REVIEW 1 major objections 2 minor 9 references
Full-polarization millimeter wavelength variability of Sagittarius A* during the 2018 EHT campaign
T0 review · 1 major / 2 minor · reviewed 2026-05-10 · grok-4.3
Pith's one-line read Near-simultaneous X-ray and millimeter peaks in Sagittarius A* point to continuous energy injection in optically thin plasma.
desk verdict This paper adds new ALMA full-polarization light curves and flare timing from the 2018 EHT run on Sgr A*, but the claim that simultaneity rules out cooling delays rests on timing coincidence whose significance against red noise is not shown in the abstract. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
Inter-band delay measurements in polarized intensity light curves combined with direct timing comparison to the simultaneous X-ray flare.
What would settle it
A repeat high-cadence campaign that measures a clear multi-minute lag between X-ray and millimeter peaks matching expected synchrotron cooling times in several flares would restore the standard transient model.
Extended reading notes
Core claim
The near-simultaneous X-ray and millimeter peaks on April 24, together with enhanced variability and stable polarization timescales, indicate that the flare emission arises from continuous energy injection within an optically thin region rather than from a transient synchrotron event subject to cooling delays.
Load-bearing premise
The X-ray flare and millimeter peak must be produced by the same plasma parcel so that their timing can distinguish cooling delays from continuous injection.
Editorial extensions
If this is right
- Accretion flow models must include steady energy replenishment to reproduce flare simultaneity across wavelengths.
- Polarization arises from a spatially coherent structure separate from the more variable total-intensity region.
- Power spectra with indices between -2 and -3 describe the variability in all Stokes parameters on intra-day scales.
- Stable polarized-intensity timescales constrain the emitting region's size or magnetic coherence.
Reading between the lines
- Similar timing tests in other black-hole systems could reveal whether continuous injection is a general feature of near-horizon plasma.
- Multi-wavelength monitoring at higher cadence might isolate whether magnetic reconnection supplies the ongoing energy.
- If the pattern holds, flare models for Sgr A* would shift emphasis from single injection events to sustained energization processes.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript presents an analysis of full-polarization millimeter-wavelength light curves of Sagittarius A* obtained with ALMA during the 2018 EHT campaign. Key findings include low fractional variability in total intensity (<10%), higher variability in linear (~30%) and circular (~50%) polarization, red-noise power spectral densities with indices between -2 and -3, and distinct variability timescales between total and polarized intensity. A notable event on 2018 April 24 is highlighted, featuring near-simultaneous X-ray and millimeter peaks along with a statistically significant inter-band delay in polarized intensity, which the authors interpret as evidence favoring continuous energy injection over standard cooling-delay models in transient synchrotron emission.
Significance. If the reported simultaneity and its interpretation are robust, the paper would provide significant new constraints on the physical processes governing variability and emission in the accretion flow around Sgr A*. It contributes observational evidence that can help discriminate between competing models of plasma heating and synchrotron radiation near the event horizon. The full-polarization approach and multi-wavelength comparison are particularly valuable strengths.
major comments (1)
- [Abstract (discussion of April 24 event)] The assertion of a 'statistically significant inter-band delay in polarized intensity' coinciding with the X-ray and millimeter peaks is central to the claim that this event deviates from typical delayed-flare behavior and supports continuous energy injection. However, no cross-correlation coefficient, significance level, or test against the red-noise background (PSD slopes -2 to -3) is provided. This omission makes it difficult to evaluate whether the observed simultaneity is statistically meaningful or consistent with chance alignment of independent processes.
minor comments (2)
- [Abstract] The acronym 'Srg A*' appears to be a typographical error and should be corrected to 'Sgr A*'.
- [Abstract] The abstract would benefit from a brief mention of the number of epochs, frequency bands, and any data quality cuts applied to the ALMA observations to allow better assessment of the reported variability levels.
Simulated Author's Rebuttal
We thank the referee for their careful and constructive review of our manuscript. We address the single major comment below and will revise the paper accordingly to strengthen the statistical support for our claims.
read point-by-point responses
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Referee: [Abstract (discussion of April 24 event)] The assertion of a 'statistically significant inter-band delay in polarized intensity' coinciding with the X-ray and millimeter peaks is central to the claim that this event deviates from typical delayed-flare behavior and supports continuous energy injection. However, no cross-correlation coefficient, significance level, or test against the red-noise background (PSD slopes -2 to -3) is provided. This omission makes it difficult to evaluate whether the observed simultaneity is statistically meaningful or consistent with chance alignment of independent processes.
Authors: We agree with the referee that the manuscript would benefit from explicit quantitative details on the cross-correlation analysis to substantiate the claim of statistical significance. While the abstract and main text describe the near-simultaneous X-ray/mm peaks and the inter-band delay in polarized intensity, along with its interpretation favoring continuous energy injection, the specific cross-correlation coefficient, lag value, and formal significance test against red-noise realizations were not reported. In the revised manuscript we will add this information, including the peak cross-correlation coefficient, the measured lag, and the results of Monte Carlo simulations that generate surrogate light curves matching the observed PSD slopes (-2 to -3) to establish the significance level. This addition will allow readers to directly assess the robustness of the simultaneity against the red-noise background. revision: yes
Circularity Check
No significant circularity; claims rest on direct observational timing and variability metrics
full rationale
The paper reports ALMA light-curve statistics (variability amplitudes, PSD slopes between -2 and -3, intra-day timescales) and a timing coincidence between an X-ray flare and millimeter peak. These quantities are extracted from the data themselves and compared against pre-existing standard synchrotron models; no equation or parameter is fitted inside the paper and then re-labeled as a 'prediction' of the same data. No self-citation chain is invoked to justify uniqueness or an ansatz, and the central claim (simultaneity favoring continuous injection) is presented as an empirical challenge rather than a derived identity. The analysis is therefore self-contained against external benchmarks.
Assumptions & free parameters
Cite this review
Pith. "Pith review of Full-polarization millimeter wavelength variability of Sagittarius A* during the 2018 EHT campaign." pith.science (2026). https://pith.science/paper/2604.10287
@misc{pith2026260410287,
author = {Pith},
title = {Pith review of: Full-polarization millimeter wavelength variability of Sagittarius A* during the 2018 EHT campaign},
year = {2026},
howpublished = {\url{https://pith.science/paper/2604.10287}},
note = {Machine review of arXiv:2604.10287}
}
abstract
Sagittarius A* (Srg A*), the supermassive black hole at the center of the Milky Way, provides a unique laboratory to study accretion dynamics and plasma processes near the event horizon. We investigated the variability and polarization properties of Srg A* using ALMA observations during the 2018 Event Horizon Telescope campaign. We analyzed high-cadence full-polarization light curves from ALMA at millimeter wavelengths, performed time-series analysis, and investigated the temporal behavior during an X-ray flare observed by Chandra on 2018 April 24. The variability characteristics are compared with expectations from standard accretion flow models. We find low variability in total intensity ($\sigma/\mu < 10\%$), but significantly higher variability in linear and circular polarization (~ 30% and ~ 50%, respectively). A time-series analysis reveals red-noise variability, with power spectral densities between -2 and -3 across all Stokes parameters. Polarized intensity shows stable intra-day timescales, while total intensity exhibits more variable timescales, suggesting distinct emission regions, with polarization likely arising from a coherent structure. On April 24, a statistically significant inter-band delay in polarized intensity coincides with a near-simultaneous X-ray and millimeter peak that deviates from the typical delayed flare scenario. This event also features enhanced millimeter variability and coherent polarization loop evolution. The observed simultaneity challenges standard models of transient synchrotron emission with cooling delays, favoring instead a scenario of continuous energy injection in an optically thin region. Our results offer new constraints on the physical mechanisms driving variability in Srg A*, and provide key observational input for refining theoretical models of accretion and plasma behavior in the vicinity of supermassive black holes.
Figures
Figures from the paper (11 more)
Reference graph
Works this paper leans on
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[1]
Albentosa-Ruiz, E. & Marchili, N. 2024, PASP, 136, 114502 Aschenbach, B., Grosso, N., Porquet, D., & Predehl, P. 2004, A&A, 417, 71 Balick, B. & Brown, R. L. 1974, ApJ, 194, 265 Blackburn, L., Chan, C.-k., Crew, G. B., et al. 2019, ApJ, 882, 23 Bower, G. C., Broderick, A., Dexter, J., et al. 2018, ApJ, 868, 101 Bower, G. C., Falcke, H., Wright, M. C., & B...
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[2]
We display both the sum and difference of the parallel-hand correlations (XXandYY) and the cross-hand cor- relations (XYandY X), which correspond to the Stokes I, Q, U, and V light curves, respectively, according to the Radio Interfer- ometer Measurement Equation formalism (Smirnov 2011). The visibility amplitude light curves confirm that, unlike Sgr A∗, ...
work page 2011
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[3]
On April 21, the LNDCF is slightly lower, around 0.6±0.1. The reduced correlation on April 21 and 25 is likely due to the lower elevation of Sgr A ∗ during ALMA observations coinciding with SMA coverage, leading to lower observed flux densities and increased noise in the light curves. Given this overall consistency, also in agreement with the findings of ...
work page 2018
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[4]
Appendix F: Details of the structure function analysis Section 4.2 presents the main results of the SF analysis of the Sgr A∗ 2018 ALMA light curves. In this appendix we examine in greater detail the behavior of the SF and describe the methodol- ogy employed to estimate PSD slopes and variability timescales. In Fig. 10 the SF of total intensity for April ...
work page 2018
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[5]
were computed after denoising the light curve. This process involved binning the data using a window approximately two times the observational cadence and fitting a spline to the re- sulting light curve. This effectively reduced noise while preserv- ing the main features of the signal, mitigating noise that could af- fect the SF slope estimates for the to...
work page 2018
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[6]
Fig. F .1.Sample of SF fitted to retrieve the slope (blue line, with an area corresponding to the 3σlevel), the plateau level (orange line) and the timescales (red point, marking the intersection of the slope and the plateau). Left: SF of the April 21 Stokes I denoised light curve. Right: SF of the April 22 Stokes I light curve; we note two distinct slope...
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[7]
Appendix H: Polarization properties and accretion rate of Sgr A ∗ The RM andm ′ observables provide insight into the structure of the Faraday depth across the source (e.g., Sokoloffet al. 1998). In Table H.1, we present the polarized observables averaged for each day. For the total flux density, polarized intensity, EVPA and Stokes V , we report the daily...
work page 1998
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[8]
using the time-series analysis techniques introduced in this work. The lack of significant cor- relation between the RM and depolarization curves, as indicated by the correlation function analysis, suggests that bandwidth de- polarization is unlikely to be the dominant mechanism. Article number, page 21 A&A proofs:manuscript no. aa56759-25corr Table H.1.A...
work page 2018
Show all 9 references
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[9]
H.2.Sgr A ∗ accretion rate evolution through the ALMA 2018 observations, estimated from the RM light curve presented in Fig
5.0 7.5 10.0 12.5 5.0 7.5 10.0 7.5 10.0 12.5 2018 April 21 2018 April 22 2018 April 24 2018 April 25 UT (h) Fig. H.2.Sgr A ∗ accretion rate evolution through the ALMA 2018 observations, estimated from the RM light curve presented in Fig. 4, using the accretion flow model prese...
2018
Reviewed May 10, 2026 · model on record in the stance chip above.
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