Hintze Centre for Astrophysical Surveys

Abstract:

We report a detailed spectral analysis of the population of low-massX-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) LX for these sources is ∼(3-8) × 10³⁸ erg s^-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 ∼0.7-1.7 keV and Λ ∼ 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-lawdominated emission, pointing to spectral/luminosity variability, reminiscent of Galactic BHBs. The BH masses derived from a comparison of our spectral results with the LX ≥ T in ⁴ relation of Galactic BHBs are in the 5-15M⊙ range, as observed in the Milky Way. The analysis of joint spectra of sources selected in three luminosity ranges (LX ≥ 1.5 × 10³⁸ erg s^-1, 6 × 10³⁷ erg s^-1 ≤ LX < 1.5 × 10³⁸ erg s^-1, and LX < 6 × 10³⁷ erg s^-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 NH 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 X-RG correlation (where RG 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 LX-RG 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. © 2010. The American Astronomical Society. All rights reserved.

Abstract:

From a deep multi-epoch Chandra observation of the elliptical galaxy NGC 3379 we report the spectral properties of eight luminous LMXBs (LX < 1.2 × 10³⁸ 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 ∼0.7 and 1.55 keV. The large variability observed in the brightest GC source (LX< 4×10³⁸ erg s^-1) suggests the presence of a BH binary. At its most luminous this source is observed in a TD state with kTin = 1.5 keV, consistent with a BH mass of ∼4M ⊙. This observation provides further evidence that GCs are able to retain such massive binaries. We also observed a source transitioning from a bright state (LX ∼ 1 × 10³⁹ erg s^-1), with prominent thermal and non-thermal components, to a less luminous hard state (LX = 3.8 × 10³⁸ erg s^-1, Λ = 1.85). In its high flux emission, this source exhibits a cool-disk component of ∼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. © 2010. The American Astronomical Society. All rights reserved.