The Megasecond Chandra X-Ray Visionary Project Observation of NGC 3115. II. Properties of Point Sources

Dacheng Lin, University of New Hampshire, Durham
Jimmy A. Irwin, University of Alabama - Tuscaloosa
Ka-Wah Wong, Minnesota State University Mankato
Zachary G. Jennings, University of California Observatories
Jeroen Homan, MIT Kavli Institute for Astrophysics and Space Research
Aaron J. Romanowsky, San Jose State University
Jay Strader, Michigan State University
Gregory R. Sivakoff, University of Alberta
Jean P. Brodie, University of California Observatories
Ronald A. Remillard, MIT Kavli Institute for Astrophysics and Space Research


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 LX ~ 1036–1039 erg s−1. Other than 13 transient candidates, the sources overall have less long-term variability at higher luminosity, at least at 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 LX versus photon index and LX versus disk temperature kTMCD plots, most of our sources fall on a narrow track in which the spectral shape hardens with increasing luminosity below erg s−1, but is relatively constant ( or 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 erg s−1 as atolls in the soft state and those with 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.