SS 433's Jets: A Dance with a Glitch express
The binary system SS 433 ejects two thin jets of gas at nearly the speed of light. One flies toward us, the other away from us. In April 2024 and March 2025, three instruments watched them simultaneously: the X-ray telescope XRISM, ordinary optical telescopes, and a near-infrared camera.
When comparing how fast the jets move in X-rays versus visible light, a shift was noticed: the optical emission seems to lag a step behind. Like a translator who hears a phrase and only a moment later speaks it aloud. Perhaps the light is born a bit farther from the center than the X-rays, and it takes time to reach our instruments.
The jet speed wasn't constant. In 2024, it oscillated around 26% of the speed of light; in 2025, it was already about 30%. It varied with a period of roughly 6.3 days. This resembles a rhythm with which the system wobbles — the so-called nutation cycle. But the phase shift was about minus 90 degrees: the jet reaches its peak speed not when you'd expect from the wobble.
Visible-light flares lasted about 400 seconds in 2024 and 1600 seconds in 2025. Brightness jumped by up to 15%. But the X-rays stayed quiet during that time — no noticeable flares. The infrared channel also saw nothing similar. So flares are born only in visible light, while X-rays remain calm.
These observations are like three different witnesses to the same event. Each sees their own thing, and only together do they form the full picture. In the future, XRISM will continue to watch SS 433, and then perhaps it will become clear why the optical light lags and where these flares come from.
Original abstract
as written by the authors arXiv:2602.21661
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.
SS 433's jets move at about 26–30% of the speed of light: that's roughly 80,000 kilometers per second. In one second, such a jet would cross the Atlantic Ocean eight times.