THE X-RAY SPECTRA OF THE LUMINOUS LMXBs IN NGC 3379: FIELD AND GLOBULAR CLUSTER SOURCES

N J Brassington,G Trinchieri,G Fabbiano,R L Davies,A R King,A Zezas,S Zepf,J Gallagher,D.-W Kim,S Blake,V Kalogera,L Angelini,A Kundu

doi:10.1088/0004-637x/725/2/1805

Abstract

From a deep multi-epoch Chandra observation of the elliptical galaxy NGC 3379 we report the spectral properties of eight luminous LMXBs (LX>1.2E38 erg/s). 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, and one is a confirmed field source. Due to the nature of the luminosity cut all sources are either neutron star binaries emitting at or above the Eddington luminosity or black hole 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 dominated sources have inner disc temperatures between ~0.7-1.55 keV. The large variability observed in the brightest globular cluster source (LX>4E38 erg/s) suggests the presence of a black hole binary. At its most luminous this source is observed in a thermally dominated state with kT=1.5 keV, consistent with a black hole mass of ~4 Msol. This observation provides further evidence that globular clusters are able to retain such massive binaries. We also observed a source transitioning from a bright state (LX~1E39 erg/s), with prominent thermal and non-thermal components, to a less luminous hard state (LX=3.8E38 erg/s, Gamma=1.85). In its high flux emission this source exhibits a cool-disc component of ~0.14 keV, similar to spectra observed in some ultraluminous X-ray sources. Such a similarity indicates a possible link between `normal' stellar mass black holes in a high accretion state and ULXs.

Highlights

The discovery with Chandra of several low-mass X-ray binary (LMXB) populations in early-type galaxies and the associations of these LMXBs with either globular clusters (GCs) or the stellar field have provided new impetus to the study of the formation and evolution of LMXBs in GCs and to the possible relation of field LMXBs to the GC population (Grindlay & Hertz 1985; Verbunt & van den Heuvel 1995; see Fabbiano 2006 and references therein)
We report the results of the spectral analysis of eight luminous sources (LX > 1.2 × 1038 erg s−1 in the 0.3–8.0 keV band), detected in the nearby elliptical galaxy NGC 3379 with Chandra ACIS-S (Weisskopf et al 2000)
From the spectral analysis of the eight sources presented in this paper, a diverse range of temporal characteristics has been identified in the bright (LX 1.2 × 1038 erg s−1) LMXB population of NGC 3379

Summary

Introduction

The discovery with Chandra of several low-mass X-ray binary (LMXB) populations in early-type galaxies and the associations of these LMXBs with either globular clusters (GCs) or the stellar field have provided new impetus to the study of the formation and evolution of LMXBs in GCs and to the possible relation of field LMXBs to the GC population (Grindlay & Hertz 1985; Verbunt & van den Heuvel 1995; see Fabbiano 2006 and references therein). In the few cases of detection of luminous sources in deep enough observations, detailed spectral and variability studies can be pursued, to provide more direct constraints on the nature of the X-ray sources One such example is the recent discovery of a variable luminous GC source in NGC 4472, with temporal and spectral characteristics supporting a stellar black hole (BH) binary (Maccarone et al 2007; Shih et al 2008). We report the results of the spectral analysis of eight luminous sources (LX > 1.2 × 1038 erg s−1 in the 0.3–8.0 keV band), detected in the nearby elliptical galaxy NGC 3379 (in the poor group Leo, D = 10.6 Mpc; Tonry et al 200112) with Chandra ACIS-S (Weisskopf et al 2000) These sources were observed at five different epochs, as part of a monitoring campaign with Chandra (PI: Fabbiano) providing the rare opportunity of long-term spectral monitoring of LMXBs in an elliptical galaxy. These sources are part of the sample of 132 sources detected in NGC 3379 from these observations with luminosities greater than a few 1036 erg s−1 (Brassington et al 2008—hereafter B08)

Results

Discussion

Conclusion