Browsing by Author "Randall, SW"

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    Chandra observation of diffuse gas and low-mass X-ray binaries in the elliptical galaxy NGC 4649 (M60)
    (IOP Publishing, 2004-01-10) Randall, SW; Sarazin, CL; Irwin, JA; University of Virginia; University of Michigan System; University of Michigan; University of Alabama Tuscaloosa
    We present a Chandra X-ray observation of the X-ray bright E2 elliptical galaxy NGC 4649. In addition to bright diffuse emission, we resolve 165 discrete sources, most of which are presumably low-mass X-ray binaries. As found in previous studies, the luminosity function of the resolved sources is well-fitted by a broken power law. In NGC 4697 and NGC 1553, the break luminosity was comparable to the Eddington luminosity of a 1.4 M-. neutron star. One possible interpretation of this result is that those sources with luminosities above the break are accreting black holes and those below are mainly accreting neutron stars. The total X-ray spectrum of the resolved sources is well fitted by a hard power law, while the diffuse spectrum requires a hard and a soft component, presumably due to the relatively soft diffuse gas and the harder unresolved sources. We also find evidence for structure in the diffuse emission near the center of NGC 4649. Specifically, there appear to be bright "fingers'' of emission extending from the center of the galaxy and a 500 long bar at the center of the galaxy. The fingers are morphologically similar to radial features seen in two-dimensional hydrodynamic simulations of cooling flows in elliptical galaxies, and although their other properties do not match the predictions of the particular simulations used, we conclude that the radial fingers might be due to convective motions of hot outflowing gas and cooler inflowing gas. The bar is coincident with the central extended radio source; we conclude that the bar may be caused by weak shocks in the diffuse gas from an undetected low-luminosity active galactic nucleus.
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    Low-mass X-ray binaries and globular clusters in early-type galaxies
    (University of Chicago Press, 2003-10-01) Sarazin, CL; Kundu, A; Irwin, JA; Sivakoff, GR; Blanton, EL; Randall, SW; University of Virginia; Michigan State University; University of Michigan System; University of Michigan; University of Alabama Tuscaloosa
    A high fraction of the low-mass X-ray binaries (LMXBs) in early-type galaxies are associated with globular clusters (GCs). Here we discuss the correlations between LMXBs and GCs in a sample of four early-type galaxies with X-ray source lists determined from Chandra observations. There is some evidence that the fraction of LMXBs associated with GCs (f(X-GC)) increases along the Hubble sequence from spiral bulges ( or spheroids) to S0s to Es to cDs. On the other hand, the fraction of GCs that contain X-ray sources appears to be roughly constant at f(GC-X) similar to 4%. There is a strong tendency for the X-ray sources to be associated with the optically more luminous GCs. However, this correlation is consistent with a constant probability of finding an LMXB per unit optical luminosity; that is, it seems to result primarily from the larger number of stars in optically luminous GCs. The probability of finding a bright LMXB per unit optical luminosity in the GCs is about 1.5 x 10(-7) LMXBs per L-.,L-I for L-X greater than or similar to 1 x 10(38) ergs s(-1) (0.3 - 10 keV) and rises to about 2.0 x 10(-7) LMXBs per L-.; I at lower X-ray luminosities, L-X greater than or similar to 3 x 10(37) ergs s(-1). This frequency appears to be roughly constant for different galaxies, including the bulges of the Milky Way and M31. There is a tendency for the X-ray sources to be found preferentially in redder GCs, which is independent of optical luminosity correlation. This seems to indicate that the evolution of X-ray binaries in a GC is affected by either the metallicity or the age of the GC, with younger and/or more metal rich GCs having more LMXBs. There is no strong difference in the X-ray luminosities of GC and non-GC LMXBs. There is a weak tendency for the brightest LMXBs, whose luminosities exceed the Eddington luminosity for a 1.4 M-. neutron star, to avoid GCs. That may indicate that black hole X-ray binaries are somewhat less likely to be found in GCs, as seems to be true in our Galaxy. On the other hand, there are some luminous LMXBs associated with GCs. There is no clear evidence that the X-ray spectra or variability of GC and non-GC X-ray sources differ. We also find no evidence for a difference in the spatial distribution of GC and non-GC LMXBs. Many of these results are similar to those found in NGC 1399 and NGC 4472 by Angelini et al. and Kundu et al., respectively.