The First Proper Motion Measurement of the Acceleration Regions in the Large-scale Jets of SS 433 Powering the W50 Nebula

Abstract We report on new Chandra ACIS-I observations of the X-ray knots located in the western and eastern lobes of W50 associated with the parsec-scale jets of the Galactic microquasar SS 433. These knots are likely counterparts of the recently detected very-high-energy ( E > 100 GeV) gamma-ray emission by HAWC and HESS. These findings, together with the ultrahigh-energy signal recently reported by the LHAASO Collaboration, have established the SS 433/W50 system as a unique jet-driven PeVatron candidate. Combining new and archival Chandra data, we perform the first proper motion search of the X-ray knot structures over a time interval spanning approximately 20 yr. We found no statistically significant motion of these knots at the 3 σ confidence level, and place an upper limit of <0.019–0.033 c (5800–9800 km s −1 ) for the speed of the innermost knots at an assumed distance d = 5.5 kpc. Combined with the velocities reported in the literature, the upstream speed in the shock rest frame would reach several 10 4 km s −1 , suggesting that highly efficient particle acceleration, approaching the Bohm limit, is occurring. The absence of significant motion of the knots suggests the presence of a standing recollimation shock, formed by the balance between the jet pressure and the external pressure. This interpretation is consistent with the expected occurrence of such shocks at 20–30 pc from SS 433, matching the location of the observed knots.