Ultraluminous X-Ray Sources in External Galaxies

Abstract

We investigate models for the class of ultraluminous non-nuclear X-ray sources (ULXs) seen in a number of galaxies and probably associated with star-forming regions. Models where the X-ray emission is assumed to be isotropic run into several difficulties. In particular formation of sufficient numbers of the required ultramassive black-hole X-ray binaries is problematic, and the likely transient behaviour of the resulting systems is not in good accord with observation. The assumption of mild X-ray beaming suggests instead that ULXs may represent a shortlived but extremely common stage in the evolution of a wide class of X-ray binaries. The best candidate for this is the phase of thermal-timescale mass transfer inevitable in many intermediate and high-mass X-ray binaries. This in turn suggests a link with the Galactic microquasars. The short lifetimes of high-mass X-ray binaries would explain the association of ULXs with episodes of star formation. These considerations still allow the possibility that individual ULXs may contain extremely massive black holes.Comment: 4 pages, no figures; accepted for ApJ Letter

From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link

“…In the last decade it has been proposed that a substantial fraction of the population of ultraluminous X-ray sources (ULXs) and ultraluminous supersoft sources (ULSs) are powered by super-Eddington accretion onto compact objects such as neutron stars and black holes (see, e.g., King et al 2001, Roberts 2007, Urquhart and Soria 2016, Feng et al 2016, and references therein). In particular, ULSs could be a category of ULXs observed at high inclination angles, possibly edge on, where a thick layer of material is obscuring the innermost hard X-ray emitting regions (e.g., Kylafis &Xilouris 1993 andPoutanen et al 2007).…”

X-ray observations of ultraluminous X-ray sources

“…M < 20M ⊙ ) one then needs to either make such objects actually break the Eddington limit, or to make them apparently exceed it. Lower-luminosity and/or Eddington-limited objects could appear as bright ULXs due to beaming, either through relativistic boosting of their X-ray emission along our line-of-sight (Körding, Falcke & Markoff 2002) or through collimation of their radiation -probably by a geometricallythick accretion disc -such that it only escapes into a fraction of the sky (King et al 2001). 4 Alternatively, models have been suggested whereby actual superEddington luminosities are achieved and maintained, at factors < ∼ 10 above the Eddington limit (Begelman 2002;Ebisawa et al 2003;Heinzeller & Duschl 2007).…”

The Luminous X‐Ray Source Population in M51 Observed withChandra

“…One of the most intriguing and puzzling classes of X-ray source comprises the so-called ultraluminous X-ray sources (ULXs), whose luminosities (>10 39 ergs s À1 , sometimes greater than 10 40 ergs s À1 ) considerably exceed the Eddington luminosity of a neutron star (e.g., Fabbiano 1996;Makishima et al 2000 and references therein). An ULX may be a stellar mass black hole radiating near or above the Eddington luminosity (Begelman 2002), a stellar mass black hole or neutron star with beamed X-ray emission (Reynolds et al 1997;King et al 2001;Körding, Falcke, & Markoff 2002), or an intermediate-mass ($100 M or higher, but smaller than those in AGNs) black hole radiating near or lower than the Eddington luminosity Kaaret et al 2001). …”