Optical observations and atomic environment of supernova remnant G25.1-2.3

Aytap Sezer; Ebru Aktekin; Hicran Bak{\i}\c{s}; Hidetoshi Sano; Volkan Bak{\i}\c{s}; Yasuo Fukui; Yuya Asano

arxiv: 2604.02902 · v1 · submitted 2026-04-03 · 🌌 astro-ph.HE · astro-ph.GA

Optical observations and atomic environment of supernova remnant G25.1-2.3

Ebru Aktekin , Hicran Bak{\i}\c{s} , Volkan Bak{\i}\c{s} , Yuya Asano , Hidetoshi Sano , Yasuo Fukui , Aytap Sezer This is my paper

Pith reviewed 2026-05-13 18:30 UTC · model grok-4.3

classification 🌌 astro-ph.HE astro-ph.GA

keywords supernova remnantG25.1-2.3optical spectroscopyshock-heated gaselectron densityHI distributioninterstellar medium

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The pith

Optical spectra identify shock-heated gas and electron densities in supernova remnant G25.1-2.3 along with a matching HI hole.

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

This paper presents the first optical spectroscopic analysis of SNR G25.1-2.3 using LAMOST and RTT150 data, combined with Hα imaging and HI/CO mapping. The [SII]/Hα ratios range from 0.16 to 0.83 across observed regions, marking shock-heated gas specifically in the northern and southern parts while surrounding areas show photoionized characteristics. Electron densities derived from the [SII] λ6716/λ6731 doublet range from 120-1030 cm^{-3} in the north to 490-4500 cm^{-3} in the south, with varying Hα/Hβ ratios indicating differing extinction. A newly identified hole-like HI distribution matches the radio diameter of the remnant. Radio data are used to place the remnant on the surface brightness-diameter relation for evolutionary assessment.

Core claim

The paper claims that optical spectra of G25.1-2.3 show [SII]/Hα ratios of 0.16-0.83 that identify shock-heated gas in the north and south, with [SII] doublet ratios yielding electron densities of 120-1030 cm^{-3} (north) and 490-4500 cm^{-3} (south), while Hα images display filamentary and diffuse emission and a new hole-like HI structure aligns with the SNR diameter, enabling an evolutionary stage estimate via the Σ-D relation.

What carries the argument

The [SII]/Hα emission line ratio used as a diagnostic to separate shock-heated gas from photoionized gas, together with the [SII] λ6716/λ6731 ratio for calculating local electron density.

If this is right

Shock-heated gas is confirmed in the northern and southern regions of the remnant.
Electron densities are systematically higher in the south than in the north.
The HI hole provides direct evidence of the remnant's interaction with surrounding atomic gas.
The remnant's evolutionary stage can be constrained by its position on the surface brightness-diameter relation.

Where Pith is reading between the lines

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

Density contrasts between north and south imply the remnant is expanding into a clumpy interstellar medium.
The optical filaments likely trace current locations of the shock front.
The photoionized regions may be shaped by radiation from the SNR or nearby stars.

Load-bearing premise

The assumption that standard [SII]/Hα ratios can be applied directly to identify shock-heated gas without major interference from local abundance variations, temperature differences, or other emission sources.

What would settle it

New spectra showing [SII]/Hα ratios below 0.4 throughout the northern and southern regions or an HI map whose hole-like feature fails to match the radio boundary of the SNR.

Figures

Figures reproduced from arXiv: 2604.02902 by Aytap Sezer, Ebru Aktekin, Hicran Bak{\i}\c{s}, Hidetoshi Sano, Volkan Bak{\i}\c{s}, Yasuo Fukui, Yuya Asano.

**Figure 1.** Figure 1: The 𝜆11 cm radio continuum image (Reich et al. 1990) of the area around G25.1−2.3. The radio continuum contours scale linearly from 60 to 488 mJy beam−1 . We show five regions (S1, S2, S3, NW, and N) observed with the 1-m T100 telescope, marked with boxes [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗

**Figure 2.** Figure 2: The continuum-corrected SHASSA (Gaustad et al. 2001) H𝛼 image (2.6 × 2.6 deg2 ) of the SNR’s vicinity, highlighting the radio size of the SNR (white box; 80 × 30 arcmin2 ) from Gao et al. (2011), the regions observed by the 1-m T100 telescope (brown boxes; 21 × 21 arcmin2 ), and the locations where the LAMOST and RTT150 spectra were extracted, marked with black and red crosses, respectively. The overlaid 𝜆… view at source ↗

**Figure 3.** Figure 3: The continuum-subtracted H𝛼 images for S1, S2, S3, NW, and N regions with the 1-m T100 telescope, starting from the top left, respectively. The bottom-right panel shows a continuum-subtracted H𝛼 image focusing on the filament located in the S3 region, obtained with the 1.5-m RTT150 telescope. The slit positions from the RTT150 spectroscopic observations are shown as crosses, and their central coordinates a… view at source ↗

**Figure 4.** Figure 4: Top panel: Example spectrum including the H𝛼 stellar absorption feature. Bottom panel: Residual spectrum after Gaussian fit. The yellow line shows the Gaussian fit to the stellar absorption. Pink areas refer to fitted sampling regions [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗

**Figure 5.** Figure 5: The LAMOST spectra (6540 − 6600 and 6710 − 6750 Å) for the P1, P2, P3, and P4 positions. The residual data in the upper right corner refer to the spectrum after the stellar line is removed (see [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗

**Figure 6.** Figure 6: Continued from [PITH_FULL_IMAGE:figures/full_fig_p009_6.png] view at source ↗

**Figure 7.** Figure 7: The long-slit spectra obtained with the RTT150 telescope for the S2, S3, NW, and N positions, starting from the top left, respectively. The spectra cover 4500 − 7000 Å, with fluxes expressed in units of 10−16 erg cm−2 s −1 Å −1 . MNRAS 000, 1–16 (2026) [PITH_FULL_IMAGE:figures/full_fig_p010_7.png] view at source ↗

**Figure 8.** Figure 8: Two-dimensional diagnostic diagrams developed by Kopsacheili et al. (2020), overlaid with our spectroscopic measurements from RTT150 (blue asterisks) and LAMOST (black asterisks). The pink shaded region indicates the parameter space characteristic of shock-excited gas. The diagnostic datasets were accessed through the link provided by Kopsacheili et al. (2024). (see Urošević et al. 2018), we estimated a ma… view at source ↗

**Figure 9.** Figure 9: Integrated intensity maps of (a) HI4PI H i (HI4PI Collaboration et al. 2016) and (b) NANTEN 12CO(𝐽 = 1–0) (Mizuno & Fukui 2004). The integration velocity range of Hi and CO is 40–58 km s−1 . Superposed contours represent the 2695 MHz radio continuum obtained from Effelsberg (Duncan et al. 1999). The contour levels are 100, 130, 160, 190, 220, and 250 K. The white–dashed line and red-solid line show the obs… view at source ↗

**Figure 10.** Figure 10: (a) Integrated intensity map of H i. The integration velocity range and the overlaid contours are the same as those shown in Fig. 9a. The dashed–dotted lines show the integration ranges in Galactic latitude and longitude. (b) Galactic latitude–velocity diagram of H i. The integration range in latitude is from −2.56 to −1.73. (c) Galactic longitude–velocity diagram of H i. The integration range in longitud… view at source ↗

read the original abstract

The supernova remnant (SNR) G25.1-2.3 was identified in the radio band during the Sino-German $\lambda$6 cm survey of the Galactic plane. We present a detailed investigation of the optical, HI, and CO emission towards the G25.1-2.3 to better understand its characteristics and environment. In this study, optical spectroscopic data of the remnant and its environment have been analysed for the first time, providing new insights into their emission properties. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and 1.5-m Russian-Turkish Telescope (RTT150) data show variations across the observed regions, with [SII]/H$\alpha$ ranging from 0.16 to 0.83. We identified shock-heated gas in the northern and southern regions and several photoionized regions around the SNR based on their [SII]/H$\alpha$ ratios derived from spectra. The [SII]$\lambda$6716/$\lambda$6731 ratio observed in the northern region suggests electron densities ($n_{\rm e}$) ranging from 120 to 1030 cm$^{-3}$, whereas the southern regions show higher values, between 490 and 4500 cm$^{-3}$. The variations in the observed H$\alpha$/H$\beta$ ratios indicate significant differences in extinction across the regions. H$\alpha$ images obtained using the 1-m Turkish Telescope (T100) reveal optical emission in the northern and southern, characterized by filamentary and diffuse structures. We newly found a hole-like distribution of HI, whose spatial extent is roughly consistent with the diameter of the SNR. Based on radio data, we examine the evolutionary stage of G25.1-2.3 using the surface brightness-diameter ($\Sigma-D$) relation and the equipartition method.

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.

Desk Editor's Note private letter to a colleague

First optical spectra for G25.1-2.3 give shock indicators and densities plus a matching HI hole, but the ratios need explicit extinction checks.

read the letter

The main thing to know is that this paper supplies the first optical spectra for SNR G25.1-2.3. The [SII]/Hα ratios run from 0.16 to 0.83, marking shock-heated gas in the northern and southern parts, while the [SII] λ6716/λ6731 doublet yields electron densities of 120–1030 cm^{-3} in the north and 490–4500 cm^{-3} in the south. They also report a new hole-like HI distribution whose size lines up with the radio diameter of the remnant. These are direct additions to the earlier radio-only work on this object. The observations come from LAMOST, RTT150, and T100, combined with HI and CO maps, and the analysis applies the usual line-ratio diagnostics to separate shocks from photoionized gas while noting the varying Hα/Hβ as evidence of differential extinction. That multi-wavelength mapping is the solid part. The soft spots sit in the handling of the ratios and the presentation. The abstract shows no error bars on the densities or ratios, and the full reduction pipeline is not laid out. More critically, the [SII]/Hα values appear without a clear correction for the extinction differences they themselves flag. The lines sit about 150 Å apart, so even 1–2 mag of differential A_V can move the ratio by 10–20 percent and potentially shift points across the 0.4 shock threshold. No extra checks such as [NII] λ5755 or [OIII] ratios are mentioned to test abundances or temperature in this inner-Galaxy sightline. The central claims still read as straightforward measurements rather than circular fits. This paper is useful for people who maintain catalogs of individual SNRs or study the Galactic plane in multiple bands. It adds concrete numbers where there were none before. I would send it to peer review so referees can verify the data quality and suggest the needed extinction adjustments.

Referee Report

3 major / 3 minor

Summary. The manuscript presents the first optical spectroscopic observations of SNR G25.1-2.3 using LAMOST and RTT150 data, reporting [SII]/Hα ratios from 0.16 to 0.83 that identify shock-heated gas in northern and southern regions, electron densities of 120–1030 cm^{-3} (north) and 490–4500 cm^{-3} (south) from the [SII] λ6716/λ6731 ratio, extinction variations from Hα/Hβ, filamentary Hα structures from T100 imaging, a new HI hole matching the SNR diameter, and evolutionary stage assessment via radio Σ-D relation and equipartition.

Significance. If the shock identifications hold after proper corrections, the work adds the first optical spectroscopy for this SNR along with a spatially matching HI cavity, strengthening multi-wavelength characterization of its ISM interaction and evolutionary status. The combination of spectroscopy, imaging, and radio analysis is a clear strength.

major comments (3)

[Abstract] Abstract and spectroscopic results section: The [SII]/Hα ratios (0.16–0.83) used to classify shock-heated gas (threshold ~0.4) are presented without explicit dereddening corrections, despite noted significant Hα/Hβ variations implying differential extinction. With [SII] and Hα separated by ~150 Å, A_V differences of 1–2 mag can shift the ratio by 10–20 %, risking misclassification near the diagnostic boundary.
[Results] Spectroscopic analysis and density derivation: No uncertainties or error bars are reported for the [SII]/Hα ratios or the derived n_e values, and there is no discussion of potential systematics from local abundance variations, temperature gradients, or inner-Galaxy line-of-sight effects on the standard solar-abundance shock thresholds.
[HI observations] HI distribution section: The claim of a newly identified hole-like HI distribution whose extent matches the SNR diameter lacks quantitative metrics (e.g., overlap statistics, position-velocity analysis, or comparison to random fields) to establish that the match is not coincidental.

minor comments (3)

[Abstract] Abstract: The title refers to 'atomic environment' but the text emphasizes HI and CO; clarify the intended meaning of 'atomic'.
[Observations] Methods: Expand data reduction, flux calibration, and any applied extinction or telluric corrections for LAMOST and RTT150 spectra to ensure reproducibility.
[Figures] Figures: Add SNR radio contours, north/south region labels, and scale bars to the Hα and HI maps for direct visual comparison.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the detailed and constructive report. We have addressed each major comment below with revisions to strengthen the manuscript. All changes will be incorporated in the revised version.

read point-by-point responses

Referee: [Abstract] Abstract and spectroscopic results section: The [SII]/Hα ratios (0.16–0.83) used to classify shock-heated gas (threshold ~0.4) are presented without explicit dereddening corrections, despite noted significant Hα/Hβ variations implying differential extinction. With [SII] and Hα separated by ~150 Å, A_V differences of 1–2 mag can shift the ratio by 10–20 %, risking misclassification near the diagnostic boundary.

Authors: We agree that differential extinction should be accounted for to ensure robust classification. In the revised manuscript we will compute and report dereddened [SII]/Hα ratios using the observed Hα/Hβ values for each region, following the standard extinction law. We will also add a short discussion quantifying the maximum possible shift in the ratio for the observed A_V range and confirm that the shock identifications remain valid after correction. revision: yes
Referee: [Results] Spectroscopic analysis and density derivation: No uncertainties or error bars are reported for the [SII]/Hα ratios or the derived n_e values, and there is no discussion of potential systematics from local abundance variations, temperature gradients, or inner-Galaxy line-of-sight effects on the standard solar-abundance shock thresholds.

Authors: We accept this criticism. The revised version will include 1σ uncertainties on all [SII]/Hα ratios and n_e values propagated from the spectral fitting and continuum subtraction. We will also add a concise paragraph addressing possible systematics, noting that the standard solar-abundance thresholds remain appropriate given the modest deviations expected in the inner Galaxy and that our measured ratios lie sufficiently far from the 0.4 boundary in the shock-classified regions. revision: yes
Referee: [HI observations] HI distribution section: The claim of a newly identified hole-like HI distribution whose extent matches the SNR diameter lacks quantitative metrics (e.g., overlap statistics, position-velocity analysis, or comparison to random fields) to establish that the match is not coincidental.

Authors: We will strengthen this section by adding quantitative support. Specifically, we will report the fractional overlap area between the HI depression and the radio continuum boundary, together with a position-velocity slice through the cavity that shows the expected kinematic signature. These additions will demonstrate that the spatial coincidence is not random. revision: yes

Circularity Check

0 steps flagged

No circularity: direct observational measurements using established diagnostics

full rationale

The paper reports new optical spectra from LAMOST and RTT150 telescopes, directly measuring [SII]/Hα ratios (0.16–0.83) and [SII] λ6716/λ6731 ratios to derive electron densities (120–4500 cm^{-3}) and classify regions as shock-heated or photoionized. These steps apply standard, externally established diagnostic thresholds to fresh data without fitting any parameters to the target dataset or deriving results by construction from the same inputs. The HI hole is identified from independent radio observations whose spatial match to the SNR diameter is reported as a new finding. The Σ-D evolutionary analysis uses the standard radio surface-brightness relation on existing radio data. No self-citation chain, ansatz smuggling, or renaming of fitted quantities occurs; the derivation chain is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The analysis rests on established domain assumptions for emission-line diagnostics and SNR evolutionary relations rather than new free parameters or postulated entities.

axioms (2)

domain assumption Standard [SII]/Hα line ratios reliably distinguish shock-heated from photoionized gas
Invoked to classify northern/southern regions as shock-heated and other areas as photoionized
domain assumption The surface brightness-diameter (Σ-D) relation can be used to assess the evolutionary stage of SNRs
Applied to radio data to examine the remnant's evolutionary stage

pith-pipeline@v0.9.0 · 5683 in / 1504 out tokens · 55786 ms · 2026-05-13T18:30:12.736470+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

1 extracted references · 1 canonical work pages

[1]

Aktekin E., Bakış H., Bakış V., Sezer A., 2025, MNRAS, 543, 761 Alarie A., Drissen L., 2019, MNRAS, 489, 3042 Alsaberi R. Z. E., et al., 2024, MNRAS, 527, 1444 Anderson L. D., et al., 2017, A&A, 605, A58 Anderson L. D., et al., 2025, A&A, 693, A247 Araya M., 2023, MNRAS, 518, 4132 Araya M., 2024, A&A, 691, A225 ArbutinaB.,UroševićD.,AndjelićM.M.,PavlovićM...

work page 2025

[1] [1]

Aktekin E., Bakış H., Bakış V., Sezer A., 2025, MNRAS, 543, 761 Alarie A., Drissen L., 2019, MNRAS, 489, 3042 Alsaberi R. Z. E., et al., 2024, MNRAS, 527, 1444 Anderson L. D., et al., 2017, A&A, 605, A58 Anderson L. D., et al., 2025, A&A, 693, A247 Araya M., 2023, MNRAS, 518, 4132 Araya M., 2024, A&A, 691, A225 ArbutinaB.,UroševićD.,AndjelićM.M.,PavlovićM...

work page 2025