The first resolved imaging of milliarcsecond-scale jets in Circinus X-1

TL;DR

This paper reports the first high-resolution imaging of milliarcsecond-scale jets in the neutron star X-ray binary Circinus X-1, revealing symmetric, mildly relativistic jets and constraining their physical properties.

Contribution

It provides the first resolved imaging of the jets in Circinus X-1 at milliarcsecond resolution, offering new insights into their structure and motion.

Findings

Jets are symmetric and unresolved along the minor axis.

Proper motion suggests mildly relativistic speeds.

Constraints on jet opening angle are less than 20 degrees.

Abstract

We present the first resolved imaging of the milliarcsecond-scale jets in the neutron star X-ray binary Circinus X-1, made using the Australian Long Baseline Array. The angular extent of the resolved jets is ~20 milliarcseconds, corresponding to a physical scale of ~150 au at the assumed distance of 7.8 kpc. The jet position angle is relatively consistent with previous arcsecond-scale imaging with the Australia Telescope Compact Array. The radio emission is symmetric about the peak, and is unresolved along the minor axis, constraining the opening angle to be less than 20 degrees. We observe evidence for outward motion of the components between the two halves of the observation. Constraints on the proper motion of the radio-emitting components suggest that they are only mildly relativistic, although we cannot definitively rule out the presence of the unseen, ultra-relativistic (Lorentz factor >15) flow previously inferred to exist in this system.