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

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 ∼ 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 ± 0.03c at a propagation distance of 2 pc. We demonstrate that these shocks are likely powered by a hidden relativistic ( 0.3c) wide-angle conical outflow launched in 1972 ± 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 ∼ 0.1L_Edd, while the time-averaged power required forthe kinetic energy of the shocksis ∼ 40  5/10^-2 cm^-3 L_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 ± 6 per cent in the shocks demonstrating the presence of an ordered magnetic field of strength ∼ 200 μG. We show that the shocks are potential PeVatrons, capable of accelerating electrons to ∼ 0.7PeV and protons to ∼ 20 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.