The first proper motion measurement of the acceleration regions in the large-scale jets of SS 433 powering the W50 nebula

TL;DR

This study presents the first proper-motion constraints for X-ray knots in the SS 433/W50 large-scale jets using archival and new Chandra data over ~20 years, finding no significant motion at 3σ and setting upper limits on $v_{app}$ of about $0.012$–$0.033\,c$ for $d=5.5$ kpc. Interpreting the knots as quasi-stationary features associated with a recollimation shock, the authors infer upstream shock speeds $v'_{ m u}$ up to $\sim(1.5$–$7.6)\times10^{4}$ km s$^{-1}$ (depending on downstream speeds), implying highly efficient particle acceleration potentially near the Bohm limit for plausible magnetic fields $B\sim5$–$40\,\mu$G and electron cutoffs $E_0$ in the $100$ TeV–1 PeV range. The results favor a standing shock scenario at ~20–30 pc from SS 433, consistent with both the knot locations and multiwavelength emission, while acknowledging alternative formation mechanisms and highlighting future high-resolution monitoring (e.g., AXIS) to tighten dynamical constraints. These findings advance our understanding of jet dynamics and the conditions required for efficient acceleration in Galactic PeVatrons.

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$σ$ 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.

Figures (4)

• Figure 1: Left: The projection profiles, extracted from the regions shown in Figure \ref{['fig:image']}. Right: the $\chi^2$ profiles, where the positive shift indicates a direction to the west.
• Figure 2: The velocity profile in the SS 433/W50 system, assuming $d=5.5$ kpc. The blue and lightblue errorbars show the 90% and 3$\sigma$ uncertainties, respectively. The positive velocity is defined as the direction toward the outer lobe away from SS 433 (i.e., the positive values of the eastern knots mean motions to the east, while the positive values of the western knots mean motions to the west). The velocities of the knots (e0--e3 and w1--w2) were measured by proper motion studies (this work, and sakemi_energy_2021 for e3), while that of SS 433 was derived by the shift of line emission in the arcsec-scale X-ray jets marshall_high-resolution_2002.
• Figure 3: The relation between the shock velocity ($v^{\prime}_{\rm u}$) and the electron cutoff energy ($E_0$) in Equation \ref{['eq:eta']} with different values of the magnetic field. The colored diagonal lines indicate the Bohm-limit cases with $B$ of 5, 12, 20, and 40 $\mu \mathrm{G}$, and the arrows show the allowed parameter space with $\eta_{\rm B} > 1$. The vertical lines denote the constrained values of $v^{\prime}_{\rm u}$ for the e0 knot, assuming the constant-speed and expanding jet model ($v_{\rm d}$ = 0.045$c$), the constant-speed and non-expanding jet ($v_{\rm d}$ = 0.061$c$), and the lower limit of $v_{\rm d}$ = 0.021$c$hess_2024_ss43.
• Figure 4: The constraints on the upstream velocity at the observer frame $v_{\rm u}$ (left) and at the shock rest frame $v^{\prime}_{\rm u}$ (right) as functions of $v_{\rm sh}$ and $v_{\rm d}$ (top: e0 and bottom: w1). The vertical solid and dashed lines show the 90% and 3$\sigma$ uncertainties of $v_{\rm sh}$, respectively. The horizontal lines show the constraints on the downstream velocity, assuming decreasing speed profile and expanding jet ($v_{\rm d}$ = 0.083$c$), constant speed and non-expanding jet (0.061$c$), and constant speed and expanding jet (0.045$c$), and the lower and upper limits including the systematic uncertainties (0.021$c$ and 0.12$c$) in hess_2024_ss43. The region enclosed by the tick black line indicates the allowed parameter space.