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

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Singha A. C. , Devi A. S.

tom 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.