We present results from the first radio observations of a complete orbit ( \sim 17 days ) of the neutron star X-ray binary Circinus X-1 using the Australia Telescope Compact Array Broadband Backend , taken while the system was in an historically faint state .
We have captured the rapid rise and decline of a periastron passage flare , with flux densities for 9 days prior to the event stable at \sim 1 mJy at 5.5 GHz and \sim 0.5 mJy at 9 GHz .
The highest flux densities of 43.0 \pm 0.5 mJy at 5.5 GHz and 29.9 \pm 0.6 mJy at 9 GHz were measured during the flare ’ s decline ( MJD 55206.69 ) which continues towards pre-flare flux densities over the following 6 days .
Imaging of pre-flare data reveals steady structure including two stable components within 15 arc-seconds of the core which we believe may be persistent emission regions within the system ’ s outflows , one of which is likely associated with the system ’ s counter-jet .
Unlike past observations carried out in the system ’ s brighter epochs , we observe no significant structural variations within \approx 3 arc-seconds of the core ’ s position .
Model subtraction and difference mapping provide evidence for variations slightly further from the core : up to 5 ” away .
If related to the observed core flare , then these variations suggest very high outflow velocities with \Gamma > 35 , though this can be reduced significantly if we invoke phase delays of at least one orbital period .
Interestingly , the strongest structural variations appear to the north west of the core , opposite to the strongest arcsec-scale emission historically .
We discuss the implications of this behaviour , including the possibility of precession or a kinked approaching jet .