THE EFFECT of VARIABILITY on X-RAY BINARY LUMINOSITY FUNCTIONS: MULTIPLE-EPOCH OBSERVATIONS of NGC 300 with CHANDRA

B. Binder, J. Gross, B. F. Williams, M. Eracleous, T. J. Gaetz, P. P. Plucinsky, E. D. Skillman

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Abstract

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.

Original languageEnglish (US)
Article number128
JournalAstrophysical Journal
Volume834
Issue number2
DOIs
StatePublished - Jan 10 2017

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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