We have 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 2 \sigma detection , with 0.3–8 keV luminosity L _ { \mathrm { X } } \sim 10 ^ { 36 } –10 ^ { 39 } erg s ^ { -1 } .
Other than 13 transient candidates , the sources overall have less long-term variability at higher luminosity , at least at L _ { \mathrm { X } } \gtrsim 2 \times 10 ^ { 37 } erg s ^ { -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 _ { \mathrm { X } } versus photon index \Gamma _ { \mathrm { PL } } and L _ { \mathrm { X } } versus disk temperature kT _ { \mathrm { MCD } } plots , most of our sources fall on a narrow track in which the spectral shape hardens with increasing luminosity below L _ { \mathrm { X } } \sim 7 \times 10 ^ { 37 } erg s ^ { -1 } but is relatively constant ( \Gamma _ { \mathrm { PL } } \sim 1.5 or kT _ { \mathrm { MCD } } \sim 1.5 keV ) above this luminosity , 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 _ { \mathrm { X } } \lesssim 7 \times 10 ^ { 37 } erg s ^ { -1 } as atolls in the soft state and those with L _ { \mathrm { X } } \gtrsim 7 \times 10 ^ { 37 } erg s ^ { -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 .