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 H.E.S.S. These findings, together with the ultra-high-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 years. We found no statistically significant motion of these knots at the 3$\sigma$ confidence level, and place an upper limit of $<$ 0.019-0.033$c$ (5,800-9,800 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.