Study of X-ray point sources in NGC 5643 and NGC 7457 with Chandra

Abstract

In the present study we have analysed Chandra Observational data of 2 galaxies: NGC 5643 and NGC 7457. Four point sources from NGC 5643 and two point sources from NGC 7457 with net counts ${\ge} 100$ were considered for the present study. The spectra of these sources were fitted using two spectral models- an absorbed powerlaw and an absorbed disk blackbody. The spectrum of all the sources can be explained almost equally by both the models. We report here the discovery of 3 Ultraluminous X-ray sources (ULXs), X-1, X-2 and X-3 in the galaxy NGC 5643 and one ULX, X-5 in the galaxy NGC 7457. The spectral parameters suggest that all the above four ULX sources are in hard state ($\varGamma \sim 1.42\mbox{--}1.86$), which may be due to thermal comptonization. If explained by absorbed diskblackbody model, the Black Hole (BH) mass of these sources are estimated to be stellar mass BHs with X-2, & X-5 accreting at super-Eddington rate while X-1 and X-3 at sub-Eddington rate. Another ULX, X-4 in NGC 5643 which is also accreting at super-Eddington rate is found to be a variable ULX with its luminosity reducing from $4.4 \times 10^{40}~\mbox{ergs}\,\mbox{s}^{-1}$ to $2.27 \times 10^{40}~\mbox{ergs}\,\mbox{s}^{-1}$ in the 0.3–10.0 keV energy range within a period of 11 years.