An asymmetric arcsecond radio jet from Circinus X-1

In observations with the Australia Telescope Compact Array, we have resolved the radio counterpart of the unusual X-ray binary Circinus X-l into an asymmetric, extended structure on arcsecond scales. In order to quantify the asymmetry, we have redetermined as accurately as possible both the optical and radio coordinates of the source. The extended emission can be understood as a compact, absorbed core at the location of the Xray binary and as extended emission up to 2" to the southeast of the core. The arcsecond-scale-extended emission aligns with the larger, more symmetric arcminute-scale-collimated structures in the surrounding synchrotron nebula. This suggests that the transport of mass and/or energy from the X-ray binary to the synchrotron nebula is occurring via the arcsecond-scale structures. The ratio of extended flux from the southeast to that from the northwest of the core is at least 2:1. Interpreted as a relativistic aberration of an intrinsically symmetric jet from the source, this implies a minimum outflow velocity of 0.1c. Alternatively, the emission may be intrinsically asymmetric, perhaps as a result of the high space velocity of the system.