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

TL;DR

XRISM/Resolve provides high-resolution spectroscopy of SS 433's jets, resolving Doppler-shifted Fe and Ni K lines and enabling precise measurements of line widths. Time-resolved analysis in the Fe K band reveals the jet velocity dispersion increases from eclipse to non-eclipse, indicating a decreasing dispersion with distance along the jet and supporting a picture of progressive jet collimation or reduced turbulence. A Fe K versus Si/S K-band comparison shows Fe K lines are broader than Si/S K lines, consistent with spatial variation in jet properties, and receding-jet widths suggest partial occultation effects. The study also confirms Ni overabundance and uncovers residual line structure around 6.4–7 keV, highlighting the need for multi-temperature modeling and further broad-band XRISM studies to fully characterize the jet environment.

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$σ$ 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$α$ 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.

Figures (6)

• Figure 1: Resolve (gray) and Xtend (black) light curves with 512 s bins. The errors indicate 1$\sigma$ statistical uncertainties. The shaded region indicates the time interval used for the spectral analysis described in Sec. \ref{['subsec:and_Fe_vs_Si']}. Alt text: a graph with error bars in the x- and y-axis directions. The x-axis shows the time and orbital phase and the y-axis shows the count rate. The top panel plots three different datasets.
• Figure 2: Time-averaged Resolve 2--4 keV (top) and 5.5--9 keV (bottom) spectra, corrected for the effective area of the instrument, in the eclipse and non-eclipse phases. The identifications of main lines are also presented, where (r), (b), and (disk) denote those from the approaching jet, the receding jets, and the accretion disk (or the disk wind), respectively. Note that the non-eclipse phase is longer than the eclipse phase and therefore the lines in the former appear broader, likely due to variations in Doppler shifts caused by the jet precession. Alt text: Two graphs with error bars in the x- and y-axis directions. Two data (in eclipse and non-eclipse phases) are presented in each graph. The x- and y-axes show the energy in units of keV and the count rate, respectively.
• Figure 3: Representative spectra in the (a) eclipse and (b) non-eclipse phases and their best-fit models. Contributions of the approaching jet, receding jets, and the narrow FeI K$\alpha$ line are presented with the blue, red, and green lines, respectively. The spectra are corrected for the effective area of the instrument. The data versus model ratios are shown in the bottom panels. Alt text: Two figures lined-up horizontally, each with two graphs combined vertically. The upper panels show graphs with error bars in the x- and y-axis directions. The x and y axes show the time and the count rate, respectively. In the lower panels the y-axis is the data versus model ratio.
• Figure 4: Evolution of the jet parameters: the plasma temperature (a), the Doppler shifts of approaching and receding jet components (b and c, respectively), and their velocity dispersion (d). Alt text: four graphs with error bars in the x- and y-axis directions are aligned vertically. The x-axis shows the time and y-axis shows the jet parameters.
• Figure 5: Time-averaged spectra for $\phi_{\rm orb} = 0.13$--$0.20$ in (a) 5.5--9 keV and (b) 2--4 keV and the best-fit models, corrected for the effective area of the instrument. The contributions of the approaching jet, receding jet, and the Gaussian line components are shown with the blue, red, and green lines, respectively. The middle and bottom panels in (a) present the data versus model ratio of the best-fit models without and with two broad Gaussian components, respectively. Alt text: two figures lined-up vertically, each with two graphs combined vertically. The upper panels show graphs with error bars in the x- and y-axis directions. The x and y axes show the time and the count rate, respectively. In the lower panels the y-axis is the data versus model ratio.