Browsing by Author "Seitzer, P"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    Galactic globular clusters with luminous X-ray binaries
    (IOP Publishing, 2006-03-20) Bregman, JN; Irwin, JA; Seitzer, P; Flores, M; University of Michigan System; University of Michigan; University of Alabama Tuscaloosa
    Luminous X-ray binaries (LMXBs; > 10(34) ergs s(-1)) have a neutron star or black hole primary, and in globular clusters, most of these close binaries are expected to have evolved from wider binaries through dynamical interactions with other stars. We attempt to find a predictor of this formation rate that is representative of the initial properties of globular clusters rather than of the highly evolved core quantities. Models indicate the half-light quantities best reflect the initial conditions, so we examine whether the associated dynamical interaction rate, proportional to L-1.5/r(h)(2.5), is useful for understanding the presence of luminous LMXBs in the Galactic globular cluster system. We find that while LMXB clusters with large values of L-1.5/r(h)(2.5) preferentially host LMXBs, the systems must also have half-mass relaxation times below t(h,relax) similar to 10(9) yr. This relaxation time effect probably occurs because several relaxation times are required to modify binary separations, a timescale that must be shorter than cluster ages. The likelihood of finding an LMXB cluster is enhanced if the cluster is metal-rich and if it is close to the bulge region. The dependence on metallicity is most likely either due to differing initial mass functions at the high-mass end or because bulge systems evolve more rapidly from tidal interactions with the bulge. This approach can be used to investigate globular cluster systems in external galaxies, where core properties are unresolved.
  • Thumbnail Image
    Item
    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.