Models for the Observable System Parameters of Ultraluminous X‐Ray Sources

Abstract

We investigate the evolution of model populations of ultraluminous X-ray sources (ULXs), consisting of a black hole accretor in a binary with a donor star. Two of the models we consider invoke stellar-mass (up to $25 M ) black hole binaries (LMBHs), generated with a binary population synthesis code, while a third model uses intermediatemass ($1000 M ) black hole accretors ( IMBHs). For each model, we computed 30,000 binary evolution sequences. A scheme for calculating the optical flux from ULXs is discussed. We present ''probability images'' for the colormagnitude diagrams (CMDs) and for the orbital periodYX-ray luminosity (P orb -L x ) plane. We show how a population of luminous X-ray sources in a cluster of stars evolves with time. The most probable ULX system parameters correspond to high-mass donors (of initial mass k25 M ) with effective O through late B spectral types, and P orb between 1 and 10 days. Estimates of the numbers of ULXs in a typical galaxy as a function of L x are also presented. We find that if LMBHs are allowed to have super-Eddington L x , the binding energy parameter for the stellar envelope of the black hole progenitor must be k P 0:03 in order not to overproduce ULXs. Comparison of six known ULX counterparts with our model CMDs indicates that the IMBH model somewhat more closely matches the observations. We find that a significant contribution to the optical flux from the IMBH systems comes from intrinsic accretion disk radiation. In effect, IMBH systems, when operating at their maximum luminosities (10 41 Y10 42 ergs s À1 ), are milli-AGNs. While models of IMBH systems during the X-ray phase are attractive, their formation mechanism remains uncertain.

The Effect of Starburst Metallicity on Bright X-Ray Binary Formation Pathways

“…Our simulations yield a number of key results: (1) the young, extragalactic HMXB population is dominated by systems moving through two specific pathways, RLO of MS donors onto reasonably sized BHs, and wind accretion from SG donors onto massive BHs formed via direct collapse (consistent with Van Bever & Vanbeveren 2000); (2) while metallicity does not greatly affect the characteristics of HMXBs moving through either pathway, it affects the relative strength (in number of systems) of each pathway, creating a total HMXB population which looks very different at different metallicities; (3) with mild allowances for super-Eddington accretion, the majority of the ULX population can be explained as an extension of the stellar HMXB population-consistent with both observations (Irwin et al 2004;Pakull & Mirioni 2002;Zampieri et al 2004;Swartz et al 2008) and theoretical models (Madhusudhan et al 2008;Mapelli et al 2009;Zampieri & Roberts 2009);and (4) as opposed to previous models, we find this phenomenon to be connected to the binary evolution of HMXB progenitors, rather than to the fact that low-metallicity stars produce more massive BHs.…”

“…While our own modeling codes agree with the correlation between low metallicity and large BH mass (see Belczynski et al 2006), there are several problems in correlating this result to explain the ULX population, as done in Madhusudhan et al (2008), Mapelli et al (2009), andRoberts (2009). It is important to note that ULX-HMXBs form from only a very small percentage of the initial binary population and are thus likely to have properties quite different than the average binary (or average HMXB for that matter).…”

“…Because some of these systems have isotropic luminosities above the Eddington luminosity for black holes (BHs) thought to be produced via single-star evolution (SSE), much work has centered on the possibility that ULXs contain an intermediate-mass BH (IMBH; Colbert & Mushotzky 1999; van der Marel 2004; Madhusudhan et al 2008). Other arguments point to ULXs as a natural extension of the HMXB population (Grimm et al 2003b).…”

Ultraluminous X-Ray Sources in Arp 147

“…This is demonstrated in Figure 4, which shows a sample of the results of the population synthesis study of candidate ULXs carried out by Madhusudhan et al (2008;see also Portegies Zwart et al 2004b for a related earlier study). The candidate ULXs were (1) conventional highmass X-ray binaries consisting of a massive donor star initially of mass 10-34 M and an accreting stellar-mass, i.e., 6-24 M , black hole, and (2) donor stars initially of mass 5-50 M feeding a 1000 M black hole (IMBH).…”

“…At the higher end of the ULX luminosity function, i.e., at L x 10 40 erg s −1 , and especially as the inferred luminosities approach ∼10 41 erg s −1 (e.g., ESO 243-49;Farrell et al 2009;Godet et al 2009), it becomes increasingly difficult to see how the requisite emission, even if somewhat beamed, could be produced around a stellar-mass black hole (see, e.g., Madhusudhan et al 2008). By contrast, the X-ray luminosities of accreting IMBHs could easily exceed 10 41 erg s −1 without violating the Eddington limit, but this explanation of ULXs is confronted by other serious problems.…”