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arxiv: 2605.22573 · v1 · pith:4V4WSSZAnew · submitted 2026-05-21 · 🌌 astro-ph.SR · astro-ph.IM

Iron Fluorescence in X-class Solar Flares: Aditya-L1/SoLEXS Observations

Abhilash R. Sarwade , K. Sankarasubramanian , Monoj Bug , Vaishali Sharan , Kiran Lakshmipathaiah , Ankur Kushwaha , M.C. Ramadevi , Smrati Verma This is my paper

Pith reviewed 2026-05-22 03:33 UTC · model grok-4.3

classification 🌌 astro-ph.SR astro-ph.IM
keywords iron fluorescencesolar flaresX-class flarescoronal source heightX-ray spectroscopycenter-to-limb variationphotospherefluorescence efficiency
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The pith

Observations from 47 X-class solar flares show iron fluorescence efficiency varying with position on the solar disk in a way that can help constrain coronal source heights.

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

The paper examines how X-rays from the corona during solar flares produce iron K-alpha fluorescence by irradiating the photosphere. Using broadband measurements from the SoLEXS instrument on Aditya-L1, the authors quantify the fluorescent line flux and the exciting flux above 7.11 keV across 47 X-class events and establish a clear relationship between the two. The derived efficiencies display a center-to-limb dependence that aligns with theoretical predictions for different source heights and viewing angles. A sympathetic reader would care because this offers a potential new way to estimate the height of the X-ray source region in the corona using detectors that are simpler than traditional crystal spectrometers. The mean efficiency at flare peak supplies a constraint on that height, though the estimate depends on the assumed iron abundance in the photosphere.

Core claim

The paper establishes a well-determined relationship between Fe Kα flux and the exciting flux above 7.11 keV for 47 X-class flares. The derived fluorescence efficiencies exhibit a center-to-limb dependence consistent with theoretical models, offering a potential diagnostic to probe coronal source heights and viewing geometries. The mean fluorescence efficiency during the flare peak provides a potential constraint on the effective coronal source height, although this derivation remains subject to a fundamental degeneracy because the estimated source height cannot be uniquely determined without assuming a specific value for the photospheric iron abundance. These findings demonstrate that SDDs,

What carries the argument

fluorescence efficiency, defined as the ratio of observed Fe Kα line flux to the incident X-ray flux above the iron K-edge at 7.11 keV, together with its variation as a function of heliocentric angle

If this is right

  • The center-to-limb dependence of fluorescence efficiency can serve as a diagnostic for coronal source heights and viewing geometries.
  • The mean efficiency value during the flare peak supplies a constraint on the effective height of the coronal X-ray source.
  • Silicon drift detectors can function as a diagnostic tool for iron fluorescence even though they have lower spectral resolution than crystal spectrometers.
  • Statistical uncertainties currently restrict the ability to follow rapid changes in source height on short timescales.

Where Pith is reading between the lines

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

  • Independent measurements of photospheric iron abundance from other methods could remove the degeneracy and allow absolute source-height determinations.
  • Applying the same analysis to a larger sample that includes weaker flares would test whether the diagnostic works across a wider range of event sizes.
  • Pairing fluorescence data with simultaneous imaging or spectroscopy at other wavelengths could help separate height effects from abundance effects.
  • If the center-to-limb pattern holds, future missions equipped with similar detectors could use it for routine estimates of flare geometry.

Load-bearing premise

The estimated coronal source height cannot be uniquely determined without assuming a specific value for the photospheric iron abundance.

What would settle it

A set of flares observed at a range of positions from disk center to limb that produce fluorescence efficiencies showing no systematic center-to-limb trend matching any combination of source height and iron abundance.

Figures

Figures reproduced from arXiv: 2605.22573 by Abhilash R. Sarwade, Ankur Kushwaha, Kiran Lakshmipathaiah, K. Sankarasubramanian, M.C. Ramadevi, Monoj Bug, Smrati Verma, Vaishali Sharan.

Figure 1
Figure 1. Figure 1: Schematic illustrating the geometry of photospheric iron fluorescence excited by a coronal X-ray source. (a) Limb flare configuration: The coronal source (black star), located at a height h above the photosphere, illuminates a spherical cap on the solar surface (highlighted in red). This illuminated region is the site of fluorescence production. A blue arrow indicates the escaping Fe Kα fluorescent photons… view at source ↗
Figure 2
Figure 2. Figure 2: (a) The intrinsic fluorescence efficiency (Γ) as a function of coronal plasma temperature, showing that hotter flares are less efficient at producing fluorescence. (b) The dependence of the intrinsic fluorescence efficiency (Γ) on the photospheric iron abundance, which shows a slightly sub-linear scaling. (c) The geometric factor (f(θ, h)) as a function of heliocentric angle (θ) for several different coron… view at source ↗
Figure 3
Figure 3. Figure 3: In-flight calibration spectrum and background characterization. The plot shows the spectrum recorded by SDD2 during the pre-commissioning phase with the aperture door closed, integrated over 48 hours. The observed spectrum (black points) is dominated by characteristic lines from the on board 55Fe calibration source, including Ti Kα, Ti Kβ, Mn Kα, and Mn Kβ (fitted model in solid red). Several weaker fluore… view at source ↗
Figure 4
Figure 4. Figure 4: Simulated line intensities using the CHIANTI database for a plasma at 20 MK. (a) The region around the neutral Fe Kα energy (6.40 keV), showing the weak contributions from lower ionization states (Fe xix-Fe xxiv). (b) The region around the main thermal Fe line complex (6.7 keV), showing the dominant high-temperature lines (Fe xxv). Note the difference in intensity scales, illustrating that the lines near 6… view at source ↗
Figure 5
Figure 5. Figure 5: (a) Spatial distribution of the X-class solar flares selected for this study. The heliographic latitude and longitude of each of the 47 flares in our sample are shown as red data points. The solid circle represents the solar limb (θ = 90◦), with the inner dashed circles representing contours corresponding to heliocentric angles of 30◦ and 60◦. The plot demonstrates the comprehensive coverage of the dataset… view at source ↗
Figure 6
Figure 6. Figure 6: (a) An AIA 94 ˚A difference image showing the flare’s location at S17E24 (heliocentric angle θ ≈ 29◦). (b) The corresponding GOES light curve for the X1.0-class flare (XRS-A and XRS-B) plotted alongside the SoLEXS 7.11–22 keV flux (red). (c) The SoLEXS spectrum, integrated for 10 s starting at the flare’s peak (SOL-2024-06-01T18:34:40, indicated by the dashed line in panel b). The right side of panel (c) s… view at source ↗
Figure 7
Figure 7. Figure 7: (a) An AIA 94 ˚A difference image showing the flare’s location at S17W89 (heliocen￾tric angle θ ≈ 89◦). (b) The corresponding GOES light curve for the X1.2-class flare (XRS-A and XRS-B) plotted alongside the SoLEXS 7.11–22 keV flux (red). (c) The SoLEXS spectrum, integrated for 10 s starting at the flare’s peak (SOL-2024-05-14T12:49:50, indicated by the dashed line in panel b). The right side of panel (c) … view at source ↗
Figure 8
Figure 8. Figure 8: Simulated line flux in the SoLEXS spectral window centered at the Fe Kα fluorescence energy (6.400 ± 0.085 keV) as a function of plasma temperature. The simulation assumes an isothermal plasma with coronal abundances and a fixed emission measure of 1051/cm3 . The parameters for these Gaussian components are constrained based on known atomic physics and instrumental characteristics. The Fe Kα component is c… view at source ↗
Figure 9
Figure 9. Figure 9: Composite model fit to the SoLEXS spectrum for a near-disk-center X1.0-class flare (location: S17E24, heliocentric angle: θ = 29◦). This spectrum is same as the one shown in Figure 6c, integrated for 10 s starting at SOL-2024-06-01T18:34:40. The observed spectrum (black points) is shown with the total model fit (solid red line), which includes the thermal plasma component (vth abun, dashed green line) and … view at source ↗
Figure 10
Figure 10. Figure 10: Temporal evolution of observed and simulated fluxes for the near-disk-center X1.0-class flare (peak: SOL-2024-06-01T18:34, location: S17E24, heliocentric angle: θ = 29◦). (Top) The observed Fe Kα fluorescence flux (FKα), measured exciting flux (F>7.11), and the theoretically predicted flux for source heights of h = 0 and h = 0.1 R⊙ with photospheric iron abundance of 5.5 × 10−5 . The black curve (right ax… view at source ↗
Figure 11
Figure 11. Figure 11: Temporal evolution of observed and simulated fluxes for the limb X1.2-class flare (peak: SOL-2024-05-14T12:51, location: S17W89, heliocentric angle: θ = 89◦). (Top) The observed Fe Kα fluorescence flux (FKα), measured exciting flux (F>7.11), and the theoretically predicted flux for source heights of h = 0 and h = 0.1 R⊙ with photospheric iron abundance of 5.5 × 10−5 . The black curve (right axis) shows th… view at source ↗
Figure 12
Figure 12. Figure 12: Observed fluorescence efficiency calculated at the peak of each flare, plotted against its heliocentric angle (θ). Each data point is color-coded by the flare’s peak plasma temperature. Note that Flare No. 8 ( [PITH_FULL_IMAGE:figures/full_fig_p023_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: Observed fluorescence efficiency as a function of heliocentric angle (θ) for the flare sample. To minimize temperature-related effects, each data point is derived from a 10 second interval where the flare plasma temperatures are in the range of 18.5–21.5 MK. The solid curves represent the theoretically expected efficiency for a 20 MK plasma at various coronal source heights (h), calculated using the forma… view at source ↗
Figure 14
Figure 14. Figure 14: Comparison of best-fit thermal parameters, (a) temperture and (b) iron abundance, derived from the isothermal-only and the composite (isothermal + fluorescence) models. accepted photospheric value of 3.16 × 10−5 (Asplund et al. 2009), it perfectly matches the effective baseline adopted by Parmar et al. (1984) to obtain similar consistency with their observational data. The high-cadence capabilities of SoL… view at source ↗
read the original abstract

Iron fluorescence is produced by the irradiation of the solar photosphere by coronal X-rays during flares. This study presents the first comprehensive analysis of iron K$\alpha$ fluorescence characteristics in 47 X-class flares observed during the inaugural year of the Solar Low Energy X-ray Spectrometer (SoLEXS) on board India's Aditya-L1 mission. Leveraging the capability of modern silicon drift detectors (SDDs) for simultaneous broadband continuum and line measurements, the Fe K$\alpha$ flux and the exciting flux ($F_{>7.11 \text{ keV}}$) are quantified for each event, establishing a well-determined relationship between them across the sample. The derived fluorescence efficiencies exhibit a center-to-limb dependence consistent with theoretical models, offering a potential diagnostic to probe coronal source heights and viewing geometries. While statistical uncertainties currently limit the ability to track rapid height variations on short timescales, the mean fluorescence efficiency during the flare peak provides a potential constraint on the effective coronal source height. However, this derivation remains subject to a fundamental degeneracy, as the estimated source height cannot be uniquely determined without assuming a specific value for the photospheric iron abundance. These findings demonstrate that SDDs, despite having lower spectral resolution than traditional crystal spectrometers, provide a new diagnostic for the solar iron fluorescence observations.

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

2 major / 2 minor

Summary. The manuscript presents the first comprehensive analysis of iron Kα fluorescence in 47 X-class solar flares observed by the SoLEXS instrument on Aditya-L1. It quantifies the Fe Kα flux and the exciting flux above 7.11 keV for each event, establishes their relationship, derives fluorescence efficiencies that show a center-to-limb dependence consistent with theoretical models, and discusses the mean efficiency during flare peak as a potential constraint on effective coronal source height, while explicitly noting the degeneracy with assumed photospheric iron abundance. The work highlights the utility of silicon drift detectors for such observations despite lower spectral resolution than crystal spectrometers.

Significance. If the reported center-to-limb trend and flux relationship hold after detailed verification of methods, this provides a new diagnostic capability for probing coronal source heights and viewing geometries in solar flares using modern SDD instrumentation. The explicit flagging of the iron abundance degeneracy and the use of a large sample of X-class events strengthen the analysis; the work could be significant for solar flare physics if the efficiencies can be used to test models of photospheric irradiation.

major comments (2)
  1. The central relationship between Fe Kα flux and F>7.11 keV is presented as well-determined, but without access to the specific data-selection criteria, background subtraction procedure, or error budget in the methods section, it is difficult to assess whether post-hoc choices influence the reported trend or the center-to-limb variation. A dedicated subsection detailing these steps, including any cuts on flare phase or signal-to-noise, would be required to support the claim.
  2. While the degeneracy with photospheric iron abundance is correctly flagged in the abstract for the source-height inference, the manuscript should include a quantitative sensitivity analysis (e.g., showing how assumed abundance values shift the derived height range) in the discussion to make the potential constraint on coronal source height more robust and falsifiable.
minor comments (2)
  1. Figure captions should explicitly state the number of flares contributing to each bin in the center-to-limb plot and whether error bars include both statistical and systematic uncertainties.
  2. The abstract states that SDDs provide a 'new diagnostic'; a brief comparison in the introduction to prior crystal-spectrometer results (e.g., from earlier missions) would clarify the incremental advance.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments and positive assessment of our manuscript. We address each major comment below and will revise the manuscript to incorporate the suggested improvements for greater clarity and robustness.

read point-by-point responses

  1. Referee: The central relationship between Fe Kα flux and F>7.11 keV is presented as well-determined, but without access to the specific data-selection criteria, background subtraction procedure, or error budget in the methods section, it is difficult to assess whether post-hoc choices influence the reported trend or the center-to-limb variation. A dedicated subsection detailing these steps, including any cuts on flare phase or signal-to-noise, would be required to support the claim.

    Authors: We agree that additional methodological details are necessary to allow full assessment of the analysis. In the revised manuscript we will insert a new dedicated subsection in the Methods section that explicitly describes the data-selection criteria, background subtraction procedure, full error budget, and any applied cuts on flare phase or signal-to-noise ratio. revision: yes

  2. Referee: While the degeneracy with photospheric iron abundance is correctly flagged in the abstract for the source-height inference, the manuscript should include a quantitative sensitivity analysis (e.g., showing how assumed abundance values shift the derived height range) in the discussion to make the potential constraint on coronal source height more robust and falsifiable.

    Authors: We appreciate this suggestion. To strengthen the discussion of the source-height diagnostic, we will add a quantitative sensitivity analysis in the Discussion section that illustrates how the inferred coronal source height range changes under different assumed photospheric iron abundance values, using the observed mean fluorescence efficiency during flare peak. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

The paper performs direct observational analysis of SoLEXS SDD spectra for 47 X-class flares, measuring Fe Kα line flux and the exciting continuum flux above 7.11 keV on a per-event basis. Fluorescence efficiency is computed as the ratio of these two independently extracted quantities, and the resulting efficiencies are then plotted against heliocentric angle to reveal an observed center-to-limb trend that is compared with existing theoretical models. No parameter is fitted to a subset of the data and then re-labeled as a prediction; no self-citation supplies a uniqueness theorem or ansatz that the present work relies upon; and the degeneracy between source height and photospheric iron abundance is explicitly stated rather than concealed inside a fitted value. The derivation chain therefore consists of measurement, ratio formation, and empirical comparison, all of which remain independent of the reported results.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The analysis rests on standard solar-atmosphere fluorescence models and the assumption that the observed center-to-limb variation is purely geometric. The principal free parameter is the photospheric iron abundance needed to convert efficiency into source height.

free parameters (1)
  • photospheric iron abundance
    Required to resolve the degeneracy between fluorescence efficiency and coronal source height; no independent value is supplied in the abstract.
axioms (1)
  • domain assumption Theoretical fluorescence efficiency versus heliocentric angle follows standard models without significant deviations from non-uniform photospheric iron or scattering effects.
    Invoked when stating that observed efficiencies are consistent with theory and can be inverted for source height.

pith-pipeline@v0.9.0 · 5805 in / 1437 out tokens · 46341 ms · 2026-05-22T03:33:59.299142+00:00 · methodology

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