THE X-RAY SPECTRA OF THE LUMINOUS LMXBs IN NGC 3379: FIELD AND GLOBULAR CLUSTER SOURCES
- 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)
- INAF-Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milan (Italy)
From a deep multi-epoch Chandra observation of the elliptical galaxy NGC 3379 we report the spectral properties of eight luminous LMXBs (L{sub X} >1.2 x 10{sup 38} erg s{sup -1}). We also present a set of spectral simulations, produced to aid the interpretation of low-count single-component spectral modeling. These simulations demonstrate that it is possible to infer the spectral states of X-ray binaries from these simple models and thereby constrain the properties of the source. Of the eight LMXBs studied, three reside within globular clusters (GCs) and one is a confirmed field source. Due to the nature of the luminosity cut, all sources are either neutron star (NS) binaries emitting at or above the Eddington luminosity or black hole (BH) binaries. The spectra from these sources are well described by single-component models, with parameters consistent with Galactic LMXB observations, where hard-state sources have a range in photon index of 1.5-1.9 and thermally dominant (TD) sources have inner-disk temperatures between {approx}0.7and1.55 keV. The large variability observed in the brightest GC source (L{sub X} >4 x 10{sup 38} erg s{sup -1}) suggests the presence of a BH binary. At its most luminous this source is observed in a TD state with kT{sub in} = 1.5 keV, consistent with a BH mass of {approx}4 M{sub sun}. This observation provides further evidence that GCs are able to retain such massive binaries. We also observed a source transitioning from a bright state (L{sub X} {approx}1 x 10{sup 39} erg s{sup -1}), with prominent thermal and non-thermal components, to a less luminous hard state (L{sub X} = 3.8 x 10{sup 38} erg s{sup -1}, {Gamma}= 1.85). In its high flux emission, this source exhibits a cool-disk component of {approx}0.14 keV, similar to spectra observed in some ultraluminous X-ray sources (ULXs). Such a similarity indicates a possible link between 'normal' stellar-mass BHs in a high accretion state and ULXs.
- OSTI ID:
- 21476672
- Journal Information:
- Astrophysical Journal, Vol. 725, Issue 2; Other Information: DOI: 10.1088/0004-637X/725/2/1805; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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