A comparative study of occurrence rates and nature of Ultraluminous X-ray sources in spiral and elliptical galaxies

Abstract

Ultraluminous X-ray sources (ULXs) are mostly extragalactic non-nuclear point sources having X-ray luminosity exceeding the Eddington luminosity of 10 $M_{\odot }$ black hole i.e., $L_X\ge$ 10${}^{39}$ erg ~s${}^{-1}$. They are observed in all types of galaxies; spirals, ellipticals and dwarf irregulars. But the rate of occurrence of ULXs per galaxy varies, some might host a single ULX, whereas some host a large number. In this work we attempt to identify possible differences in ULX properties between two extreme categories in spirals and ellipticals, i.e. ULXs occurring at a rate of one per galaxy ($N=1$) and those occurring at larger rate. We adopt an effective scheme to generate flux limited, credible samples corresponding to the two groups in spirals and ellipticals. From this study, we infer the presence of a separate population of ULXs in the $N=1$ spiral group which contains a reasonable fraction of both soft and hard sources, while the remaining categories contain mostly harder sources. We also find six ULXs in $N=1$ ellipticals with globular cluster association. In addition, we identify few luminous candidates likely hosting massive accretors. This study provides crucial hints of a potential link between ULX types and their occurrence rates and host morphology, a finding that warrants validation via targeted observations and detailed spectral analysis of these sources.

14 pages, 7 figures

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