2015MNRAS.452.1112E

The examination of two 2010 Chandra ACIS (Advanced CCD Imaging Spectrometer) exposures of the Circinus galaxy resulted in the discovery of two pulsators: CXO J141430.1-651621 and CXOU J141332.9-651756. We also detected 26 ks pulsations in CG X-1, consistently with previous measures. For ∼ 40 other sources, we obtained limits on periodic modulations. In CXO J141430.1-651621, which is ∼ 2 arcmin outside the Circinus galaxy, we detected signals at 6120±1 s and 64.2±0.5 ks. In the longest observation, the source showed a flux of ~1.1x10^–13 erg/ cm2/s (absorbed, 0.5-10 keV) and the spectrum could be described by a power law with photon index Γ ≃ 1.4. From archival observations, we found that the luminosity is variable by ~50 per cent on time-scales of weeks to years. The two periodicities pin down CXO J141430.1-651621 as a cataclysmic variable of the intermediate polar subtype. The period of CXOU J141332.9-651756 is 6378±3 s. It is located inside the Circinus galaxy, but the low absorption indicates a Galactic foreground object. The flux was ~5x10^–14 erg/ cm2/s in the Chandra observations and showed ~50 per cent variations on weekly/yearly scales; the spectrum is well fitted by a power law with Γ ≃ 0.9. These characteristics and the large modulation suggest that CXOU J141332.9-651756 is a magnetic cataclysmic variable, probably a polar. For CG X-1, we show that if the source is in the Circinus galaxy, its properties are consistent with a Wolf-Rayet (WR) plus black hole (BH) binary. We consider the implications of this for ultraluminous X-ray sources and the prospects of Advanced LIGO and Virgo. In particular, from the current sample of WR-BH systems, we estimate an upper limit to the detection rate of stellar BH-BH mergers of ∼ 16/yr.