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Description
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 1.1Ms). In total we found 136 candidate LMXBs in the field and 49 in globular clusters (GCs) above 2{sigma} detection, with 0.3-8keV luminosity L_X_~10^36^-10^39^erg/s. Other than 13 transient candidates, the sources overall have less long-term variability at higher luminosity, at least at L_X_>~2x10^37^erg/s. 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_X_ versus photon index {Gamma}_PL_ and L_X_versus disk temperature kT_MCD_ plots, most of our sources fall on a narrow track in which the spectral shape hardens with increasing luminosity below L_X_~7x10^37^erg/s, but is relatively constant ({Gamma}_PL_~1.5 or kT_MCD_~1.5keV) 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_X_<~7x10^37^erg/s as atolls in the soft state and those with L_X_>~7x10^37^erg/s 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.
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