CG X-1: An Eclipsing Wolf–Rayet ULX in the Circinus Galaxy

Abstract

We investigated the time-variability and spectral properties of the eclipsing X-ray source Circinus Galaxy X-1 (GG X-1), using Chandra, XMM-Newton and ROSAT. We phase-connected the lightcurves observed over 20 years, and obtained a best-fitting period P = (25, 970.0±0.1) s ≈7.2 hr, and a period derivativeṖ /P = (10.2±4.6)×10 −7 yr −1 . The X-ray lightcurve shows asymmetric eclipses, with sharp ingresses and slow, irregular egresses. The eclipse profile and duration vary substantially from cycle to cycle. We show that the X-ray spectra are consistent with a power-law-like component, absorbed by neutral and ionized Compton-thin material, and by a Compton-thick, partial-covering medium, responsible for the irregular dips. The high X-ray/optical flux ratio rules out the possibility that CG X-1 is a foreground Cataclysmic Variable; in agreement with previous studies, we conclude that it is the first example of a compact ultraluminous X-ray source fed by a Wolf-Rayet star or stripped Helium star. Its unocculted luminosity varies between ≈4 ×10 39 erg s −1 and ≈3 ×10 40 erg s −1 . Both the donor star and the super-Eddington compact object drive powerful outflows: we suggest that the occulting clouds are produced in the wind-wind collision region and in the bow shock in front of the compact object. Among the rare sample of Wolf-Rayet X-ray binaries, CG X-1 is an exceptional target for studies of super-critical accretion and close binary evolution; it is also a likely progenitor of gravitational wave events.

A full characterization of the supermassive black hole in IRAS 09149–6206

“…Matt et al 1996;Bianchi et al 2002;Arévalo et al 2014), meaning the intrinsic AGN continuum is not seen below 10 keV. The variability seen by XMM-Newton is instead almost certainly related to the bright X-ray binaries that are within the 40 arcsec extraction region used by González-Martín & Vaughan (2012), which make a significant contribution to the total soft X-ray emission; most notable is the variable ultraluminous X-ray source CG X-1 which can reach luminosities in excess of 10 40 erg s −1 and is separated from the nucleus by ∼15 arcsec (Bauer et al 2001;Qiu et al 2019). However, we stress that the González-Martín & Vaughan (2012) evaluation ultimately still agrees with the result presented here, giving log [M BH /M ] = 7.7 ± 0.6.…”

Ultraluminous X-Ray Sources

“…The values of the error bars takes into account the systematic error associated with the uncertainty of subtracting the nebula contribution. For comparison, the ratios of the widths of these lines are given for the transitional stars WR 22, WR 24 and WR 25 (all three are of the type WN 6ha (Walborn and Fitzpatrick, 2000), luminosities only in the case of sufficiently short orbital periods, such as those of M101 ULX-1 and CG X-1 (Liu et al, 2013;Qiu et al, 2019). A more powerful argument against the idea of the stellar origin of the ULX spectra is the strong variability of the width of the emission lines at times less than one day, which is not typical for stars, since the terminal wind velocity is determined mainly by gravity on the star surface and its luminosity, and therefore can only change over large time intervals.…”

Quasi-periodic dipping in the ultraluminous X-ray source, NGC 247 ULX-1

“…The PSD was estimated using the standard method of calculating the periodogram (e.g. Priestley 1981), with an [rms/mean] 2 normalisation, then averaging over segments at each Fourier frequency. Non-overlapping neighbouring frequency bins are then averaged (see e.g.…”

“…It is well known that the periodogram estimate at each frequency follows a 2 =2 distribution so large scatter around the true underlying power spectrum is expected (e.g. Priestley 1981). We therefore also show a Gaussian smoothed with periodogram over = 3 neighbouring frequency bins.…”