Hierarchical Merging, Ultraluminous and Hyperluminous X-ray Sources
Abstract
Various arguments strongly suggest that the population of ultraluminous X-ray sources (ULXs: apparent X-ray luminosity > Eddington limit for 10 Msun ~1039 erg/s) in nearby galaxies are mostly stellar-mass X-ray binaries in unusual evolutionary stages. However there are indications that the very brightest systems may be difficult to explain this way. Accordingly we consider the class of hyperluminous X-ray sources (HLXs) (i.e. those with apparent bolometric luminosities >~1041 erg/s). Because this class is small (currently only the M82 object is a secure member) we do not need to invoke a new formation mechanism for its black holes. We explore instead the idea that HLXs may be the nuclei of satellite galaxies captured during hierarchical merging. The observed correlation between AGN and tidal interactions implies that HLX activity would switch on during passage through the host galaxy, close to pericentre. This suggests that HLXs should appear near the host galaxy, be associated with star formation, and thus possibly with ULXs.
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