FIELD AND GLOBULAR CLUSTER LOW-MASS X-RAY BINARIES IN NGC 4278
- Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
- School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ (United Kingdom)
- Laboratory for X-ray Astrophysics, NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)
- Sub-Department of Astrophysics, University of Oxford, Oxford OX1 3RH (United Kingdom)
- Department of Astronomy, University of Wisconsin, Madison, WI 53706-1582 (United States)
- Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208 (United States)
- Theoretical Astrophysics Group, University of Leicester, Leicester LE1 7RH (United Kingdom)
- Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824-2320 (United States)
- Dipartimento di Astronomia, Universita di Bologna, Via Ranzani 1, 40127 Bologna (Italy)
- INAF-Osservatorio Astronomico di Brera, Via Brera 28, 20212 Milano (Italy)
We report a detailed spectral analysis of the population of low-mass X-ray binaries (LMXBs) detected in the elliptical galaxy NGC 4278 with Chandra. Seven luminous sources were studied individually, four in globular clusters (GCs) and three in the stellar field. The range of (0.3-8 keV) L{sub X} for these sources is {approx}(3-8) x 10{sup 38} erg s{sup -1}, suggesting that they may be black hole binaries (BHBs). Fitting the data with either single thermal accretion disk or power-law (PO) models results in best-fit temperatures of {approx}0.7-1.7 keV and {Gamma} {approx} 1.2-2.0, consistent with those measured in Galactic BHBs. Comparison of our results with simulations allows us to discriminate between disk and power-law-dominated emission, pointing to spectral/luminosity variability, reminiscent of Galactic BHBs. The BH masses derived from a comparison of our spectral results with the L{sub X} {approx} T {sup 4}{sub in} relation of Galactic BHBs are in the 5-15 M{sub sun} range, as observed in the Milky Way. The analysis of joint spectra of sources selected in three luminosity ranges (L{sub X} {>=} 1.5 x 10{sup 38} erg s{sup -1}, 6 x 10{sup 37}ergs{sup -1} {<=} L{sub X} < 1.5 x 10{sup 38} erg s{sup -1}, and L{sub X} < 6 x 10{sup 37} erg s{sup -1}) suggests that while the high-luminosity sources have prominent thermal disk emission components, power-law components are likely to be important in the mid- and low-luminosity spectra. Comparing low-luminosity average spectra, we find a relatively larger N{sub H} in the GC spectrum; we speculate that this may point to either a metallicity effect or to intrinsic physical differences between field and GC accreting binaries. Analysis of average sample properties uncovers a previously unreported L{sub X}-R{sub G} correlation (where R{sub G} is the galactocentric radius) in the GC-LMXB sample, implying richer LMXB populations in more central GCs. No such trend is seen in the field LMXB sample. We can exclude that the GC L{sub X}-R{sub G} correlation is the by-product of a luminosity effect and suggest that it may be related to the presence of more compact GCs at smaller galactocentric radii, fostering more efficient binary formation.
- OSTI ID:
- 21476673
- Journal Information:
- Astrophysical Journal, Vol. 725, Issue 2; Other Information: DOI: 10.1088/0004-637X/725/2/1824; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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