THE MEGASECOND CHANDRA X-RAY VISIONARY PROJECT OBSERVATION OF NGC 3115. II. PROPERTIES OF POINT SOURCES

Abstract

We carried out an in-depth study of low-mass X-ray binaries (LMXBs) detected in the nearby lenticular galaxy NGC 3115 using the Megasecond Chandra X-ray Visionary Project observation (total exposure time 1.1 Ms). In total we found 136 candidate LMXBs in the field and 49 in globular clusters (GCs) above 2σ detection, with 0.3–8 keV luminosity L[subscript X] ~ 10[superscript 36]–10[superscript 39] erg s[superscript −1]. Other than 13 transient candidates, the sources overall have less long-term variability at higher luminosity, at least at L[subscript X] [> over ~] 2 X 10[superscript 37] erg s[superscript −1]. In order to identify the nature and spectral state of our sources, we compared their collective spectral properties based on single-component models (a simple power law or a multicolor disk) with the spectral evolution seen in representative Galactic LMXBs. We found that in the L[subscript X] versus photon index Γ[subscript PL] and L[subscript X] versus disk temperature kT[subscript MCD] plots, most of our sources fall on a narrow track in which the spectral shape hardens with increasing luminosity below L[subscript X] ~ 7 X 10[superscript 37] erg s[superscript −1], but is relatively constant (Γ[subscript PL] ~ 1.5 or kT[subscript MCD] ~ 1.5 keV) above this luminosity, which is similar to the spectral evolution of Galactic neutron star (NS) LMXBs in the soft state in the Chandra bandpass. Therefore, we identified the track as the NS LMXB soft-state track and suggested sources with L[subscript X] [< over ~] 7 X 10[superscript 37] erg s[superscript −1] as atolls in the soft state and those with L[subscript X] [> over ~] 7 X 10[superscript 37] erg s[superscript −1] as Z sources. Ten other sources (five are transients) displayed significantly softer spectra and are probably black hole X-ray binaries in the thermal state. One of them (persistent) is in a metal-poor GC.