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arxiv: 2605.13990 · v1 · submitted 2026-05-13 · 🌌 astro-ph.HE · astro-ph.GA

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Infrared Line Diagnostics Fail to Constrain Sgr A*'s UV Output

Mayura Balakrishnan , Sebastiano D. von Fellenberg , Daryl Haggard , Joseph M. Michail , Nicole M. Ford , Joseph L. Hora , Laurent Loinard , Sera Markoff
show 6 more authors
Joey Neilsen Giacomo Principe Nadeen B. Sabha Howard A. Smith Zach Sumners Shuo Zhang
Authors on Pith no claims yet

Pith reviewed 2026-05-15 02:15 UTC · model grok-4.3

classification 🌌 astro-ph.HE astro-ph.GA
keywords Sgr A*mid-infrared linesflaresphotoionizationGalactic CenterUV fluxvariability
0
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The pith

Infrared emission lines near Sgr A* remain steady despite flares, preventing constraints on the black hole's instantaneous UV output.

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

The paper tests whether mid-infrared emission lines can reveal the ultraviolet output of Sgr A* during its X-ray flares by taking time-resolved JWST spectroscopy of the central 0.3 arcsec region. No statistically significant changes appear in lines such as [Fe II] 5.34 micron, [Ne II] 12.813 micron, [Fe II] 17.936 micron, or [S III] 18.713 micron. CLOUDY photoionization models for flare UV luminosities from 10^32 to 10^39 erg s^-1 predict clear responses in a dense medium, yet the observed constancy matches expectations once light-crossing times of 0.1-10 days and recombination/cooling times longer than flare durations are included. The lines therefore register only the time-averaged radiation field.

Core claim

The absence of detectable variability in mid-infrared emission lines, despite expectations from photoionization models, arises because the spatially extended line-emitting gas has light-crossing timescales of 0.1-10 days and recombination/cooling timescales much longer than individual flare durations; the resulting emission is continuum-dominated, intrinsically weak, and velocity-broadened to ~10^3 km s^-1, so infrared line diagnostics cannot constrain the instantaneous UV flux of Sgr A*.

What carries the argument

The mismatch between short flare durations and the long light-crossing plus recombination timescales of the extended line-emitting gas, which averages any transient response into a steady-state signal.

If this is right

  • The predicted line emission is continuum-dominated and broadened by velocities of order 10^3 km s^-1, which reduces contrast and limits detectability.
  • Extending the search to higher-ionization mid-infrared or near-infrared lines yields no gain in sensitivity.
  • Infrared line diagnostics can only trace the steady-state radiation field and are unsuitable for measuring flare-by-flare UV output.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Steady-state photoionization calculations could still be used to bound the time-averaged UV luminosity over weeks or longer.
  • Similar averaging arguments may apply to line diagnostics around other accreting black holes with short variability timescales.
  • Direct constraints on Sgr A* UV output will require wavelengths or techniques that respond on the flare timescale itself.

Load-bearing premise

The line-emitting gas is distributed over scales where light takes days to cross and where recombination and cooling times greatly exceed flare lengths.

What would settle it

Detection of statistically significant changes in any of the listed mid-infrared lines that are synchronized with known Sgr A* flare times on hourly-to-daily scales would contradict the central claim.

Figures

Figures reproduced from arXiv: 2605.13990 by Daryl Haggard, Giacomo Principe, Howard A. Smith, Joey Neilsen, Joseph L. Hora, Joseph M. Michail, Laurent Loinard, Mayura Balakrishnan, Nadeen B. Sabha, Nicole M. Ford, Sebastiano D. von Fellenberg, Sera Markoff, Shuo Zhang, Zach Sumners.

Figure 1
Figure 1. Figure 1: JWST/MIRI Channel 1 5.6 micron image of the Galactic Center field. Prominent sources are labeled in black. The position of Sgr A⋆ is marked by the yellow “X”, and the white circle indicates the 0. ′′3 radius aperture used for spectral extraction. The color scale shows the calibrated surface brightness in units of MJy sr−1 . et al. 2025a) and staring-mode aperture corrections (see Appendix A of J. M. Michai… view at source ↗
Figure 2
Figure 2. Figure 2: Representative lightcurves showing the absence of variability in both [S III] (18.713 µm) and [Ne II] (12.813 µm), shown in blue, in response to the Sgr A⋆ flare described in S. D. von Fellenberg et al. (2025b) and J. M. Michail et al. (2026). The corresponding continuum lightcurve is plotted in orange. Uncertainties on the integrated line flux are estimated via a Monte Carlo procedure in which the spectru… view at source ↗
Figure 3
Figure 3. Figure 3: Comparison of CLOUDY models to the observed JWST/MIRI spectrum of the central 0. ′′3 around Sgr A⋆ . The crimson and gray curve show the MIRI MRS spectra, dereddened and observed, respectively, during the flare window and the dashed crimson curve shows the local continuum-subtracted RMS. Colored curves show the predicted flare-peak spectra for flare luminosities Lflare = 1032–1039 erg s−1 . The black curve… view at source ↗
Figure 4
Figure 4. Figure 4: Predicted time-dependent line fluxes for representative high-ionization MIR (left) and NIR (right) transitions at log Lflare = 39, computed with CLOUDY. The flare is injected for the first hour of the simulation. Fluxes are shown as equivalent Gaussian peak flux densities assuming a FWHM of 1000 km s−1 . In all cases, the emission is extremely faint (≲ 10−5 mJy) and exhibits only weak temporal evolution fo… view at source ↗
Figure 5
Figure 5. Figure 5: Recombination time (blue points), cooling time (orange squares; computed assuming a representative cooling coefficient of 10−22 erg cm3 s −1 ), and density (green diamonds) as a function of the radius at which each ion reaches peak emissivity. Ion labels are shown above the recombination-time points. Even the shortest recombination timescale is of order ∼ 8 days, far exceeding the characteristic variabilit… view at source ↗
read the original abstract

Sgr A*, the 4 x 10^6 solar-mass supermassive black hole at the Galactic Center, exhibits frequent flaring with X-ray luminosities of L_X ~ 10^35--10^36 erg s^-1, while its ultraviolet (UV) emission remains unconstrained due to extreme extinction (A_V ~ 30 mag). We use JWST/MIRI time-resolved spectroscopy of the central Galactic Center's 0.3 arcsec region to search for mid-infrared emission-line variability driven by Sgr A* flares, comparing the results to CLOUDY photoionization models spanning flare luminosities of L_UV = 10^32--10^39 erg s^-1 in a dense medium. We detect no statistically significant variability in any mid-infrared line, including [Fe II] 5.34 micron, [Ne II] 12.813 micron, [Fe II] 17.936 micron, and [S III] 18.713 micron. Despite expectations of a flare-driven response, we show that the lack of variability is consistent with the physical conditions in the spatially extended line-emitting gas, where light-crossing timescales of ~0.1--10 days and recombination and cooling timescales much longer than the flare timescale suppress any observable response to individual flares. We further find that the predicted emission is continuum dominated and that even the brightest lines are intrinsically weak and broadened by velocities of order 10^3 km s^-1, reducing their contrast against the continuum and limiting their detectability. Extending the analysis to higher-ionization mid-infrared and near-infrared lines does not improve sensitivity. These results demonstrate that infrared emission lines trace a steady-state radiation field rather than individual flaring events, and therefore infrared line diagnostics cannot be used to constrain the instantaneous UV flux of Sgr A*.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

0 major / 2 minor

Summary. The paper reports a non-detection of statistically significant variability in mid-infrared emission lines ([Fe II] 5.34 μm, [Ne II] 12.813 μm, [Fe II] 17.936 μm, [S III] 18.713 μm) from JWST/MIRI time-resolved spectroscopy of the central 0.3 arcsec region around Sgr A*. It compares this null result to CLOUDY photoionization models for L_UV spanning 10^32–10^39 erg s^{-1} and attributes the lack of response to light-crossing timescales of ~0.1–10 days across the extended gas plus recombination/cooling timescales exceeding flare durations, concluding that the lines trace a steady-state radiation field and cannot constrain the instantaneous UV flux of Sgr A*.

Significance. If the result holds, the work is significant for establishing that infrared line diagnostics are insensitive to individual flares from Sgr A* owing to the physical conditions in the spatially extended line-emitting gas. The combination of the reported null detection, explicit model comparison, and timescale argument provides a clear, falsifiable explanation for why such lines are continuum-dominated and velocity-broadened, limiting their utility for transient UV constraints. This is a useful observational limit for Galactic Center studies.

minor comments (2)
  1. The abstract states that lines are 'broadened by velocities of order 10^3 km s^{-1}'; the main text should explicitly state the adopted line profile (Gaussian or otherwise) and how this broadening is incorporated into the CLOUDY predictions for contrast against the continuum.
  2. The manuscript would benefit from a brief table summarizing the exact flare durations and luminosities used in the CLOUDY grid, to allow direct comparison with the observed non-variability limits.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive assessment of our work and for recommending acceptance. We are pleased that the combination of the null detection, model comparison, and timescale arguments is viewed as providing a clear explanation for the lack of observable line variability.

Circularity Check

0 steps flagged

No significant circularity identified

full rationale

The paper reports a direct observational null result from JWST/MIRI time-resolved spectroscopy showing no statistically significant variability in mid-IR lines. This is compared to independent CLOUDY photoionization models run over a range of L_UV values. The explanation for the null result invokes standard physical timescales (light-crossing time across 0.3 arcsec and recombination/cooling times) derived from atomic physics and geometry, without any parameter fitting to the target data that would make the conclusion self-referential. No self-citations, ansatzes, or renamings reduce the central claim to its own inputs by construction; the derivation remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The analysis rests on standard photoionization modeling and domain assumptions about gas geometry and timescales in the Galactic Center; no new free parameters or entities are introduced.

axioms (2)
  • domain assumption CLOUDY photoionization models accurately predict line responses to UV illumination in dense gas
    Invoked to interpret the expected flare-driven variability across L_UV = 10^32-10^39 erg s^-1.
  • domain assumption Light-crossing, recombination, and cooling timescales in the spatially extended line-emitting gas exceed flare durations
    Used to explain the absence of observable response.

pith-pipeline@v0.9.0 · 5699 in / 1302 out tokens · 49084 ms · 2026-05-15T02:15:28.959995+00:00 · methodology

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