Swift J1727.8-1613 has the Largest Resolved Continuous Jet Ever Seen in an X-ray Binary

Multi-wavelength polarimetry and radio observations of Swift J1727.8-1613 at the beginning of its recent 2023 outburst suggested the presence of a bright compact jet aligned in the north-south direction, which could not be confirmed without high angular resolution images. Using the Very Long Baseline Array and the Long Baseline Array, we imaged Swift J1727.8-1613, during the hard/hard-intermediate state, revealing a bright core and a large, two-sided, asymmetrical, resolved jet. The jet extends in the north-south direction, at a position angle of \(-0.60\pm0.07\deg\) East of North. At 8.4 GHz, the entire resolved jet structure is \(\sim110 (d/2.7\,\text{kpc})/\sin i\) AU long, with the southern approaching jet extending \(\sim80 (d/2.7\,\text{kpc})/\sin i\) AU from the core, where \(d\) is the distance to the source and \(i\) is the inclination of the jet axis to the line of sight. These images reveal the most resolved continuous X-ray binary jet, and possibly the most physically extended continuous X-ray binary jet ever observed. Based on the brightness ratio of the approaching and receding jets, we put a lower limit on the intrinsic jet speed of \(\beta\geq0.27\) and an upper limit on the jet inclination of \(i\leq74\deg\). In our first observation we also detected a rapidly fading discrete jet knot \(66.89\pm0.04\) mas south of the core, with a proper motion of \(0.66\pm0.05\) mas hour\(^{-1}\), which we interpret as the result of a downstream internal shock or a jet-ISM interaction, as opposed to a transient relativistic jet launched at the beginning of the outburst.