 2276 which has been observed five times by the Chandra ACIS-S over a time period of 16 yr. We detect seven X-ray point sources from the recent Chandra observations of 2019-2020 and compare the results with earlier observations from 2004 and 2013. The spectra of these sources are fitted using two empirical models- an absorbed power law and an absorbed disk blackbody. The X-ray luminosity of all these sources are estimated to be Lx≥ 1039 erg/s, thus falling under the ULX category, with five sources having Lx≈1040 erg/s. The power law photon index (Γ) and the inner disk temperature (kTin) of these sources (except for one - Src6) are found to be in the hard states implying the radiative mechanism of these sources to be the inverse comptonization of soft photons and the emission is dominated by the inner regions of the accretion disk. Comparing the long term luminosity of these sources, we observe variations in two sources while three sources have gradually increasing luminosity and two other sources have almost consistent luminosity. Short term variability is absent in these sources. Assuming the emission to be isotropic, we estimate the BH mass from the disk blackbody model. The upper limit of the BH mass of these seven sources are found to be in the stellar mass BHs range with MBH<70 M☉, accreting at higher rates than their respective Eddington limit.

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.