Relativistic precessing jets powered by an accreting neutron star

Abstract

Precessing relativistic jets launched by compact objects are rarely directly measured, and present an invaluable opportunity to better understand many features of astrophysical jets. In this Letter we present MeerKAT radio observations of the neutron star X-ray binary system (NSXB) Circinus X-1 (Cir X-1). We observe a curved S-shaped morphology on $\sim 20''$ $(\sim1\:\text{pc})$ scales in the radio emission around Cir X-1. We identify flux density and position changes in the S-shaped emission on year timescales, robustly showing its association with relativistic jets. The jets of Cir X-1 are still propagating with mildly relativistic velocities $\sim1\:\text{pc}$ from the core, the first time such large scale jets have been seen from a NSXB. The position angle of the jet axis is observed to vary on year timescales, over an extreme range of at least $110°$. The morphology and position angle changes of the jet are best explained by a smoothly changing launch direction, verifying suggestions from previous literature, and indicating that precession of the jets is occurring. Steady precession of the jet is one interpretation of the data, and if occurring, we constrain the precession period and half-opening angle to $>10$ years and $>33°$ respectively, indicating precession in a different parameter space to similar known objects such as SS~433.