Energy-dependent orbital phases in NGC 300 X-1

Abstract

NGC 300 X-1 is a Wolf Rayet + black hole binary that exhibits periodic decreases in X-ray flux. We present two new observations of NGC 300 X-1 from the Chandra X-ray Observatory (totaling ∼130 ks) along with ACS imaging data from the Hubble Space Telescope. We observe significant short-term variability in the X-ray emission that is inconsistent with an occultation by the donor star, but is consistent with structure in the outer accretion disk or the wind of the donor star. We simultaneously fit a partially-covered disk blackbody and Comptonized corona model to the eclipse egress and noneclipsing portions of the X-ray spectrum. We find that the only model parameters that varied between the eclipse egress and non-eclipsing portions of the spectra were the partial covering fraction (∼86% during eclipse egress and ∼44% during non-eclipse) and absorbing column (∼12.3×10 22 cm −2 during eclipse egress, compared to ∼1.4×10 22 cm −2 during non-eclipse). The X-ray spectra are consistent with the movement of the X-ray source through the dense stellar winds of the companion star. From our new HST imaging, we find the WR star within the X-ray error circle, along with additional optical sources including an AGB star and an early-type main sequence star. Finally, we use our egress measurement to rephase previous radial velocity measurements reported in the literature, and find evidence that the velocities are strongly affected by the ionization of the wind by the compact object. Thus, we argue the inferred mass of the black hole may not be reliable.

Soft extragalactic X-ray binaries at the Eddington Threshold

“…The WR candidate is marked with a cyan circle. We indicate the other two possible counterpart candidates identified by Binder et al (2015) with cyan arrows, and one further bright star and possible candidate within our error circle with a green arrow. Centre left, NGC 4395 ULX-1 is marked with a 25 arcsec white circle on XMM-Newton observation X3.…”

The Effect of Variability on X-Ray Binary Luminosity Functions: Multiple-Epoch Observations of NGC 300 With Chandra

“…NGC 300 does not contain such luminous X-ray sources; the brightest X-ray source, NGC 300 X-1, has a luminosity of ∼4×10 38 erg s −1 (Binder et al 2011(Binder et al , 2015. We therefore use a high-luminosity cut-off of 10 41 erg s −1 for the HMXB XLF.…”

“…This component is needed to explain the bright-end of the observed logN -logS distributions, which is dominated by the bright source NGC 300 X-1 (Binder et al 2011(Binder et al , 2015, and references therein). Although previously thought to be a Wolf Rayet + black hole HMXB, recent observations by Binder et al (2015) have suggested that the donor star may be significantly less massive than previously believed, making X-1 a persistently bright black hole-LMXB.…”

Two Eclipsing Ultraluminous X-Ray Sources in M51

“…For IC 10 X-1, it is still disputed whether the accreting compact object is a BH or a neutron star (Laycock et al 2015b). Outside the Local Group, NGC 300 X-1 ( »Ĺ 5 10 X 38 erg s −1 ; binary period »33 hr) shows X-ray dips, consistent with occultation from geometrically thick structures in the outer disk, or absorption in the wind of the donor star, but not with true eclipses (Binder et al 2015). A strong candidate for a true eclipse is the sharp dip in the Swift/X-ray Telescope flux recorded once from the ULX P13 in NGC 7793, at an orbital phase consistent with the inferior conjunction of its supergiant donor star (Motch et al 2014).…”