SST-1M: Recent results and prospects for observation of the Galactic Center region

The useful thing in this proceedings paper is the new forecast: 250 h of simulated SST-1M observations of the Galactic Center from a proposed Malargüe site, with the 2–4σ sensitivity to a spectral cutoff of the diffuse Ridge. That specific number is not in prior literature, and the authors get there with a reasonable pipeline: MC IRFs, gammapy 3D likelihood, and external spectral models from H.E.S.S., MAGIC, VERITAS, HAWC, and LST-1. The recent results section is a competent summary of the collaboration’s Crab, Cygnus, CTA 1, and Mrk 421 work; it is not new but it supports credibility.

The soft spots are real, mostly about what is not shown. The load-bearing step is the transfer of the MC model from Ondřejov (510 m, 155.2 m baseline) to Malargüe (1420 m, 120 m). The paper cites a companion study on altitude performance, but does not show it or quantify any residual IRF uncertainty. Given the quoted significances are only 2–4σ, a few percent energy bias or a ~30% excess-count shift could change the conclusion. The safety mask on effective area and energy bias mitigates bad bins, not model transfer error.

Second, the likelihood ratio test applies Wilks’ theorem with the cutoff parameter at the boundary λ=0, so the quoted significance is likely optimistic. Third, no systematic uncertainties are propagated into the forecast. These are not fatal for a proceedings contribution, but they mean the headline number should be read as an upper end of sensitivity, not a prediction.

One more thing: the abstract calls the instrument 'ideal' for PeVatron searches. With a 2–4σ marginal detection claim, 'suitable' would be more honest. Minor, but it set my teeth on edge.

Bottom line: this is a modest but honestly presented simulation study, with a clearly stated new result and standard methods. The lack of site-specific validation and the boundary issue make me want to see the companion paper before betting on the 2–4σ number. I would accept it for peer review as a proceedings paper: the forecast is concrete, testable, and worth a referee’s time. I would not cite it as a final sensitivity statement until the IRF transfer is validated.

The paper's central forecast—2–4σ detection of a multi-TeV cutoff in 250 hours of Galactic Center observations from Malargüe—depends on the Instrument Response Functions used in the simulation matching the relocated telescopes. Section 3 says these IRFs were prepared by taking the MC model 'validated with data from the Ondrejov Observatory' and moving it to Malargüe (1420 m a.s.l., 120 m separation), but the text provides no site-specific validation. The altitude change from 510 m to 1420 m alters the Cherenkov light pool, atmospheric transmission, and night-sky background; the baseline change from 155.2 m to 120 m changes stereo reconstruction and effective area. The safety mask on effective area and energy bias mitigates bad energy bins but does not validate the model transfer. Since the quoted significances are only 2–4σ, an IRF-induced shift of even ~30% in excess counts or a few percent in energy bias could move a 3σ claim below 2σ. A companion study on altitude performance is cited, so the transfer may be defensible, but as presented the forecast is conditional on that unshown validation.

I read the strongest claim as the 250-hour cutoff sensitivity forecast. The weakest premise is indeed the transfer of the telescope MC model from Ondrejov to Malargüe: the claim requires that the IRFs used in the simulation describe the actual relocated system, and the paper does not show site-specific validation. This matches the reader's weakest_assumption. I considered the Wilks-theorem boundary issue: for a one-sided λ ≥ 0 test, quoting √TS as a one-sided significance is standard and does not inflate the quoted significance, so I do not raise it. I also considered the FoVBackgroundModel absorbing diffuse flux, but because the simulation and fit use the same spatial template, this is a smaller effect than the model-transfer uncertainty. The cited companion study on altitude performance may contain the needed validation, which is why the appropriate verdict remains CONDITIONAL rather than REJECT; the proposed test would settle whether the transfer concern actually lands.

If the forecast is reliable, the paper makes a useful scientific case: SST-1M would provide an independent, southern-hemisphere measurement of the Galactic Center Ridge and a direct probe of the PeVatron hypothesis. The work builds on external benchmarks (H.E.S.S., MAGIC, VERITAS, HAWC, LHAASO comparisons), a Crab-validated Monte Carlo, and a transparent gammapy-based 3D analysis. The significance of the result is, however, conditional on two load-bearing assumptions that are not fully demonstrated in the manuscript: the transferability of the Monte Carlo performance model from Ondřejov to Malargüe, and the validity of the likelihood-ratio significance estimate when the null hypothesis lies on the boundary of the parameter space.

The paper argues that a relocated pair of small single-mirror Cherenkov telescopes, SST-1M, can make a central measurement in the most crowded part of the gamma-ray sky: separating the diffuse Ridge emission of the Galactic Center from its point sources and measuring the energy at which its spectrum cuts off. Using instrument response functions derived from a Monte Carlo model validated on earlier Crab Nebula data, the authors simulate 250 hours of observations from a proposed Argentine site and find that the spectral cutoff in the Ridge emission would be detected at 2-4σ significance, depending on the cutoff energy. If correct, this would let a modest 4-m-class observatory test whether protons are accelerated to PeV energies in the Galactic Center, a question currently tangled in source confusion and the northern locations of most instruments. The paper also reports Crab, Cygnus, CTA 1, and Mrk 421 results supporting the instrument's ability to detect and resolve extended emission.

What carries the argument. The central machinery is the set of instrument response functions (IRFs) built from the Monte Carlo model of the SST-1M telescopes, which was validated against Crab Nebula data at the current northern site and re-derived for the proposed southern site's altitude of 1420 m and telescope separation of 120 m. These IRFs feed a 3D likelihood analysis of the entire Galactic Center region in which all bright sources and the diffuse component are fitted simultaneously, with the background estimated from source-free parts of the field of view. The cutoff significance is obtained from the likelihood-ratio test statistic $\mathrm{TS}=-2\ln(L(\lambda=0)/L(\lambda))$, where $\lambda=1/E_{\mathrm{cutoff}}$, and the square root of TS is interpreted, via Wilks' theorem for $\Delta\mathrm{dof}=1$, as a detection significance. The large, flat gamma-ray acceptance of SST-1M over a field of view of about 2.5 degrees is what lets a small telescope see the extended Ridge without being blinded by its point sources.

Core claim. On the paper's own terms, the central claim is that the SST-1M stereo system, once moved to a southern site, will resolve the individual very-high-energy gamma-ray sources in the Galactic Center region—Sgr A*, SNR G0.9+0.1, and the diffuse Ridge—and, within 250 hours of exposure, detect a spectral cutoff in the diffuse component at 2-4σ significance. The detection significance comes from a likelihood-ratio test between a pure power-law model and an exponentially-cutoff power-law model, applied in a simultaneous 3D fit of all sources in the region. The simulation adopts published spectral models for the known point sources and a spatial template for the diffuse emission derived from radio maps, and the authors model the crowded region as a whole rather than fitting the diffuse component alone. The paper presents this as evidence that the large field of view and multi-TeV sensitivity of the small telescopes make them effective PeVatron search instruments.

The premise it rests on. The load-bearing premise is that the Monte Carlo model of the telescopes, validated at the current site's lower altitude and wider telescope separation, remains accurate at the proposed Argentine site's higher altitude and narrower separation; if the instrument response or background rejection changes there, the predicted 2-4σ cutoff sensitivity is not reliable.

If the cutoff is detected at 2-4σ in 250 hours, the measurement would discriminate between the hard power-law spectrum reported by H.E.S.S. and VERITAS and the softer or cutoff spectra reported by MAGIC and hinted by LST-1 and the updated H.E.S.S. analysis.

Successful separation of Sgr A* from the diffuse Ridge would give each component its own spectrum, removing the source confusion that has complicated previous modeling of the region.

The same large-field-of-view performance, demonstrated on extended sources such as VER J2019+368 and CTA 1, would make the relocated SST-1M a practical survey instrument for southern PeVatron candidates.

A cutoff measurement above 10 TeV would directly constrain the maximum energy of accelerated protons in the Galactic Center, while a non-detection in the same exposure would push the cutoff energy beyond the instrument's reach and strengthen the PeVatron interpretation.

Because the Galactic Center is visible for about 680 dark hours per year from the proposed site, extending the simulated 250-hour campaign would raise the cutoff significance beyond the 2-4σ range or allow searches for fainter cutoffs.

The same simultaneous-fit machinery could be repurposed to test whether the Ridge morphology is energy dependent, since the large field of view covers the whole central molecular zone in a single pointing.

A southern relocation would also open multi-TeV time-domain studies of Sgr A* flares, which the current northern site cannot observe; the paper does not discuss this application.

The assumed transferability of the validated Monte Carlo model could be checked cheaply during commissioning: comparing the Crab Nebula spectrum and angular resolution measured from the Argentine site with the IRF predictions would flag any significant atmospheric or geometric mismatch before the full 250-hour campaign.

The 2-4σ range can be read as a sensitivity curve against cutoff energy; concentrating exposure in the 10-50 TeV band, where the cutoff is expected to appear, might improve the detection significance for the same total observing time.

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