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

Journal article

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 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 at a propagation distance of 2 pc. We demonstrate that these shocks are likely powered by a hidden relativistic () wide-angle conical outflow launched in , 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 , while the time-averaged power required for the kinetic energy of the shocks is , 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 in the shocks demonstrating the presence of an ordered magnetic field of strength . We show that the shocks are potential PeVatrons, capable of accelerating electrons to and protons to , 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.

Authors: Cowie, FJ; Fender, RP; Heywood, I; Carotenuto, F; Matthews, JH

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Oxford University Press
• Journal: Monthly Notices of the Royal Astronomical Society
• Volume: 546
• Issue: 4
• Pages: stag163
• Article number: stag163
• Publication date: 2026-01-22
• Acceptance date: 2026-01-21
• DOI: 10.1093/mnras/stag163
• EISSN: 1365-2966
• ISSN: 0035-8711
• Language: English
• Keywords: shock waves; X-rays: individual: Cir X–1; ISM: jets and outflows; stars: neutron; acceleration of particles; X-rays: binaries