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2006ApJ...649..730W - Astrophys. J., 649, 730-752 (2006/October-1)

XMM-Newton archival study of the ultraluminous X-ray population in nearby galaxies.

WINTER L.M., MUSHOTZKY R.F. and REYNOLDS C.S.

Abstract (from CDS):

We present the results of an archival XMM-Newton study of the bright X-ray point sources (LX>1038 ergs/s) in 32 nearby galaxies. From our list of approximately 100 point sources, we attempt to determine if there is a low-state counterpart to the ultraluminous X-ray (ULX) population, searching for a soft-hard state dichotomy similar to that known for Galactic X-ray binaries and testing the specific predictions of the intermediate-mass black hole (IMBH) hypothesis. To this end, we searched for low-state'' objects, which we defined as objects within our sample that had a spectrum well fitted by a simple absorbed power law, and high-state'' objects, which we defined as objects better fitted by a combined blackbody and a power law. Assuming that low-state objects accrete at approximately 10% of the Eddington luminosity (as found by Done & Gierlinski) and that high-state objects accrete near the Eddington luminosity, we further divided our sample of sources into low- and high-state ULX sources. We classify 16 sources as low-state ULXs and 26 objects as high-state ULXs. As in Galactic BH systems, the spectral indices, Γ, of the low-state objects, as well as the luminosities, tend to be lower than those of the high-state objects. The observed range of blackbody temperatures for the high state is 0.1-1 keV, with the most luminous systems tending toward the lowest temperatures. We therefore divide our high-state ULXs into candidate IMBHs (with blackbody temperatures of approximately 0.1 keV) and candidate stellar mass BHs (with blackbody temperatures of approximately 1.0 keV). A subset of the candidate stellar mass BHs have spectra that are well fitted by a Comptonization model, a property similar to Galactic BHs radiating in the very high'' state near the Eddington limit.

Journal keyword(s): Accretion, Accretion Disks - Galaxies: General - Surveys - X-Rays: Binaries

VizieR on-line data: <Available at CDS (J/ApJ/649/730): table1.dat table6.dat table10.dat>

Nomenclature: Table 6: [WMR2006] NGCNNNN XMMN, [WMR2006] ICNNNN XMMN, [WMR2006] Mrk71 XMMN, [WMR2006] Ho R XMMN, [WMR2006] MNNN XMMN, [WMR2006] Sextans A XMMN, [WMR2006] Circinus XMMN, N=109.

CDS comments: table 7 : Star in NGC 5408 = EQ J1403-4125