We have obtained three epochs of Chandra ACIS-I observations ( totaling \sim 184 ks ) of the nearby spiral galaxy NGC 300 to study the log N -log S distributions of its X-ray point source population down to \sim 2 \times 10 ^ { -15 } erg s ^ { -1 } cm ^ { -2 } in the 0.35-8 keV band ( equivalent to \sim 10 ^ { 36 } erg s ^ { -1 } ) .
The individual epoch log N -log S distributions are best described as the sum of a background AGN component , a simple power law , and a broken power law , with the shape of the log N -log S 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 \sim 2 ) .
The differential power law index of \sim 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 \sim 1.7 , a bright-end index of \sim 2.8–4.9 , and a break flux of \sim 8 \times 10 ^ { -15 } erg s ^ { -1 } cm ^ { -2 } ( \sim 4 \times 10 ^ { 36 } erg s ^ { -1 } ) , suggesting they are mostly outbursting , wind-fed high mass X-ray binaries , although the log N -log S distribution of variable sources likely also contains low-mass X-ray binaries .
We generate model log N -log S 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 \sim 1–3 % of the Eddington rate .