Radio observations of Circinus X-1 over a complete orbit at an historically faint epoch

Abstract

We present results from the first radio observations of a complete orbit (~\n17 days) of the neutron star X-ray binary Circinus X-1 using the Australia\nTelescope Compact Array Broadband Backend, taken while the system was in an\nhistorically faint state. We have captured the rapid rise and decline of a\nperiastron passage flare, with flux densities for 9 days prior to the event\nstable at ~ 1 mJy at 5.5 GHz and ~ 0.5 mJy at 9 GHz. The highest flux densities\nof 43.0 +/- 0.5 mJy at 5.5 GHz and 29.9 +/- 0.6 mJy at 9 GHz were measured\nduring the flare's decline (MJD 55206.69) which continues towards pre-flare\nflux densities over the following 6 days. Imaging of pre-flare data reveals\nsteady structure including two stable components within 15 arc-seconds of the\ncore which we believe may be persistent emission regions within the system's\noutflows, one of which is likely associated with the system's counter-jet.\nUnlike past observations carried out in the system's brighter epochs, we\nobserve no significant structural variations within \\approx 3 arc-seconds of\nthe core's position. Model subtraction and difference mapping provide evidence\nfor variations slightly further from the core: up to 5" away. If related to the\nobserved core flare, then these variations suggest very high outflow velocities\nwith {\\Gamma} > 35, though this can be reduced significantly if we invoke phase\ndelays of at least one orbital period. Interestingly, the strongest structural\nvariations appear to the north west of the core, opposite to the strongest\narcsec-scale emission historically. We discuss the implications of this\nbehaviour, including the possibility of precession or a kinked approaching jet.\n