Abstract
We investigate models for the class of ultraluminous nonnuclear X-ray sources (i.e., ultraluminous compact X-ray sources [ULXs]) seen in a number of galaxies and probably associated with star-forming regions. Models in which the X-ray emission is assumed to be isotropic run into several difficulties. In particular, the formation of sufficient numbers of the required ultramassive black hole X-ray binaries is problematic, and the likely transient behavior 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 short-lived 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 that is 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.
Highlights
The existence in spiral galaxies of off-nuclear X-ray sources whose luminosities appear significantly larger than the Eddington limit for a 1 M, object has been known for some time (Fabbiano 1989)
It is possible that a moderate-mass black hole produced in a central cluster of M82 has gained a stellar companion by some dynamical process, this scenario is unlikely to work for the systems observed in the Antennae, where the ultraluminous compact X-ray sources (ULXs) are observed to be strongly associated with the young star clusters that are located some distance from the galactic nuclei (Fabbiano et al 2001)
A population of ∼100 M, black holes originating from a much earlier generation of effectively zero-metallicity stars seems unlikely to explain the ULXs observed in the Antennae (Fabbiano et al 2001) since these black holes would be distributed throughout the galactic halo, and so the probability of picking up stars from the young stellar clusters via dynamical encounters within the last ∼107 yr is extremely low
Summary
The existence in spiral galaxies of off-nuclear X-ray sources whose luminosities appear significantly larger than the Eddington limit for a 1 M, object has been known for some time (Fabbiano 1989). We denote the mean observed number of ULXs per galaxy as n, the beaming factor as b (pQ/4p, where Q is the solid angle of emission), the duty cycle (which equals the time that the source is active as a fraction of its lifetime) as d, and define an “acceptance rate” a as the ratio ocrfemtioanssraatcecrMet1edpbay(ϪMM1 ̇ t2o). M2, i.e., define L the mean sph as the acapparent X-ray (assumed bolometric) luminosity of a source, given by the assumption of isotropic (spherical) emission, and let L 40 p L sph /1040 ergs sϪ1. It is important to note here that the required birthrate is independent of beaming (and the duty cycle): the greater intrinsic source population N required by beamed sources with bd ! 1 (see eq [4]) is compensated by their longer lifetimes (see eq [7])
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