An asymmetric arcsecond radio jet from Circinus X-1

In observations with the Australia Telescope Compact Array, we have resolve d the radio counterpart of the unusual X-ray binary Circinus X-l into an as ymmetric, 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 understoo d as a compact, absorbed core at the location of the Xray binary and as ext ended emission up to 2" to the southeast of the core. The arcsecond-scale-e xtended emission aligns with the larger, more symmetric arcminute-scale-col limated structures in the surrounding synchrotron nebula. This suggests tha t the transport of mass and/or energy from the X-ray binary to the synchrot ron nebula is occurring via the arcsecond-scale structures. The ratio of ex tended 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 sy mmetric jet from the source, this implies a minimum outflow velocity of 0.1 c. Alternatively, the emission may be intrinsically asymmetric, perhaps as a result of the high space velocity of the system.