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arXiv · 2304.11216

Statistical study of a large and cleaned sample of ultraluminous and hyperluminous X-ray sources

Hugo Tranin·Natalie Webb·Olivier Godet·Erwan Quintin
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Abstract

Ultra-/hyper-luminous X-ray sources (ULX/HLX) could be interesting laboratories to improve our understanding of the supermassive black hole growth through super-Eddington accretion episodes and successive mergers of lighter holes. ULXs are thought to be powered by super-Eddington accretion on stellar-mass compact objects, while HLXs may be explained by accretion on intermediate-mass black holes (IMBHs). However, a significant portion of ULX/HLX derived from catalog searches are background AGN. Here we build samples of ULX/HLX from recent XMM-Newton, Swift-XRT and Chandra catalogs and the GLADE catalog of galaxies. We aim to characterize the frequency, environment and X-ray properties of ULXs and HLXs to better assess their differences and understand their populations. After a thorough classification of these X-ray sources, we remove 42% of S/N>3 sources, shown to be contaminants, to obtain the cleanest sample of ULX/HLX to date. From a sample of 1342 ULXs and 191 HLXs, we study their frequency, hardness, variability, radial distribution, and preferred environment. We build their Malmquist-corrected X-ray luminosity functions (XLF) and compare them with previous studies. The large sample size allows us to statistically compare ULXs and HLXs and assess differences in their nature. The interpretation of HLXs as IMBHs is investigated. A significant break is seen in the XLF at ~$10^{40}$erg/s. Our ULX sample, containing ~2% of contaminants, confirms that ULXs are located preferentially in spiral or star-forming galaxies. Some HLXs are significantly softer that ULXs and XRBs. Unlike ULXs, HLXs seem to reside equally in spiral as well as lenticular and elliptical galaxies and 35% of them have an optical counterpart. We estimate their mass and find 120 of them in the range of $2000-10^5 M_\odot$. Most HLXs seem consistent with an accreting massive black hole in a dwarf galaxy satellite.

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Hugo Tranin, Natalie Webb, Olivier Godet, Erwan Quintin. 2023-10-25. Statistical study of a large and cleaned sample of ultraluminous and hyperluminous X-ray sources. https://doi.org/10.1051/0004-6361%2F202244952

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