The X-ray spectra of the luminous LMXBs in NGC 3379 : field and globular cluster sources

Abstract

From a deep multi-epoch Chandra observation of the elliptical galaxy NGC 3379 we report the spectral properties of eight luminous LMXBs (L-X > 1.2 x 10(38) erg s(-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 similar to 0.7 and 1.55 keV. The large variability observed in the brightest GC source (L-X > 4 x 10(38) erg s(-1)) suggests the presence of a BH binary. At its most luminous this source is observed in a TD state with kT(in) = 1.5 keV, consistent with a BH mass of similar to 4 M-circle dot. 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-X similar to 1 x 10(39) erg s(-1)), with prominent thermal and non-thermal components, to a less luminous hard state (L-X = 3.8 x 10(38) erg s(-1), Gamma = 1.85). In its high flux emission, this source exhibits a cool-disk component of similar to 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.