We have obtained three epochs of Chandra ACIS-I observations (totaling ∼184 ks) of the nearby spiral galaxy NGC 300 to study the logN-logS distributions of its X-ray point-source population down to ∼2 ×10-15 erg s-1 cm-2 in the 0.35-8 keV band (equivalent to ∼1036 erg s-1). The individual epoch logN-logS distributions are best described as the sum of a background active galactic nucleus (AGN) component, a simple power law, and a broken power law, with the shape of the logN-logS distributions sometimes varying between observations. The simple power law and AGN components produce a good fit for "persistent" sources (i.e., with fluxes that remain constant within a factor of ∼2). The differential power-law index of ∼1.2 and high fluxes suggest that the persistent sources intrinsic to NGC 300 are dominated by Roche-lobe-overflowing low-mass X-ray binaries. The variable X-ray sources are described by a broken power law, with a faint-end power-law index of ∼1.7, a bright-end index of ∼2.8-4.9, and a break flux of ∼ 8 × 10-15 erg s-1 cm-2 (∼4 ×1036 erg s-1), suggesting that they are mostly outbursting, wind-fed high-mass X-ray binaries, although the logN-logS distribution of variable sources likely also contains low-mass X-ray binaries. We generate model logN-logS distributions for synthetic X-ray binaries and constrain the distribution of maximum X-ray fluxes attained during outburst. Our observations suggest that the majority of outbursting X-ray binaries occur at sub-Eddington luminosities, where mass transfer likely occurs through direct wind accretion at ∼1%-3% of the Eddington rate.
Bibliographical noteFunding Information:
Support for this work was provided by the National Aeronautics and Space Administration through Chandra Award no. G04-15088X issued by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060. P.P.P. and T.J.G. acknowledge support under NASA contract NAS8-03060. This research has made use of software provided by the Chandra X-ray Center (CXC) in the application packages CIAO, ChIPS, and Sherpa and the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
- X-rays: binaries
- galaxies: individual (NGC 300)
- galaxies: spiral