2025ApJ...993L..24T

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 3σ confidence level, and place an upper limit of <0.019-0.033c (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 10⁴ 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.