{"id":"dddc8545-98b6-4edc-9c54-0c2fd463c935","arxiv_id":"2412.02552","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Using simulated sky-averaged 21-cm observations, the REACH time-separated Bayesian pipeline recovers an injected Gaussian signal with RMSE below 30 percent of its amplitude for polarization fractions below about 3 percent.","lead":"This paper tests whether the REACH Bayesian analysis pipeline can recover a simulated cosmic dawn 21-cm signal when polarized galactic foregrounds leak into the data. It finds recovery works when the polarization fraction stays below roughly three percent, and that more complicated Faraday rotation structures are easier to separate than a single slow oscillation.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Simulation omits parallactic-angle rotation and Stokes U, so the pipeline is not tested against a physically faithful polarized leakage.","rationale":"The reader's weakest assumption is that the simulated polarized foregrounds are representative of the real low-frequency polarized sky, focusing on the lack of all-sky Q/U/V maps and the simplified Faraday depth models. I partially agree, but I identify a more specific and more directly testable technical gap: even for Stokes Q alone, the simulation does not apply the time-dependent parallactic-angle rotation that a real fixed dipole antenna would impose on the polarized sky. This is an internal modeling omission, not just an external data limitation, and it affects the validity of the simulated observations that underpin the central claim. The paper's Appendix A derives the correct antenna response (E_xx ∝ I+Q' with Q' in the antenna frame), but Section 2 implements only a scalar beam convolution with the sky-frame Q, omitting the rotation. Adding Stokes U and the parallactic-angle modulation is a concrete change that can be implemented and tested directly; if the recovery threshold shifts, the paper's stated ~3% bound would need to be revised. Because the reader's CONDITIONAL verdict already flags the realism of the polarized foreground models, my concern does not change the verdict; it sharpens the condition under which the claim should be accepted. The proposed test is computationally straightforward using the existing pipeline and would settle whether the omission is material.","tokens_in":14642,"tokens_out":14207,"duration_ms":155562,"concrete_test":"Re-run the end-to-end simulation with the full polarized antenna response: for each time bin, compute T_data(ν,t) = 1/(4π) ∫ D(θ,φ,ν) [I + Q cos(2χ(t,θ,φ)) + U sin(2χ(t,θ,φ))] dΩ + noise, where χ is the parallactic angle for the REACH site (lat -30.7°) at each 5-minute time bin. Generate a Stokes U map from the same Faraday models (e.g., by rotating the simulated Stokes Q vector by 45° or using a Faraday dispersion function with non-zero U). Use the same injected Gaussian signal and polarization fractions p = 1/10, 1/30, 1/70, and the same pipeline. If the signal RMSE for p ≤ 1/30 remains below the 30% threshold, the current conclusion stands; if not, the pipeline's robustness to real polarized leakage is unverified.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The most load-bearing concern is that the simulated polarized foregrounds do not exercise the antenna's time-dependent polarization response, so the central recovery claim is not established for a real observation. In Section 2.1, the simulated antenna temperature is T_data(ν)=1/(4π)∫ D T_sky dΩ + noise, with T_sky = T_f + T_Q + T_21 (Eqs. 5–6). Here T_Q is the all-sky Stokes Q map in the sky frame (Section 2.2), and D is a scalar power pattern. No rotation of Q into the antenna polarization basis is applied, nor is any parallactic-angle term introduced as the sky rotates over the observing window. However, a fixed linearly polarized dipole measures E_xx ∝ I + Q', where Q' = Q cos 2χ + U sin 2χ, with χ the position angle of the antenna polarization axis on the sky (Appendix A, Eqs. A3–A7). Because χ changes with time and pointing, the real leakage is modulated at ~2χ(t); Stokes U contributes even when the sky has no intrinsic U. The current simulations thus omit a time-dependent chromatic effect that the time-separated pipeline is designed to exploit. The paper's conclusion that the pipeline recovers the signal in the presence of unaccounted-for linear polarization contamination is therefore only validated for an idealized antenna-polarization alignment, not for the physical REACH observation. This is a more specific and load-bearing defect than the general lack of all-sky polarization measurements.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript studies whether the REACH time-separated Bayesian data analysis pipeline can recover an injected Gaussian global 21-cm signal (amplitude 0.157 K, width 10-15 MHz, centre 80-120 MHz) from simulated antenna temperature data contaminated by linearly polarized Galactic foregrounds. Three all-sky Stokes Q models are used: a rotation-measure-synthesis model based on the MWA survey, an interpolated rotation-measure catalogue (RMTable2023), and toy models with random low Faraday-depth regions. The pipeline jointly fits all time bins with physically motivated foreground models using PolyChord, and recovery is evaluated with a signal RMSE criterion (good fit if RMSE < 30% of the injected amplitude). The paper reports successful recovery in most tested cases for polarization fractions below roughly 3%, and concludes that the REACH pipeline is robust to this class of contamination.","tokens_in":14881,"tokens_out":11578,"duration_ms":115082,"significance":"If correct, the result would be a valuable validation of the REACH pipeline against a chromatic systematic that is known to degrade standard smooth-foreground analyses. The paper's main strengths are its explicit injection-recovery design, the use of three qualitatively distinct polarized foreground models, the physically informed foreground and beam modelling, and the clear RMSE-based success metric; the injected signal is not used as a prior, so the test is not circular. The principal limitation is that the simulated leakage is not a faithful model of what a real fixed dipole measures, which makes the quantitative recovery claim conditional on a forward-model upgrade rather than directly applicable to the physical observation.","major_comments":[{"comment":"The forward model omits the rotation of Stokes Q into the antenna polarization frame and omits Stokes U (and V). A linearly polarized dipole responds to E_xx proportional to I + Q', with Q' = Q cos(2 chi) + U sin(2 chi), where chi is the position angle of the antenna polarization axis on the sky and varies with time as the sky rotates. In the simulation, a sky-frame Stokes Q map is added directly to T_f and then convolved with a scalar power pattern, so the leakage has no time-dependent parallactic-angle modulation. Because the time-separated pipeline is specifically designed to exploit time-varying chromatic structure, this omission is load-bearing for the central recovery claim. I recommend replacing the scalar-beam treatment with a Jones-matrix forward model (at minimum a time-dependent Q' rotation; ideally including U and V), or explicitly restricting the conclusions to the idealized co-aligned static case.","section":"Section 2.1, Eqs. (5)-(6); Appendix A, Eqs. (A3)-(A7)"},{"comment":"The polarization-fraction thresholds are internally inconsistent. The text states that 'signal RMSE of less than 10% ... can be achieved for linear polarisation fractions of 1/30 and 1/10', while Fig. 4 shows the p = 1/10 case completely missing the signal; Section 5 states a threshold of '<=3%' even though p = 1/30 is 3.3%; and the Abstract says 'below ~3%'. The exact p values that satisfy the 30% and 10% RMSE criteria should be reconciled and stated consistently across the abstract, results, and conclusions.","section":"Section 4.2, Figs. 4-5, Section 5, Abstract"},{"comment":"The reported RMSE values are based on a single noise realization per configuration, and the adopted noise level sigma_n is not stated. Because the paper's quantitative success criterion is RMSE < 30% of the injected signal strength, the absence of any noise-realization scatter (or at least the value of sigma_n) leaves the 'all tested cases' claim without a statistical uncertainty. Repeating the analysis over multiple noise draws, or stating sigma_n and showing representative stability, would strengthen the central claim.","section":"Section 4, Eq. (18), Figs. 5 and 8"}],"minor_comments":[{"comment":"There are several typographical issues, including the duplicated phrase 'parameters parameters' in Section 3.1, 'theflattened Gaussian' in the Introduction, and a stray quotation mark after 'nearest-neighbour interpolation.' in Section 5.","section":"Section 3.1 and throughout"},{"comment":"The statement that the REACH pipeline 'significantly outperforms standard approaches' is not directly demonstrated by this paper, since no standard pipeline is run on the same simulated data; the support comes only from cited earlier work. Either add a baseline comparison or soften the claim.","section":"Section 5"},{"comment":"The caption of Fig. 5 and the text of Section 4.2 disagree on the injected-signal width used for the RMSE curves (10 MHz in the caption versus 15 MHz in the text); please make the figure-specific parameters explicit.","section":"Fig. 5 and Section 4.2"},{"comment":"The time dependence of the simulated sky/beam is not written explicitly in Eq. (6), although the pipeline is time-separated; a sentence clarifying how the 5-minute time bins are produced would help reproducibility.","section":"Section 2.1, Eq. (6)"}],"recommendation":"major_revision","confidential_remarks":"The paper fits the journal's scope. The stress-test concern about parallactic-angle rotation and missing Stokes U is, in my reading, well-founded and is reflected in major comment 1; the issue is fixable within the scope of a revision and does not warrant rejection. No concerns about citation patterns or novelty beyond the overextended comparison to standard pipelines."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The paper is a solid simulation exercise and the first test of the REACH time-separated pipeline against polarized foreground leakage. The systematic scan over polarization fraction and the three foreground models—rotation-measure synthesis, the RMTable2023 point-source catalogue, and a toy Faraday-depth screen—are well chosen. The result that a recovery threshold of about 3% polarization fraction holds in the point-source model is plausible, and the observation that spatially mixed Faraday depths are easier to separate is a useful insight. The authors are honest about the lack of low-frequency all-sky polarization data and about ignoring Stokes U and V, but they do not flag the larger problem: the sky is static in their simulations.\n\nIn Section 2.1, the antenna temperature is computed by convolving a scalar beam with Tsky = Tf + TQ + T21, with no parallactic-angle rotation and no time dependence in the Stokes Q map. All 24 hours of data see the same foreground (apart from noise). A real dipole antenna measures Q' = Q cos 2\\chi + U sin 2\\chi, where \\chi changes with hour angle, so the polarized leakage is modulated in time. The time-separated pipeline is specifically designed to exploit this time structure to separate foregrounds from the constant 21-cm signal. By omitting it, the paper tests only a static, idealized case and does not actually validate the pipeline against the physical effect that makes the analysis challenging. This is a load-bearing limitation, more fundamental than the missing baseline comparison or the single noise realizations.\n\nThe other issues are real but minor by comparison: the claim of outperforming standard pipelines relies on earlier work rather than a direct comparison, RMSE values come from one noise realization each, and the data and code are not public. The abstract's 'all the tested cases' is technically accurate, but the conclusion that the pipeline is robust to 'unaccounted-for linear polarisation contamination' is too broad.\n\nThe paper deserves a serious referee because it addresses a genuine problem and the methodology is clear, but it needs major revision before the central claim can be trusted. The authors should either add a time-dependent simulation with a rotating sky (and ideally Stokes U) or explicitly limit their conclusions to static polarized foregrounds. I would not cite the central recovery threshold until that is done. Send it to review, but expect heavy revision.","headline":"Useful simulation study, but the missing time-dependent polarization rotation means the pipeline is not actually validated for real observations.","tokens_in":15454,"tokens_out":3581,"would_cite":false,"duration_ms":40311,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Polarised foreground leakage does not prevent recovery of the global 21-cm signal in most simulated cases.","keywords":["global 21-cm signal","Cosmic Dawn","polarised foregrounds","Faraday rotation","Bayesian inference","Stokes Q leakage","time-separated analysis","radio cosmology"],"falsifier":"Using a measured all-sky Stokes $Q$, $U$, and $V$ map at 50–150 MHz, inject a 0.157 K Gaussian signal into the simulated antenna temperature and run the time-separated pipeline; if the recovered-signal RMSE exceeds 30% of the injected amplitude at a polarisation fraction below $\\sim 3\\%$, the paper's stated bounds would not extend to the real sky.","tokens_in":14381,"feed_emoji":"📡","tokens_out":7030,"duration_ms":68110,"temperature":0.7,"pith_summary":"This paper asks whether leakage from linearly polarised Galactic foregrounds can destroy extraction of the global 21-cm absorption signal from Cosmic Dawn. The authors inject a Gaussian signal of about 0.16 K centred between 80 and 120 MHz into simulated antenna temperature data, contaminate the sky with three different models of Stokes $Q$ polarisation, and run REACH's time-separated Bayesian analysis pipeline. Their central claim is that the pipeline recovers the injected signal in the majority of cases, with root-mean-square error below 30% of the signal strength, whenever the polarisation fraction stays below roughly 3%. The reason to care is that polarised foregrounds rotated by Faraday effects create frequency-dependent structures that look like the signal; if the pipeline can separate them, one of the main known systematic threats to global 21-cm experiments is bounded.","feed_headline":"21-cm signal survives polarized foreground leakage in simulations","feed_subtitle":"A Bayesian time-separated pipeline pulls a 0.16 K absorption trough out of polarized foreground leakage below 3 percent.","key_machinery":"The machinery is a Bayesian pipeline that fits all time bins jointly. The likelihood in equation (17) gives each 5-minute time bin its own foreground model while sharing one set of global-signal parameters, and the fit is run with PolyChord nested sampling to compute evidences. Foregrounds are modelled physically by dividing the sky into regions of similar spectral index and convolving the scaled maps with the REACH dipole beam. The polarisation leakage itself is simulated through Stokes $Q$ maps whose Faraday rotation angle scales as $\\lambda^2$, so its frequency structure is determined by the distribution of Faraday depths; that structure, and the antenna's chromatic response, are what the time-separated fit exploits to distinguish leakage from the signal.","core_discovery":"The discovery claim is that unaccounted-for linear polarisation contamination does not, in most simulated scenarios, prevent the time-separated REACH data analysis pipeline from reconstructing a 0.157 K Gaussian 21-cm absorption feature centred between 80 and 120 MHz. The paper shows this for polarised diffuse emission built by rotation measure synthesis, for an interpolated rotation-measure catalogue of point sources, and for toy maps with randomly assigned low Faraday depths. In the point-source case the signal is only missed at the highest polarisation fraction tested ($p=1/10$); it is recovered with low RMSE at $p=1/30$ and $1/70$. It also reports a structural result: contamination produced by the linear mixing of many Faraday-depth patches oscillates more rapidly and is easier to separate from the smooth signal than contamination from a single, slow oscillation, which becomes the hardest case.","pith_inferences":["If the 3% threshold carries over to real data, experiments could monitor the sky's polarisation fraction rather than build leakage into the signal model; this is an extension, since the paper only tests simulated skies.","Because the simulations ignore Stokes $U$ and $V$ leakage, a testable next step is to inject full polarisation maps and see whether the recovered-signal RMSE remains below 30%.","The hardest real case implied here is a sky dominated by one slowly oscillating Faraday screen; targeting that structure with an extra foreground term may be the most effective safeguard.","A future all-sky polarisation survey at 50–150 MHz would let the same pipeline test be run on data-driven maps, converting the paper's bounds into a forecast for actual observations."],"forward_implications":["Below a polarisation fraction of roughly 3%, the pipeline recovers a 0.16 K signal without explicitly modelling polarisation leakage.","Polarised contamination that is spectrally fast, because it mixes many Faraday-depth components, is less dangerous than a single slow oscillatory component.","At $p=1/10$ the point-source model defeats the pipeline, placing an explicit bound on tolerable contamination.","These results give quantified performance bounds for a global 21-cm experiment against a class of known foreground systematics."],"supporting_citations":[{"why":"Defines REACH and its physically informed modelling approach; the pipeline tested here is built for that experiment.","marker":"de Lera Acedo et al. (2022)"},{"why":"Introduces the physically motivated foreground model with spectral-index sky regions used in the fitting.","marker":"Anstey et al. (2021)"},{"why":"Supplies the time-separated analysis method that jointly fits all time bins.","marker":"Anstey et al. (2023)"},{"why":"Provides the rotation-measure-synthesis simulation of polarised diffuse Galactic emission used as the first contamination model.","marker":"Spinelli et al. (2018)"},{"why":"Shows the low-Faraday-depth oscillation behaviour that this paper compares against.","marker":"Spinelli et al. (2019)"},{"why":"Provides the RMTable2023 rotation-measure catalogue used to build the point-source polarisation model.","marker":"Van Eck et al. (2023)"},{"why":"Provides PolyChord, the nested sampling algorithm used for Bayesian evidence and parameter estimation.","marker":"Handley et al. (2015)"},{"why":"Gives the roughly 2% upper limit on polarisation fraction of radio sources, the observational anchor for the 3% threshold.","marker":"Bernardi et al. (2013)"},{"why":"Supplies the Global Sky Model used for total-intensity foreground and spectral-index maps.","marker":"de Oliveira-Costa et al. (2008a)"}],"fun_headline_variants":["Bayesian analysis recovers simulated 21-cm signal amid polarized leakage","Simulated 21-cm signal emerges despite polarized foreground leakage","Bayesian pipeline pulls 21-cm signal from polarized leakage","Simulated 21-cm absorption found with up to 3% polarized leakage"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the three simulated Stokes $Q$ skies are representative of the real low-frequency polarized sky; real leakage could differ in spatial structure and Faraday-depth distribution, or include the Stokes $U$ and $V$ components these models omit.","fun_headline_variants_meta":{"raw":{"variants":["Bayesian analysis recovers simulated 21-cm signal amid polarized leakage","Simulated 21-cm signal emerges despite polarized foreground leakage","Bayesian pipeline pulls 21-cm signal from polarized leakage","Simulated 21-cm absorption found with up to 3% polarized leakage"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000885,"raw_usage":{"total_tokens":3877,"prompt_tokens":1055,"completion_tokens":2822,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":671,"completion_tokens_details":{"reasoning_tokens":2745}},"tokens_in":671,"tokens_out":2822,"duration_ms":21349,"temperature":1.0,"reasoning_tokens":2745,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-11T23:19:32.272917+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Using a measured all-sky Stokes $Q$, $U$, and $V$ map at 50–150 MHz, inject a 0.157 K Gaussian signal into the simulated antenna temperature and run the time-separated pipeline; if the recovered-signal RMSE exceeds 30% of the injected amplitude at a polarisation fraction below $\\sim 3\\%$, the paper's stated bounds would not extend to the real sky.","supporting_citations":[],"review_version":1}