Spectral and photometric variability of SS 433 observed with XRISM and simultaneous optical and near-infrared telescopes

Abstract

We present results from coordinated multiwavelength observations of the SS 433, obtained with XRISM, optical telescopes, and near-infrared camera during 2024 April and 2025 March. The XRISM exposures amounted to ~200 ks in 2024 and ~100 ks in 2025. With XRISM/Resolve's high spectral resolution and large effective area, we clearly resolved numerous emission lines even in short time segments, achieving improved accuracy in Doppler-shift measurements relative to earlier observations. The simultaneously obtained X-ray and optical Doppler shifts suggest a possible tendency for the optical emission to lag slightly behind the X-rays. In the Resolve data, the Doppler shifts of the two jet components exhibited apparent asymmetries, with jet speeds fluctuating around ~0.26$\pm$0.01$c$ in 2024 and ~0.30$\pm$0.01$c$ in 2025. The velocity variations indicated modulations on a timescale of ~6.3 d, with a phase offset of about -90$^{\circ}$ relative to the nutation cycle. The observed line widths and flux of the approaching and receding jets appear consistent with the expected geometrical effects, indicating systematically larger line widths in the inner regions of the jets, as proposed by Shidatsu et al. (2025). Optical light curves show flares of ~400 s in 2024 and ~1600 s in 2025, with amplitudes up to ~15% during out-of-eclipse intervals, while the XRISM/Xtend light curves show no significant variability within the overlapping intervals and given the statistical uncertainties. Near-infrared photometry in 2024, obtained during an out-of-eclipse interval at a different epoch from the optical observations, showed no flare-like variability, and the X-ray band also remained constant within uncertainties. These coordinated observations provide a foundation for future XRISM studies aimed at probing the dynamical properties of the relativistic jets in SS 433.

Figures (17)

• Figure 1: Overview of the multiwavelength observation campaign of SS 433 conducted in 2024--2025. (a) Doppler shifts of the approaching and receding jets ($z_\mathrm{b}$ and $z_\mathrm{r}$). The curves are calculated from the precession and nutation models based on the ephemerides and parameters of Gies_2002 and Davydov_2008, with the phase offset adjusted to match the observed Doppler shifts (see text). The dotted line indicates the nutation phase zero ($\phi_\mathrm{nut} = 0$). (b) Timeline of observations. Observing epochs of XRISM (black), Seimei (gray), LCO (purple), Tomo-e Gozen (blue), MITSuME (green), and kSIRIUS (red) are shown. The dashed line indicates the orbital phase zero ($\phi_\mathrm{orb} = 0$), based on the ephemeris of Cherepashchuk_2023.
• Figure 2: XRISM/Resolve spectra from 2024 (a, b) and 2025 (c, d) represent time-resolved data, extracted from 30 intervals in 2024 and 5 in 2025. The panels show representative intervals: ID 0 and 22 (2024), and ID 0 and 2 (2025). The spectra are fitted with two thermal jet components and a stationary Fe line. Black points show the data; orange, the total model. Blue, red, and green lines represent the approaching jet, receding jet, and Fe i K$\alpha$, respectively. Several representative jet emission lines are overplotted based on the best-fit Doppler shifts. Representative Fe and Ni lines (He$\alpha$, Ly$\alpha$) are shown according to the best-fit Doppler model. Each panel shows the spectra with residuals.
• Figure 3: Time-resolved XRISM/Resolve spectra in the 5.5--9 keV band from 2024 (a) and 2025 (b). The data are divided into 30 and 5 segments for the 2024 and 2025 observations, respectively, with vertical offsets applied to the black points for clarity. Spectra are fitted with two thermal jet components and a stationary Fe line (orange). Major emission lines are shown with the following colors: Fe xxv He$\alpha$ (blue/red), Fe xxvi Ly$\alpha$ (light blue/light red), and Ni xxvii He$\alpha$ (cyan/magenta), corresponding to the blue and red jets. The stationary Fe i K$\alpha$ line is shown in green. The right-hand axis shows the observation time in MJD. See table \ref{['tab:xray_doppler']} for the Doppler shift values.
• Figure 4: Optical spectra obtained with the Seimei and LCO telescopes. The blue- and red-shifted components of the H$\alpha$ line are shown in blue and red, respectively. Dotted green lines indicate stationary emission lines such as He i, C ii, and H$\alpha$, which are likely associated with the accretion disk wind or circumbinary material. The right-hand axis shows the observation time in MJD for each spectrum. The corresponding Doppler shift values are summarized in table \ref{['tab:optical_doppler']}.
• Figure 5: Doppler shifts observed with XRISM/Resolve (X-ray) and Seimei/LCO (optical) from 2024 to 2025. (a--c) Blue and red data points indicate the blue and red jets, respectively. Panel (a) also includes an inset showing a zoom-in view of the blue and red jets within the highlighted rectangular region. The precession and nutation models are calculated based on the ephemeris and parameters from table \ref{['tab:ephemeris']}, except for the reference epochs $t_{\mathrm{0,prec}}$ and $t_{\mathrm{nut}}$, which were fitted to the X-ray data as free parameters (see text).