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REVIEW 2 major objections 5 minor 3 references

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

T0 review · 2 major / 5 minor · reviewed 2026-08-02 · deepseek-v4-flash

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Pith's one-line read SS 433's jet speed oscillates with a ~6.3-day cycle and rose from ~0.26c to ~0.30c between 2024 and 2025, high-resolution X-ray spectra show.

desk verdict Good new XRISM data, but the ~6.3 d jet-speed modulation and −90° phase offset are inputs to Eq. (4), not outputs of the fit. read the letter →

arxiv 2602.21661 v1 pith:HSM7IQYT submitted 2026-02-25 astro-ph.HE

Yusuke Sakai , Shinya Yamada , Yuta Okada , Toshihiro Takagi , Tomoya Usuki , Megumi Shidatsu , Shogo B Kobayashi , Robert Petre
show 31 more authors
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
This is my paper
classification astro-ph.HE
keywords SS433microquasarrelativisticjetsXRISMDopplershiftnutationjetspeedvariabilitymulti-wavelengthphotometry
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper argues, from time-resolved X-ray spectroscopy with a high-resolution microcalorimeter, that the relativistic jets of the microquasar SS 433 do not leave the system at a constant speed. Reconstructed apparent jet speeds, assuming the two jets are symmetric, fluctuate sinusoidally with a period close to the known 6.3-day nutation cycle, but offset in phase by about −90°; the mean speed rose from about 0.26c in April 2024 to about 0.30c in March 2025. Simultaneous optical spectra hint that optical Doppler shifts lag the X-ray ones slightly. Optical light curves show flares of ~400–1600 seconds with amplitudes up to ~15%, while X-ray and near-infrared light curves stay steady within uncertainties. If correct, the results imply that the jet launch speed in SS 433 is variable and phase-locked to the disk's nutation, offering a new handle on jet-launching physics.

What carries the argument

The central machinery is the symmetric-jet kinematic model: from the measured Doppler shifts z_b and z_r of the blue and red jets, equations (2)–(3) recover the apparent jet angle α_sym and speed β_sym assuming both jets move at equal speeds in opposite directions. The speed is then fitted to β_sym = C_sym + δβ_sym sin(2π φ_nut − π/2), where φ_nut is the nutation phase and the −π/2 offset is fixed by hand based on the observed alignment of the modulation relative to the angle variation. The high spectral resolution and large effective area of the microcalorimeter provide Doppler-shift measurements with statistical errors below about 3×10⁻⁴ per time segment, which is what makes the ~0.01c amp

What would settle it

Fit equation (4) with both the nutation period and the phase offset as free parameters to the X-ray Doppler-shift time series; if the best-fit period deviates from ~6.3 d by more than the uncertainty, or the phase offset is consistent with zero, or the amplitude becomes consistent with zero at 2σ, the claimed nutation-locked speed modulation would be falsified. A second independent check: observe a full nutation cycle with continuous high-resolution X-ray coverage and verify that the speed curve repeats with the same phase lag.

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Extended reading notes

Core claim

The paper's central discovery is that the apparent speed of SS 433's two jets, reconstructed from the Doppler shifts of X-ray emission lines under the assumption that the jets are symmetric and ejected simultaneously in opposite directions, is not constant. The speed follows a sinusoidal curve with the period of the system's 6.3-day nutation cycle but is offset in phase by about −90°, with mean values of roughly 0.26c in 2024 and 0.30c in 2025. The paper interprets this as day-scale modulation of the jet launch speed tied to the nutation cycle, together with a modest year-scale increase. A secondary result is a suggested lag of optical Doppler shifts behind X-ray ones, plus a geometric expla

Load-bearing premise

The claim that the jet speed varies on a ~6.3-day cycle rests on a model in which the period is fixed to the known nutation period and the −90° phase offset is inserted by hand; if the phase and period were left free, the modulation could weaken or vanish, especially in the 2025 data where the amplitude is only about 0.0075±0.0035.

Editorial extensions

If this is right

  • If the jet speed really oscillates with the nutation phase, models treating the jet speed as constant need revision; nutation is not merely a geometric wobble of the disk but couples to the outflow's launch conditions.
  • The year-scale increase from ~0.26c to ~0.30c implies a noticeable change in the jet's bulk Lorentz factor between 2024 and 2025, altering Doppler beaming and intrinsic power estimates for the epochs.
  • The apparent optical lag behind X-rays, if confirmed, constrains the distance between the X-ray-emitting inner jet and the optical line-emitting region.
  • The consistency of line broadening and flux with inner-jet geometry supports the picture that X-ray lines emerge from an inner, partially obscured region, informing eclipse mapping of the jet base.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the phase offset and period in equation (4) were left free rather than fixed, the 2025 modulation (amplitude ~0.0075±0.0035) might become statistically marginal; a denser campaign spanning a full nutation cycle would settle whether the −90° lag is physical or a projection effect.
  • The apparent increase in mean jet speed between the two epochs may be connected to the higher optical brightness seen in 2025; a testable extension would be to compare radio flaring or disk-state indicators over the same epochs.
  • Since the nutation period is fixed in the fit, any long-term drift in the nutation period (analogous to known phase shifts in the precession ephemeris) would alias into the derived speed residuals; folding the data on a drifting period could reveal whether part of the apparent speed modulation is an artifact of an outdated ephemeris.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 5 minor

Summary. The paper reports coordinated XRISM/Resolve X-ray spectroscopy with simultaneous optical (Seimei, LCO, Tomo-e Gozen, MITSuME) and near-infrared (kSIRIUS) observations of SS 433 obtained in 2024 April and 2025 March. It measures time-resolved Doppler shifts of the jet emission lines, derives symmetric jet angles and velocities, and claims that the jet speed varies sinusoidally on a ~6.3-d timescale with a phase offset of about −90° relative to the nutation cycle. Additional results include an apparent X-ray/optical Doppler-shift lag, line-width and flux ratios consistent with geometric effects and inner-jet broadening, short optical flares, and updated precession/nutation reference epochs.

Significance. The data quality is high: the Resolve spectral fits are careful, the Doppler-shift uncertainties are small (Δz <~ 3e-4 in 2024), and the multiwavelength coverage is valuable. If the jet-speed modulation at the nutation period with a −90° phase offset were robust, it would be an important probe of jet-launching physics in SS 433. However, the headline modulation claim is not supported by the analysis as presented, because the period and phase offset are fixed inputs, not fitted outputs. The other results—line broadening geometry, optical flares, ephemeris updates—are useful but incremental. The paper would be publishable with a substantially revised interpretation.

major comments (2)
  1. [§4.4, Table 2] The abstract and §5 report 'modulations on a timescale of ~6.3 d, with a phase offset of about −90° relative to the nutation cycle' as measured results, but Eq. (4) hard-codes P_nut = 6.287599 d and the sin(2πφ_nut − π/2) form. The free parameters are only C_sym, δβ_sym, and t0_nut; t0_nut shifts the zero-point of the sinusoid but does not change the fixed ±90° relationship. Thus the fit cannot test the period or the phase offset—it only tests whether a sinusoid of exactly this period and phase has nonzero amplitude. In 2025 the amplitude is δβ_sym = 0.0075 ± 0.0035 (~2.1σ), marginal. This is a load-bearing circularity for the central claim and must be fixed by either refitting with free period/phase or rewriting the abstract and conclusions to explicitly state that the 6.3-d period and −90° offset are assumed, not measured.
  2. [§4.4, Fig. 12] The 2024 dataset spans only ~4.6 d (MJD 60410.684–60415.295; IDs 0–29), i.e., ~0.73 of the assumed 6.2876-d period, and the 2025 dataset spans ~2 d (~0.3 period). A sinusoid with fixed period and phase can represent a monotonic trend over such a short baseline, so the 'clear periodic modulation' described in the text is not established. The authors should compare the sinusoid against simpler alternatives (constant, linear trend) or show confidence regions for a free-period/free-phase fit. Without such a test, the claim that the velocity variations follow the nutation period is not supported.
minor comments (5)
  1. [Section 1 vs. Table 3 caption] The text states all quoted uncertainties are 1σ unless otherwise noted, but Table 3 explicitly says uncertainties are at the 90% confidence level. Please clarify which convention applies to the Doppler shifts and other parameters in Table 3.
  2. [Table 4] The optical Doppler shifts from Seimei and LCO are listed without uncertainties. Provide at least an estimated uncertainty (e.g., from the Hα line width or wavelength calibration) or state that these are approximate peak measurements.
  3. [Fig. 5 caption] The caption refers to 't_nut' while the text and Table 1 use 't0,nut'. Unify the notation.
  4. [Abstract] 'The SS 433' should be 'SS 433' (no definite article).
  5. [Acknowledgments] The acknowledgment thanking the referee is unconventional in a preprint and should be removed for the arXiv version.

Circularity Check

1 steps flagged · score 6.0 of 10

Claimed 6.3-d modulation and −90° phase offset are hard-coded in Eq. (4), not measured.

  1. self definitional [§4.4, Eq. (4), Table 2; Abstract; Conclusion]
    "To quantify this apparent periodicity in β_sym, we fit a simple sinusoid with a −π/2 phase offset, motivated by the observation ... β_sym,model = C_sym + δβ_sym sin(2πϕnut − π/2), (4) ... The nutation period was fixed to the value from Davydov et al. (2008) ... Figures 12c and 12d indicate modulations in the jet speed δβ_sym on a timescale of ∼6.3 d ... with a phase offset of about −90◦ relative to the nutation cycle."

    Equation (4) already contains sin(2πϕnut − π/2) with P_nut fixed to 6.287599 d. The only fitted parameters are C_sym, δβ_sym, and t0_nut. Therefore the '~6.3 d timescale' is the fixed input P_nut and the '−90° phase offset' is the fixed input −π/2. The abstract and conclusion report these two quantities as observational results ('The velocity variations indicated modulations on a timescale of ~6.3 d, with a phase offset of about −90° relative to the nutation cycle'), but the fit cannot validate the period or the phase: those values are true by construction. Only the mean speed and amplitude are genuinely data-derived.

full rationale

The central periodicity claim is partially circular. The XRISM/Resolve Doppler shifts are independent external data, and the β_sym values derived via Eq. (3) are genuine measurements; the mean speeds (~0.26c in 2024, ~0.30c in 2025) and modulation amplitudes are fitted outputs. However, the headline result — that jet speed varies on ~6.3 d with a ~−90° phase offset relative to nutation — is not a fit outcome: Eq. (4) hard-codes P_nut = 6.287599 d and a −π/2 phase term, with the text explicitly stating that the phase offset was 'motivated by the observation' and the period fixed to Davydov et al. (2008). Reporting these inputs as 'indicated modulations' in the abstract and conclusion makes the claimed result equivalent to the assumed model. The 2024 baseline (~4.6 d) is shorter than one nutation period and the 2025 amplitude is only 0.0075 ± 0.0035 (~2σ), so the data alone would not establish the sinusoid. No other load-bearing circularity was found: the comparisons with Shidatsu et al. (2025) use new line-width and flux measurements from Resolve, and the ephemeris offsets are fitted to the X-ray Doppler data rather than assumed. Score 6 reflects that one central 'prediction' reduces by construction while the underlying Doppler measurements remain independent.

Assumptions & free parameters 5 free parameters · 5 assumptions · 0 invented entities

The central kinematic claims rest on two free-parameter fits (Eq. 4 and ephemeris alignment) plus the standard SS 433 precession/nutation model and the symmetric-jet assumption. No new physical entities are introduced. The phase offset −π/2 is the most consequential free choice because it directly creates the reported −90° result.

free parameters (5)
  • C_sym (mean symmetric jet velocity) = 0.2646±0.0002 (2024), 0.2962±0.0031 (2025)
    Fitted in Eq. (4) to β_sym values derived from Resolve Doppler shifts; central to the jet-speed claim.
  • δβ_sym (modulation amplitude) = 0.0148±0.0003 (2024), 0.0075±0.0035 (2025)
    Fitted in Eq. (4); the 2025 amplitude is only ~2σ, so the 2025 modulation is marginal.
  • t0,nut (nutation reference epoch) = 43032.97±0.02 (2024), 43033.41±0.20 (2025)
    Fitted in Eq. (4); shifts the sinusoid relative to the Davydov et al. (2008) ephemeris.
  • t0,prec (precession reference epoch) = 51467.67±0.01
    Fitted to X-ray Doppler shifts in §3.2; used to align the precession model curves.
  • Phase offset −π/2 in Eq. (4) = fixed by hand
    Chosen a priori 'motivated by observation'; this is the source of the reported −90° phase offset.
assumptions (5)
  • domain assumption Doppler-shift model: z = γ[1 ± β(cos i cos θ + sin i sin θ cos(2πφ_prec))]^{-1}; δz = ±A_nut sin(2πφ_nut) (Eq. 1)
    Standard kinematic model for SS 433 jet precession plus sinusoidal nutation, adopted from Gies et al. (2002) and Davydov et al. (2008).
  • domain assumption Symmetric jets: blue and red jets launched simultaneously in opposite directions with the same intrinsic speed (Eq. 2)
    Invoked in §4.4 to convert measured Doppler shifts into α_sym and β_sym; the paper admits real jets may be asymmetric.
  • domain assumption APEC thermal plasma model for jet X-ray emission with Fe I Kα Gaussian (Section 3.1)
    Standard spectral model taken from Shidatsu et al. (2025); the paper notes more complex models are deferred.
  • domain assumption Doppler boosting relation F ∝ D^{n+Γ} with n=2 or 3 (Section 4.3)
    Used to interpret flux ratios as geometric; the paper does not attempt to distinguish n=2 vs n=3.
  • domain assumption Companion/disk obscuration of inner jet regions explains line-width and flux patterns (Section 4.3)
    Qualitative interpretation; not independently constrained by this dataset.

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Cite this review

Pith. "Pith review of Spectral and photometric variability of SS 433 observed with XRISM and simultaneous optical and near-infrared telescopes." pith.science (2026). https://pith.science/paper/HSM7IQYT

@misc{pith2026260221661,
  author       = {Pith},
  title        = {Pith review of: Spectral and photometric variability of SS 433 observed with XRISM and simultaneous optical and near-infrared telescopes},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/HSM7IQYT}},
  note         = {Machine review of arXiv:2602.21661}
}
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

Figures reproduced from arXiv: 2602.21661 by the authors.

Figure 1
Figure 1. Overview of the multiwavelength observation campaign of SS 433 conducted in 2024–2025. (a) Doppler shifts of the approaching and receding jets (zb and zr). The curves are calculated from the precession and nutation models based on the ephemerides and parameters of Gies et al. (2002) and Davydov et al. (2008), with the phase offset adjusted to match the observed Doppler shifts (see text). The dotted line indicates th… view at source ↗
Figure 2
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 sta￾tionary Fe line. Black points show the data; orange, the total model. Blue, red, and green lines represent the approaching jet, reced… view at source ↗
Figure 4
Figure 4. Optical spectra obtained with the Seimei and LCO telescopes. The blue- and red-shifted components of the Hα line are shown in blue and red, respectively. Dotted green lines indicate stationary emission lines such as He I, C II, and Hα, 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 s… view at source ↗
Figures from the paper (13 more)
Figure 3
Figure 3. 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 ap￾plied to the black points for clarity. Spectra are fitted with t…
Figure 5
Figure 5. Figure 5: figure 5. The updated reference epochs are summarized in table 1, [PITH_FULL_IMAGE:figures/full_fig_p007_5.png]
Figure 5
Figure 5. 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 highl…
Figure 6
Figure 6. Figure 6: Summary of simultaneous X-ray, optical and near-infrared observa￾tions conducted in 2024 and 2025. The orbital phase is calculated based on the ephemeris from Cherepashchuk et al. (2023). (a) XRISM/Xtend light curve in the 2–10 keV band. (b) Tomo-e Gozen optical light …
Figure 7
Figure 7. Figure 7: Light curves obtained with Tomo-e Gozen and XRISM/Xtend in 2024. (a)–(c) correspond to April 10, 12, and 14, 2024, respectively, showing optical data from Tomo-e Gozen (blue) and X-ray data from XRISM/Xtend (orange; 2–10 keV, 32 s bins). In panel (b), an optical flare …
Figure 8
Figure 8. Figure 8: kSIRIUS and XRISM/Xtend light curves in 2024. From top to bot￾tom, the panels show the XRISM/Xtend light curve (2–10 keV, 32 s bins), the Ks, H, and J bands, and the color indices H − Ks and J − H from kSIRIUS. For the kSIRIUS panels, the left and right columns corresp…
Figure 9
Figure 9. Figure 9: Light curves with Tomo-e Gozen, XRISM/Xtend, and MITSuME obtained in 2025. (a) Light curve from March 26, showing optical data from Tomo-e Gozen (blue) and X-ray data from XRISM/Xtend (orange; 2– 10 keV, 32 s bins). A gray bar with a width of 1600 s indicates the appro…
Figure 11
Figure 11. Figure 11: X-ray jet velocity widths and fluxes derived from Resolve spectral fitting. (a, b) Velocity widths (FWHM; Wv) of the blue (Wvb ) and red (Wvr ) jets obtained from the Resolve spectra in the 5.5–9 keV band for 2024 and 2025, respectively (see also Shidatsu et al. (2025…
Figure 12
Figure 12. Figure 12: Jet angles (αsym; panels a and b) and jet velocities (βsym; panels c and d) derived under the symmetric assumption (equation (3)) for the years 2024 and 2025. Black points and gray crosses indicate the observed blueshifted and redshifted components obtained with Resol…
Figure 13
Figure 13. Figure 13: Full-field optical image of the SS 433 region obtained with Tomo￾e Gozen. The image corresponds to the first data cube acquired on 2024 April 10 and is generated by stacking 10 consecutive frames with back￾ground subtraction applied. SS 433 and the photometric calibra…
Figure 14
Figure 14. Figure 14: Light curves obtained with MITSuME during April 5–13, 2024. From top to bottom, the panels show the IC, RC, g ′ bands, followed by the color indices RC − IC and g ′ − RC. 12.2 12.4 12.6 12.8 13.0 13.2 RC [mag] 1.25 1.50 1.75 2.00 R C ¡ IC [m a g] (a) tMJD » 60405:8; Á…
Figure 16
Figure 16. Figure 16: Scatter plots of magnitude versus color index based on the kSIR￾IUS light curve data shown in figure 8. Panel (a) shows Ks versus H −Ks, and panel (b) shows H versus J − H. Beniyama, J., Sako, S., Ohsawa, R., et al. 2022, PASJ, 74, 877 Bessell, M. S., & Brett, J. M. 1…
Figure 17
Figure 17. Figure 17: Time-resolved XRISM/Resolve spectra in the 1.7–10 keV band from 2024 (a) and 2025 (b). Black points represent the observed data, shown with vertical offsets for clarity. The 2024 data are divided into 30 time segments, and the 2025 data into 5 segments. Kodaira, K., N…

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Works this paper leans on

3 extracted references · 1 linked inside Pith

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    Panel (a) showsKs versusH−K s, and panel (b) showsHversusJ−H. Beniyama, J., Sako, S., Ohsawa, R., et al. 2022, PASJ, 74, 877 Bessell, M. S., & Brett, J. M. 1988, PASP, 100, 1134 Blanton, M. R., & Roweis, S. 2007, AJ, 133, 734 Blundell, K. M., & Bowler, M. G. 2004, The Astrophysical Journal, 616, L159 Blundell, K. M., Bowler, M. G., & Schmidtobreick, L. 20...

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    From top to bottom, the panels show theI C,R C,g ′ bands, followed by the color indicesR C −I C andg ′ −R C. 12.2 12.4 12.6 12.8 13.0 13.2 RC [mag] 1.25 1.50 1.75 2.00RC − IC [mag] (a) tMJD ∼ 60405.8, φorb ∼ 0.60 tMJD ∼ 60407.8, φorb ∼ 0.75 tMJD ∼ 60409.8, φorb ∼ 0.90 tMJD ∼ 60410.8, φorb ∼ 0.98 tMJD ∼ 60411.8, φorb ∼ 0.06 tMJD ∼ 60412.8, φorb ∼ 0.13 tMJD...

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