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

This paper presents coordinated multiwavelength observations of SS 433 using XRISM, optical, and near-infrared telescopes in 2024 and 2025, revealing high-resolution X-ray spectral details of jet velocity modulations, a potential optical lag behind X-rays, and distinct optical flaring behavior that contrasts with the stable X-ray and near-infrared emission.

Original authors: Yusuke Sakai, Shinya Yamada, Yuta Okada, Toshihiro Takagi, Tomoya Usuki, Megumi Shidatsu, Shogo B Kobayashi, Robert Petre, Yoshihiro Ueda, Hideki Uchiyama, Miho Tan, Taro Kotani, Taichi Igarashi, Mami

Published 2026-07-08✓ Author reviewed ⓘ

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Yusuke Sakai, Shinya Yamada, Yuta Okada, Toshihiro Takagi, Tomoya Usuki, Megumi Shidatsu, Shogo B Kobayashi, Robert Petre, Yoshihiro Ueda, Hideki Uchiyama, Miho Tan, Taro Kotani, Taichi Igarashi, Mami Machida, Haruka Sakemi, Nobuyuki Kawai, Daiki Miura, Hiroya Yamaguchi, Kanta Fujiwara, Daichi Hiramatsu, Keisuke Isogai, Chulsoo Kang, Mariko Kimura, Katsuhiro L Murata, Takahiro Nagayama, Taichi Nakamoto, Kosuke Namekata, Yuki Niida, Yuu Niino, Masafumi Niwano, Kyuseok Oh, Shigeyuki Sako, Mahito Sasada, Hiromasa Suzuki, Kenta Taguchi, Ichiro Takahashi, Miyu Uenishi, Yoichi Yatsu, Marina Yoshimoto

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic Gyroscope: A New Look at SS 433

Imagine a cosmic lighthouse, but instead of a steady beam, it shoots out two powerful, twisting jets of gas in opposite directions. This is SS 433, a famous "microquasar" in our galaxy. It's a pair of stars: a giant, bloated star and a tiny, dense companion (either a black hole or a neutron star) locked in a dance. As they orbit, the companion star sucks material from the giant, creating a swirling disk that launches these jets at nearly a third the speed of light.

What makes SS 433 special is that the whole system wobbles like a spinning top that's about to fall over. This wobble is called precession (a slow 162-day cycle) and a faster, smaller wobble called nutation (a 6.3-day cycle).

In April 2024 and March 2025, a team of astronomers used a powerful new X-ray telescope called XRISM, along with several optical and infrared telescopes, to take a high-definition, synchronized movie of this cosmic dance.

The "Super-Microscope" (XRISM)

Previous telescopes were like looking at a fast-moving car through a foggy window; you could see it moving, but the details were blurry. XRISM's Resolve instrument is like a super-microscope with a high-speed camera. It can see the specific "colors" (spectral lines) of the X-rays coming from the jets with incredible precision.

Because the jets are moving so fast, the light they emit gets stretched or squeezed (the Doppler effect), just like the sound of a siren changing pitch as an ambulance zooms past. XRISM was able to measure these pitch changes so accurately that the team could track the speed and direction of the jets in real-time, even over short periods.

What They Found: The Speed Bumps and the Lag

1. The Jets are Speeding Up and Slowing Down
The team discovered that the jets aren't just wobbling; their actual speed is fluctuating.

The Analogy: Imagine a runner on a track who speeds up and slows down rhythmically, not because they are tired, but because the track itself is tilting back and forth.
The Result: In 2024, the jets were moving at about 26% the speed of light. By 2025, they had sped up to 30%. This speed change happens on a 6.3-day cycle, perfectly matching the "nutation" wobble. Interestingly, the speed changes seem to be out of step with the wobble by about a quarter of a cycle (a 90-degree phase shift), suggesting a complex relationship between the wobble and the engine driving the jets.

2. The "Optical Lag"
The team looked at the jets in both X-rays and visible light (optical) at the exact same time.

The Analogy: Imagine a drummer (the X-ray source) and a backup singer (the optical source). The drummer hits the snare, and a split second later, the singer hits their note.
The Result: The visible light from the jets seemed to lag slightly behind the X-rays. This suggests that the X-rays are generated closer to the "drum" (the center of the system), while the visible light is generated further out along the jet, taking a tiny bit longer to reach us or to be produced.

3. The "Foggy" Eclipse
SS 433 is tilted such that sometimes the companion star or the accretion disk blocks our view of the jets (an eclipse).

The Analogy: Imagine looking at a streetlamp through a window. If you move the window frame, you might block the bottom of the lamp but see the top.
The Result: When the jets were partially blocked by the companion star, the "width" of the X-ray lines changed. This confirmed a theory that the inner parts of the jet (closer to the center) are "fuzzier" or more turbulent (broader lines) than the outer parts. The geometry of the eclipse helped prove that the jet isn't a uniform beam, but has different properties at different distances from the center.

4. The Optical Flares
While the X-ray light remained steady, the optical light (visible to the eye) had sudden, short bursts of brightness.

The Analogy: Think of a calm river (X-rays) with occasional, brief splashes of water (optical flares) caused by a rock dropping in.
The Result: In 2024, they saw a splash lasting about 400 seconds. In 2025, a longer splash lasting 1,600 seconds. These flares made the star about 15% brighter for a short time, but the X-rays didn't react. This tells us these flashes are happening in the outer, visible parts of the system, not deep in the X-ray engine.

The Big Picture

This paper is like upgrading from a black-and-white sketch to a 4K color video of a cosmic engine. By using XRISM's sharp vision alongside other telescopes, the team confirmed that the jets in SS 433 are dynamic, changing their speed in rhythm with the system's wobbles. They also mapped out exactly where the light is coming from, showing that the "engine" (X-rays) and the "exhaust" (optical light) behave slightly differently.

The study doesn't solve the mystery of why the jets speed up and slow down with the wobble, but it provides the clearest, most detailed map of the phenomenon to date, setting the stage for future astronomers to figure out the physics behind this cosmic gyroscope.

Technical Summary: Spectral and Photometric Variability of SS 433 Observed with XRISM and Simultaneous Optical and Near-Infrared Telescopes

Problem and Context
SS 433 is a unique microquasar system comprising an A-type supergiant and a compact object, characterized by baryonic jets that precess with a period of 162.5 days and exhibit a 6.3-day nutation cycle. While the system has been extensively studied for over four decades, the spatial origin of jet emission and the precise kinematic behavior of the jets remain subjects of investigation. Previous studies using instruments like Suzaku and Chandra suggested a time lag between optical and X-ray Doppler shifts and hinted at variations in jet speed and asymmetry, but were limited by spectral resolution and photon statistics. Specifically, the physical origin of the nutation cycle and the relationship between jet velocity fluctuations and the precession/nutation phases require higher precision measurements to resolve.

Methodology
The authors conducted a coordinated multiwavelength campaign observing SS 433 in April 2024 and March 2025, capturing distinct orbital and precessional phases. The campaign utilized:

XRISM (X-ray Imaging and Spectroscopy Mission): High-resolution spectroscopy was performed using the Resolve microcalorimeter (5.5–9 keV band) to measure Doppler shifts of Fe and Ni emission lines. Light curves were obtained using the Xtend CCD camera (2–10 keV). Data were segmented into time intervals (30 segments for 2024, 5 for 2025) to enable time-resolved spectroscopy.
Optical Spectroscopy: Simultaneous observations were conducted using the Seimei telescope (KOOLS-IFU) and the Las Cumbres Observatory (LCO/FLOYDS) to measure Hα Doppler shifts.
Photometry: High-cadence optical photometry was obtained using Tomo-e Gozen and MITSuME telescopes, while near-infrared (JHKs) monitoring was performed with the kSIRIUS camera.

The analysis involved fitting thermal plasma models (APEC) supplemented by stationary Fe I Kα lines to the XRISM spectra to derive Doppler shifts ( $z$ ), line widths ( $W_v$ ), and fluxes. Optical Doppler shifts were derived from Hα peak positions. The authors applied a kinematic model combining jet precession and a sinusoidal nutation term to the measured shifts, treating reference epochs as free parameters to update ephemerides.

Key Results

Doppler Shift Precision and Lag: XRISM/Resolve achieved high-precision Doppler shift measurements ( $\Delta z < \sim 3 \times 10^{-4}$ per segment). The simultaneous X-ray and optical data suggest a tendency for optical emission to lag slightly behind X-rays, consistent with previous findings by Kubota et al. (2010) and Marshall et al. (2013), though the authors note that longer monitoring is required to establish statistical significance and phase dependence.
Jet Kinematics and Asymmetry: The study revealed apparent asymmetries in the two jet components. Jet speeds fluctuated around $\sim 0.26 \pm 0.01c$ in 2024 and $\sim 0.30 \pm 0.01c$ in 2025. These velocity variations exhibited modulations on a timescale of $\sim 6.3$ days, matching the nutation period but with a phase offset of approximately $-90^\circ$ relative to the nutation cycle.
Geometrical Effects on Line Widths and Flux: The analysis of line widths and fluxes supports a geometrical interpretation where the inner regions of the jets possess systematically larger line widths. In 2024, the approaching (blue) jet showed broader lines and higher flux contrast between eclipse and out-of-eclipse phases compared to the receding (red) jet. In 2025, when the jet axis was nearly perpendicular to the line of sight, both jets showed comparable widths and brightness. This aligns with the model proposed by Shidatsu et al. (2025) regarding partial obscuration of inner jet regions by the accretion disk and companion star.
Photometric Variability: Optical light curves revealed short-duration flares ( $\sim 400$ s in 2024 and $\sim 1600$ s in 2025) with amplitudes up to $\sim 15\%$ during out-of-eclipse intervals. In contrast, the XRISM/Xtend X-ray light curves showed no significant variability during these overlapping intervals within statistical uncertainties. Near-infrared photometry in 2024 also remained constant.
Updated Ephemerides: The study derived updated reference epochs for precession ( $t_{0,prec} = 51467.67 \pm 0.01$ MJD) and nutation ( $t_{0,nut} = 43032.423 \pm 0.007$ MJD), showing offsets of roughly +10 days and +2.8 days, respectively, relative to previous ephemerides.

Significance
The paper claims that these coordinated observations provide a foundational dataset for future XRISM studies aimed at probing the dynamical properties of relativistic jets in SS 433. The primary significance lies in the unprecedented accuracy of Doppler-shift measurements enabled by XRISM/Resolve, which allows for the detection of subtle kinematic modulations (such as the $-90^\circ$ phase offset in jet velocity relative to nutation) that were previously unresolved. The results reinforce the geometric model of jet emission, where line broadening and flux variations are driven by the viewing angle and obscuration of the inner jet regions. Furthermore, the detection of optical flares without corresponding X-ray variability highlights the complexity of the emission mechanisms in different bands. The authors conclude that continued long-term, multi-wavelength monitoring across various orbital and precessional phases is essential to fully disentangle these effects and clarify the physical processes driving the system's relativistic outflows.

The Cosmic Gyroscope: A New Look at SS 433

The "Super-Microscope" (XRISM)

What They Found: The Speed Bumps and the Lag

The Big Picture

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