XRISM high-resolution spectroscopy of SS 433: Evidence of decreasing line-of-sight velocity dispersion along the jet

Abstract We report on the jet structure in SS 433 based on X-ray high-resolution spectroscopy with XRISM/Resolve. The source was observed over 5 d covering both inside and outside an eclipse of the compact object by the companion star. Doppler-shifted ionized Fe and Ni K emission lines were resolved, as well as lower-energy lines including Si and S K lines. Time-resolved spectral analysis showed that the Fe and Ni K line widths were $1020 \pm 40$ km s$^{-1}$ (corresponding to the 1 $\sigma$ width) in the eclipse phase, gradually increased during the egress, and reached $1740 \pm 30$km s$^{-1}$ outside the eclipse. A time-averaged spectrum outside the eclipse confirmed that the Fe and Ni K lines at 5.5–9 keV are significantly broader than the Si and S K$\alpha$ emission lines at 2–4 keV. Specifically, the width at 5.5–9 keV was measured to be $1900 \pm 80$ km s$^{-1}$, whereas the width at 2–4 keV is $1300^{+300}_{-400}$ km s$^{-1}$ for the approaching (blueshifted) jet component. These results indicate that the radial velocity dispersion of the jet plasma in SS 433 decreases as it moves outward. We interpret this variation as progressive jet collimation along its axis, as suggested by Namiki et al. (2003, PASJ, 55, 281), or a decrease in turbulence in the jet plasma flow within the X-ray emitting region. We also detected a clear difference in velocity dispersion between the approaching and receding (redshifted) jet components in the 5.5–9 keV band outside eclipse. The receding jet exhibited a smaller velocity dispersion ($1400 \pm 200$ km s$^{-1}$) than the approaching jet. Since the observation was conducted when the approaching jet was tilted toward the observer, this may suggest that the receding jet was more extensively occulted by the accretion disk.