XRISM High-resolution Spectroscopy of SS 433: Evidence of Decreasing Line-of-Sight Velocity Dispersion along the Jet

TL;DR

This study uses XRISM high-resolution X-ray spectroscopy to observe SS 433's jets, revealing decreasing line-of-sight velocity dispersion along the jet and differences between approaching and receding jets, indicating jet collimation and occultation effects.

Contribution

First high-resolution X-ray spectral analysis of SS 433's jets showing decreasing velocity dispersion and asymmetry between jets, suggesting jet collimation and occultation phenomena.

Findings

Velocity dispersion decreases from 1020 km/s to 1740 km/s outside eclipse.

Broader Fe and Ni lines compared to Si and S lines, indicating jet structure.

Receding jet shows smaller velocity dispersion, implying occultation effects.

Abstract

We report on the jet structure in SS 433 based on X-ray high resolution spectroscopy with the XRISM/Resolve. The source was observed over 5 days 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 Fe and Ni K line widths were $1020 \pm 40$ km s$^{-1}$ (corresponding 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 in 5.5-9 keV are significantly broader than the Si and S K$\alpha$ emission lines in 2--4 keV. Specifically, the width in 5.5-9 keV was measured to be $1900 \pm 80$ km s$^{-1}$, whereas the width in 2-4 keV is $1300^{+300}_{-400}$ km s$^{-1}$ for the approaching (blueshifted) jet component. These results indicate that 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), 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.