CG X-1: an eclipsing Wolf-Rayet ULX in the Circinus galaxy. (arXiv:1904.01066v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Qiu_Y/0/1/0/all/0/1">Yanli Qiu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Soria_R/0/1/0/all/0/1">Roberto Soria</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_S/0/1/0/all/0/1">Song Wang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wiktorowicz_G/0/1/0/all/0/1">Grzegorz Wiktorowicz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Liu_J/0/1/0/all/0/1">Jifeng Liu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bai_Y/0/1/0/all/0/1">Yu Bai</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bogomazov_A/0/1/0/all/0/1">Alexey Bogomazov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stefano_R/0/1/0/all/0/1">Rosanne Di Stefano</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Walton_D/0/1/0/all/0/1">Dominic J. Walton</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Xu_X/0/1/0/all/0/1">Xiaojie Xu</a>
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 $P = (25,970.0 pm 0.1)$ s $approx$7.2 hr, and a period
derivative $dot{P}/P = ( 10.2pm4.6) times 10^{-7}$ 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 $approx$4 $times 10^{39}$ erg s$^{-1}$ and $approx$3 $times
10^{40}$ 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.
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 $P = (25,970.0 pm 0.1)$ s $approx$7.2 hr, and a period
derivative $dot{P}/P = ( 10.2pm4.6) times 10^{-7}$ 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 $approx$4 $times 10^{39}$ erg s$^{-1}$ and $approx$3 $times
10^{40}$ 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.
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