Abstract
We consider observational tests for the nature of ultraluminous X-ray (ULX) sources. These tests must distinguish between thermal-timescale mass transfer onto stellar-mass black holes, leading to anisotropic X-ray emission, and accretion onto intermediate-mass black holes. We suggest that long-term transient behavior via the thermal-viscous disk instability could discriminate between these two possibilities for ULX sources in regions of young stellar populations. Thermal-timescale mass transfer generally produces stable disks and persistent X-ray emission. In contrast, mass transfer from massive stars to black holes produces unstable disks and thus transient behavior, provided that the black hole mass exceeds some minimum value MBH, min. This minimum mass depends primarily on the donor mass and evolutionary state. We show that MBH, min 50 M☉ for a large fraction (90%) of the mass transfer lifetime for the most likely donors in young clusters. Thus, if long-term monitoring reveals a large transient fraction among ULX sources in a young stellar population, these systems would be good candidates for intermediate-mass black holes in a statistical sense; information about the donor star is needed to make this identification secure in any individual case. A transient ULX population would imply a much larger population of quiescent systems of the same type.
Highlights
In the past few years high-angular–resolution observations with Chandra have revolutionized the study of X–ray binaries in nearby galaxies and have revealed whole populations of sources in a variety of galaxy types
In this Letter we suggest that long-term transient behavior due to the thermal-viscous disk instability (King, Kolb, & Burderi 1996; King & Ritter 1998) may distinguish the two possibilities for ultraluminous X-ray sources (ULXs) in regions of young stellar populations ( 108 yr)
If the black holes (BH) mass used in the mass transfer calculations exceeds this minimum the system will be transient. (Note that for a given sequence, Mdepends most sensitively on the donor mass and evolutionary stage at the onset of mass transfer and not so much on the accretor mass.) We examine the range of values for MBH,min and whether transient behavior can be associated with certain types of BH accretors (§ 3.2)
Summary
In the past few years high-angular–resolution observations with Chandra have revolutionized the study of X–ray binaries in nearby galaxies and have revealed whole populations of sources in a variety of galaxy types (for a recent review see Fabbiano & White 2003). The inferred X–ray luminosities reveal a distinct class of sources: non–nuclear point sources with apparent X–ray luminosities above the Eddington limit for a ∼ 10 M⊙ black hole ( 2 × 1039 erg s−1), often referred to as ultraluminous X-ray sources (ULXs). The existence of such sources was first noted in EINSTEIN observations (e.g., Fabbiano 1988). At present the majority of ULXs have been found mainly in young stellar populations and regions of recent star formation, a few have been identified in elliptical galaxies (e.g., Sarazin, Irwin, & Bregman 2001; Colbert & Ptak 2002) with luminosities close to the lower end of the ULX range
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