Dynamic shocks powered by a wide, relativistic, super-Eddington outflow launched by an accreting neutron star in the mid-20th century

ABSTRACT Accreting systems can launch powerful outflows which interact with the surrounding medium. We combine new radio observations of the accreting neutron star X-ray binary (XRB) Circinus X-1 (Cir X−1) with archival radio observations going back 24 yr. The $\sim 3$ pc scale wide-angle radio and X-ray emitting caps found around Cir X−1 are identified as synchrotron emitting shocks with significant proper motion and morphological evolution on decade time-scales. Proper motion measurements of the shocks reveal they are mildly relativistic and decelerating, with apparent velocity of $0.14c\pm 0.03c$ at a propagation distance of 2 pc. We demonstrate that these shocks are likely powered by a hidden relativistic ($\gtrsim 0.3c$) wide-angle conical outflow launched in $1972\pm 3$, in stark contrast to known structures around other XRBs formed by collimated jets over 1000s of years. The minimum time-averaged power of the outflow required to produce the observed synchrotron emission is $\sim 0.1L_\text{Edd}$, while the time-averaged power required for the kinetic energy of the shocks is $\sim 40 \left(\frac{n}{10^{-2} \text{cm}^{-3}}\right)L_\text{Edd}$, where n is the average ambient medium number density. This reveals the outflow powering the shocks is likely significantly super-Eddington. We measure significant linear polarisation up to $52\pm 6 {{\ \rm per\ cent}}$ in the shocks demonstrating the presence of an ordered magnetic field of strength $\sim 200\, \mu \text{G}$. We show that the shocks are potential PeVatrons, capable of accelerating electrons to $\sim 0.7 \text{PeV}$ and protons to $\sim 20\, \text{PeV}$, and we estimate the injection and energetic efficiencies of electron acceleration in the shocks. Finally, we predict that next generation gamma-ray facilities may be able to detect hadronic signatures from the shocks.