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Browsing by Author "Liu, JF"

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    A 2 hour quasi period in an ultraluminous X-ray source in NGC 628
    (IOP Publishing, 2005-03-01) Liu, JF; Bregman, JN; Lloyd-Davies, E; Irwin, J; Espaillat, C; Seitzer, P; University of Michigan System; University of Michigan; University of Alabama Tuscaloosa
    Quasi-periodic oscillations and X-ray spectroscopy are powerful probes of black hole masses and accretion disks, and here we apply these diagnostics to an ultraluminous X-ray source (ULX) in the spiral galaxy NGC 628 (M74). This object was observed four times over 2 years with the Chandra X-Ray Observatory and XMM-Newton, with three long observations showing dramatic variability, distinguished by a series of outbursts with a quasi period of 4000-7000 s. This is unique behavior among ULXs and Galactic X-ray binaries because of the combination of its burstlike peaks and deep troughs, its long quasi periods, its high variation amplitudes of >90%, and its substantial variability between observations. The X-ray spectra is fitted by an absorbed accretion disk plus a power-law component, suggesting the ULX was in a spectral state analogous to the low/hard state or the very high state of Galactic black hole X-ray binaries. A black hole mass of similar to(2-20) x 10(3) M-circle dot is estimated from the f(b)-M-circle scaling relation found in the Galactic X- ray binaries and active galactic nuclei.
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    An ultraluminous X-ray object with a 2 hour period in M51
    (University of Chicago Press, 2002-12-20) Liu, JF; Bregman, JN; Irwin, J; Seitzer, P; University of Michigan System; University of Michigan; University of Alabama Tuscaloosa
    Ultraluminous X-ray objects (ULXs) are off-nucleus point sources with L-X = 10(39)-10(41) ergs s(-1), but the nature 39 of such systems is largely unidentified. Here we report a 2.1 hr period observed in a Chandra ACIS observation for ULX M51 X-7, which is located on the edge of a young star cluster in the star-forming region in a spiral arm. In two ACIS observations separated by 1 yr, the ULX changed from a high-hard to a low-soft spectral state, in contrast to most Galactic low-mass X-ray binaries. On the basis of its period and spectral behaviors, we suggest that this ULX is a low-mass X-ray binary system, with a dwarf companion of 0.2-0.3 M. and a compact accretor, either a neutron star or a black hole, whose mass is not well constrained. Relativistic beaming effects are likely involved to produce the observed high X-ray luminosities, given its low accretion rate as inferred from a sustainable accretion scenario via Roche lobe overflow.
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    Ultraluminous X-ray sources in nearby galaxies from ROSAT HRI observations. II. Statistical properties
    (IOP Publishing, 2006-05-01) Liu, JF; Bregman, JN; Irwin, J; University of Michigan System; University of Michigan; University of Alabama Tuscaloosa
    The statistical properties of the nonnuclear X-ray point sources from the ROSAT HRI survey of nearby galaxies in Paper I are studied, with particular attention to the contamination from background and/or foreground objects. This study reveals a statistical preference for the ultraluminous X-ray sources (ULXs) to occur in late-type galaxies over early-type galaxies, and in starburst/H (II) galaxies over nonstarburst galaxies. There is a trend of greater occurrence frequencies and ULX rates for galaxies with increasing star formation rates, confirming the connection between the ULX phenomenon and the star formation. A nonlinear correlation is found between the number of ULXs and the star formation rate, with significantly more ULXs at low star formation rates than the ULX population expected from the high-mass X-ray binaries (HMXBs) as an indicator of the star formation and the accompanying young stellar population, suggestive of another population of ULXs associated with the low-mass X-ray binaries (LMXBs) and the old stellar population. There are no breaks around 10(39) ergs s(-1) in the luminosity functions of ULXs in all galaxies or in late-type galaxies, suggesting the regular ULXs below 10(40) ergs s(-1) are a high-luminosity extension of the ordinary HMXB/LMXB populations below 10(39) ergs s(-1). There is evidence that the extreme ULXs above 10(40) ergs s(-1) might be a different ULX class from the regular ULXs below 10(40) ergs s(-1), although a larger sample with more ULXs is needed to establish the statistical properties of the extreme ULXs as a class.