{"id":"4cc8fd51-b14d-492e-aabb-aa677f4d9217","arxiv_id":"2608.08579","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"TFGI commissioning data yield 8.3 microkelvin per degree polarization sensitivity with two 31 GHz detectors, extrapolated to 1 microkelvin per degree for the full 29-detector array after 5.7 years.","lead":"The QUIJOTE team presents the first on-sky commissioning results of TFGI, a 31 and 41 GHz CMB polarimeter, measuring a polarization sensitivity of about 8.3 microkelvin per degree with two 31 GHz detectors. A generalist should read it to see whether a Northern-hemisphere ground instrument can reach the sensitivity needed to constrain inflationary gravitational waves.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The 5.7-year forecast for 41 GHz is not supported by the only measured FGI pixel: pixel 63 alone implies ~11 years to 1 μK deg^-1, so the full-array claim implicitly assumes FGI detectors outperform the commissioning data.","rationale":"The paper's central measurement — 8.3 μK deg^-1 with two 31 GHz pixels, validated by HMDM noise maps, skydip white-noise levels, and cross-checks with WMAP/Planck — is credible and well supported. My concern is focused on the extrapolation to the full array. The reader already identified the assumption that remaining detectors perform comparably; I agree and would sharpen it. The quantitative issue is that the only FGI pixel measured on sky, pixel 63, is 73% efficient and has σ_Q=σ_U=16.4 μK deg^-1, roughly twice the combined TGI value. Scaling that measured number to 14 FGI detectors gives ~11 years to 1 μK deg^-1, not 5.7. The 5.7-year figure for 41 GHz requires FGI per-detector sensitivity to match TGI, which contradicts the only on-sky datum we have for FGI. Since 3/7 installed pixels were excluded for exactly the time-varying polarization-angle failure that the diode is supposed to cure, the forecast is conditional twice over: on the diode working as hoped, and on FGI detectors outperforming pixel 63. This does not undermine the commissioning results themselves, but it does mean the abstract's dual-frequency 5.7-year claim is not established by the reported data. I would keep the reader's CONDITIONAL verdict, hence UNCHANGED, with the condition made more explicit: the 41 GHz completion time should be recomputed from pixel 63 and stated as a separate, longer forecast unless a concrete calibration-diode demonstration is provided.","tokens_in":32903,"tokens_out":9270,"duration_ms":97958,"concrete_test":"Recompute the Sect. 5 forecast using the measured pixel-63 row of Table 8 (16.4 μK deg^-1 at 0.57 h deg^-2, ε=73%) as the baseline for all 14 FGI detectors, with 15 TGI detectors using the combined 31 GHz per-detector sensitivity, and the paper's own 18 h/day, 50% efficiency schedule. If the time to 1 μK deg^-1 at 41 GHz exceeds 5.7 years — the arithmetic gives ~11 years (or ~7 years at ε=90%) — then the abstract's 'both 31 and 41 GHz' claim is unsupported, and the headline forecast should be restricted to 31 GHz or explicitly conditioned on a demonstration that the diode delivers near-100% FGI polarization efficiency. A useful second check is to repeat the same calculation with the lab-quoted FGI efficiencies (70–75%) instead of the post-BEM value.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The conditioning that carries the central forecast is in Sect. 5 (Fig. 16): 15 TGI + 14 FGI detectors are assigned the per-detector sensitivity of the two good 31 GHz pixels to reach 1 μK deg^-1 in 5.7 years at 18 h/day and 50% efficiency. The only 41 GHz pixel retained in the analysis does not support that assignment. Table 8 gives σ_Q = σ_U = 16.4 μK deg^-1 for pixel 63 at 0.57 h deg^-2 with one detector, after correcting for its measured 73% polarization efficiency; the two 31 GHz pixels combined give 8.4/8.2 μK deg^-1. Scaling pixel 63 to 14 identical FGI detectors gives sqrt(14 T / 0.57) = 16.4 for a 1 μK deg^-1 target, hence T ≈ 11 years under the paper's own duty-cycle assumptions. Raising the polarization efficiency from 73% to 90% shortens this to ~7 years; only at ε = 100% does it approach 5.7 years. Laboratory efficiencies for FGI are quoted as 70–75%. The paper's own text says the FGI map 'would take a bit longer' and then relies on the diode making FGI 'match' TGI, but no on-sky commissioning measurement supports that convergence. Independently, 3 of the 7 installed pixels (TGI 5, FGI 41, FGI 42) were excluded for time-varying polarization angles, so the empirical yield of polarization-stable pixels is 3/7; the 5.7-year claim for 'both 31 and 41 GHz' hinges on the calibration diode repairing exactly that failure mode.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports the commissioning and first science verification of QUIJOTE-TFGI, a 31/41 GHz polarimeter on the second QUIJOTE telescope, using data from November 2021 to May 2022 with seven installed receivers (four TGI at 31 GHz, three FGI at 41 GHz). It characterizes pointing, beams, gain calibration via Tau A and Cas A, polarization efficiency and angle calibration, instantaneous white-noise levels, intensity-to-polarization leakage, and map-making performance on Cygnus and Galactic-plane fields (W43, W44, W47). The central quantitative result is the half-mission difference-map polarization sensitivity: about 8.3 microK deg^-1 from the two usable TGI pixels at an effective depth of 0.57 h deg^-2, already comparable to WMAP's polarized sensitivity in the same region. The paper then extrapolates this sensitivity to the full 29-detector array and claims that 1 microK deg^-1 over 3600 deg^2 at both 31 and 41 GHz can be reached after 5.7 effective years, enabling a target r <= 0.05.","tokens_in":1547,"tokens_out":3303,"duration_ms":112445,"significance":"If the measured sensitivity holds, this is a valuable result for ground-based CMB polarimetry: it demonstrates that a small Northern-hemisphere telescope with only two 31 GHz detectors can achieve polarization sensitivity comparable to space-based WMAP, and it validates the instrumental chain (phase-switching polarimetry, differential map-making, calibration strategy) with on-sky data. The paper is strong on the measurement side: the HMDM construction is clearly described, the comparison with WMAP and Planck is done on matched maps, the calibrations use external models (Tau A, Cas A), and the SED fits provide independent cross-checks. The main weakness is not the measurement but the forecasting step, which extrapolates from the two best 31 GHz pixels to 29 unmeasured detectors and, in particular, assumes that the 41 GHz channel will match TGI performance despite the only measured FGI pixel being substantially less sensitive.","major_comments":[{"comment":"The 41 GHz forecast is not supported by the on-sky data. Pixel 63, the only FGI pixel retained in the analysis, gives sigma_Q = sigma_U = 16.4 microK deg^-1 with one detector at 0.57 h deg^-2, after correcting for its measured 73% polarization efficiency. Scaling this single detector to 14 identical FGI detectors gives (16.4^2 x 0.57)/14 = 10.9 years to reach 1 microK deg^-1, not 5.7 years. Even under a hypothetical 90% polarization efficiency the time would be about 7 years; matching the TGI claim requires that the unmeasured FGI detectors perform at the level of the two best TGI pixels rather than at the level of the only FGI pixel actually measured. The text's statement that 'the FGI map would take a bit longer' understates this by roughly a factor of two, and the abstract's claim of reaching 1 microK deg^-1 at both 31 and 41 GHz after 5.7 years is therefore not established by the commissioning data.","section":"Section 5, Table 8, Eq. (11), Fig. 16"},{"comment":"The sensitivity estimate and the forecast rest on a post-hoc selection of pixels. Three of the seven installed pixels (TGI pixel 5, FGI 41, FGI 42) were excluded because of time-varying polarization angles and low on-sky polarization efficiency; the 8.3 microK deg^-1 figure uses the best two TGI pixels. The full-array forecast assumes that the calibration diode will repair exactly this failure mode, but no on-sky commissioning measurement demonstrates that convergence. The Conclusions state that the forecasts 'should be interpreted as conservative scenarios', which is difficult to sustain: the 5.7-year number assumes all 29 detectors behave like the best measured subset, and the diode correction is a planned hardware upgrade, not a measured property of the present data. The forecast should be presented as conditional on the diode and on unverified detector yield.","section":"Section 2.1, Section 3.4, Section 5"},{"comment":"The W44 polarized SED indicates that pixel 63's polarized signal is lower than expected even after the Tau-A-derived polarization efficiency correction, which the paper attributes to time variations of the polarization angle over the longer W44 dataset. This is directly relevant to the forecast: the 16.4 microK deg^-1 noise value in Table 8 is a valid noise measurement, but if slow polarization-angle drifts suppress the polarized signal on survey timescales, the usable scientific sensitivity is worse than the raw HMDM noise implies. The forecast in Fig. 16 should either account for this signal-recovery loss or explicitly state the assumption that the diode eliminates it.","section":"Section 4, Fig. 11, Appendix D"}],"minor_comments":[{"comment":"The two lines defining h1 both use 'h1'; the second line should assign the odd-index scans to h2.","section":"Section 2.3, Eq. (12)"},{"comment":"The text refers to 'TGI pixel 25' as one of the unstable pixels, but Table 1 lists the unstable TGI pixel as pixel 5; the numbering should be made consistent.","section":"Section 3.4.2 and Section 6"},{"comment":"The label '29 TGI pixels (equivalent to full-TGI configuration)' could mislead readers, since the full TFGI configuration is 15 TGI plus 14 FGI detectors; the curve is a hypothetical all-31-GHz array and should be labeled as such.","section":"Fig. 16 caption"},{"comment":"The 5.7-year forecast assumes a 50% observing efficiency, whereas the commissioning campaign reports a 35% efficiency (Section 2.3). The paper notes that 50% may be optimistic, but the sensitivity of the headline number to this assumption should be stated explicitly in the main text.","section":"Section 5"}],"recommendation":"major_revision","confidential_remarks":"The measurement content of the paper is solid and appropriate for an instrumentation paper for the journal. The central problem is the forecasting section: the 5.7-year claim for both frequencies is not supported by the only measured FGI pixel and depends on a diode-based fix that is not yet demonstrated on sky. I recommend major revision rather than rejection, provided the authors rescope the forecast as a conditional scenario and give the FGI projection explicitly using the pixel 63 sensitivity."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Two things worth knowing about this one. First, the on-sky commissioning results are the real thing: two 31 GHz pixels reach ~8.3 μK deg^-1 polarization sensitivity from half-mission difference maps, with careful flagging and appropriate WMAP/Planck cross-checks. Second, the headline forecast—1 μK deg^-1 over 3600 deg^2 in 5.7 years with the full 29-detector array—is not supported for 41 GHz. The only measured FGI pixel (pixel 63) gives 16.4 μK deg^-1 at 0.57 h deg^-2; scaling to 14 FGI detectors takes ~11 years, not 5.7. The paper says 'a bit longer' and leans on the calibration diode to make FGI match TGI, but that is an expectation, not a measurement.\n\nWhat the paper does well: the HMDM sensitivity estimate is a direct noise product, not a fit to the model being validated. The 10% gain calibration uncertainty is honestly conservative. The W44 SEDs are consistent with prior data. The treatment of the three unstable pixels is transparent. The beam FWHM, pointing model, and leakage upper limits are useful reference numbers.\n\nThe stress-test note holds up. Three of seven pixels had time-varying polarization angles, so the empirical yield of polarization-stable pixels is 3/7. The 29-detector forecast assumes the diode repairs exactly that failure mode, which is plausible but unverified. The abstract and conclusions present 5.7 years as the headline number without that caveat. I'd suggest either reporting the 41 GHz extrapolation at ~11 years under current per-pixel performance, or presenting it as a conditional goal that requires the diode to improve polarization efficiency from ~73% to ~90%+. Also, calling the forecasts 'conservative scenarios' in the conclusions oversells them; given the FGI data, 'optimistic' is closer, at least for 41 GHz.\n\nNone of this undermines the core commissioning result. The sensitivity measurement is credible and the instrument behaves as designed for the good pixels. This paper is for the CMB instrumentation and foreground community; it gives first on-sky reference numbers for TFGI and belongs in the QUIJOTE series.\n\nSend it to a referee. It's a serious, honest commissioning paper, and the forecast issue is a tractable revision: reframe the 41 GHz forecast and soften the abstract. I'd accept after minor revision.","headline":"Solid commissioning paper with a credible sensitivity measurement; the 5.7-year full-array forecast for 41 GHz is not supported by the only measured FGI pixel and should be presented as conditional.","tokens_in":33949,"tokens_out":4466,"would_cite":true,"duration_ms":45411,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"TFGI's two 31 GHz detectors already deliver about 8.3 microkelvin per degree polarization sensitivity, nearly matching WMAP with a third of the integration time, and the full array is forecast to reach 1 microkelvin per degree in 5.7 years.","keywords":["Cosmic microwave background radiation","Observational cosmology","Diffuse radiation","Radio continuum emission","Milky Way disk","Polarimetry","instrument commissioning","polarization sensitivity"],"falsifier":"After one year with the 10 TGI pixels now being installed, construct the half-mission difference maps of a $3^\\circ$-radius aperture in the Galactic-plane field and measure the RMS: if the $Q/U$ sensitivity is not close to the projected $\\sim 2.9\\,\\mu\\mathrm{K}\\,\\mathrm{deg}^{-1}$, the detector-count scaling that underlies the 5.7-year forecast is wrong. The same maps will show whether the calibration diode has reduced polarization-angle scatter to the sub-degree level needed to keep the additional detectors from cancelling in $Q$ and $U$.","tokens_in":32731,"feed_emoji":"📡","tokens_out":11523,"duration_ms":105972,"temperature":0.7,"pith_summary":"This paper reports the commissioning of TFGI, a 31/41 GHz polarimeter on the second QUIJOTE telescope, and argues that the instrument has already reached a polarization sensitivity of about 8.3 microkelvin per degree using only two 31 GHz detectors after 0.57 hours per square degree. That is comparable to WMAP's polarization noise in the same region, which used almost three times more integration time per area. Extrapolating from these two detectors to the planned 29-detector array, the paper forecasts that TFGI will reach its survey target of about 1 microkelvin per degree over 3600 square degrees in 5.7 years, enough to constrain the tensor-to-scalar ratio at the $r\\le0.05$ level from the Northern sky. The claim matters because it says a comparatively small ground-based Northern-hemisphere instrument can deliver polarization maps competitive with space missions, provided the remaining detectors perform like the three analysed pixels.","feed_headline":"Two 31 GHz detectors nearly match WMAP's polarization sensitivity","feed_subtitle":"Commissioning data show ~8.3 µK deg⁻¹ polarization noise; the full array should reach 1 µK deg⁻¹ in 5.7 years.","key_machinery":"The load-bearing mechanism is the electronic phase-switch polarimeter. TFGI switches its polarization phase states at up to 160 kHz (cycling 16 states at 4 kHz), and forms Stokes $Q$ and $U$ from differences between phase states separated by $180^\\circ$, so correlated $1/f$ noise and atmospheric signal cancel within each detector rather than between detectors with different bandpasses. The paper's quantitative engine is the half-mission difference map (HMDM): scans are split chronologically into two halves, the weighted difference is a noise map whose RMS in a $3^\\circ$ radius aperture gives the sensitivity in $\\mu\\mathrm{K}\\,\\mathrm{deg}^{-1}$. This map-based sensitivity, together with the assumption that sensitivity scales as the inverse square root of detector number and time, produces the 5.7-year forecast.","core_discovery":"The central discovery is that the TFGI polarization channel works as designed: half-mission difference maps of Galactic plane fields, made with two 31 GHz detectors (plus a 41 GHz check), give $\\sim 8.3\\,\\mu\\mathrm{K}\\,\\mathrm{deg}^{-1}$ in Stokes $Q/U$ at an effective depth of $0.57\\,\\mathrm{h}\\,\\mathrm{deg}^{-2}$. This is only about 20 percent worse than WMAP's 33 GHz polarization sensitivity in the same region, despite WMAP having almost three times the integration per unit area. The paper then scales this number linearly with detector count and observing time: 10 TGI pixels would need almost 9 years to reach $1\\,\\mu\\mathrm{K}\\,\\mathrm{deg}^{-1}$, while the full 29-detector configuration (15 TGI plus 14 FGI) would need 5.7 years over three $1200\\,\\mathrm{deg}^2$ fields. Along the way it verifies the beam shapes, gain stability, polarization efficiency, intensity-to-polarization leakage below about 0.4 percent, and consistency of W44's intensity and polarization spectral energy distributions with previous data.","pith_inferences":["If the detector-scaling model is right, the 2026 19-detector configuration should reach about $2.9\\,\\mu\\mathrm{K}\\,\\mathrm{deg}^{-1}$ on the 31 GHz TGI map after one year; measuring the HMDM noise in that configuration would test the forecast long before the full array is built.","Even with the target sensitivity reached, the reported 4–6 degree polarization-angle calibration uncertainty is far above the sub-degree accuracy future B-mode searches will need, so the calibration diode's angle-tracking performance, not raw sensitivity, is the real constraint on using these maps for cosmology.","The comparison with WMAP suggests that fast electronic phase-switching can let a small ground-based telescope compete with a space mission in polarization mapping; the same architecture could be tested at other Northern-hemisphere sites with comparable precipitable water vapour."],"forward_implications":["If the forecast is correct, TFGI's 31 GHz maps reach $\\sim 1\\,\\mu\\mathrm{K}\\,\\mathrm{deg}^{-1}$ over 3600 deg$^2$ after 5.7 years, enough to put an upper limit of $r \\le 0.05$ from the Northern sky.","With the 10 TGI detectors now installed, reaching the same 31 GHz goal would take almost 9 years; the remaining detectors reduce this to 5.7 years, assuming equal performance per detector.","The instantaneous white-noise levels measured from skydips (314–376 $\\mu\\mathrm{K}\\,\\mathrm{s}^{1/2}$) match the design value of 350 $\\mu\\mathrm{K}\\,\\mathrm{s}^{1/2}$, so per-detector sensitivity is already at the level the forecast assumes.","The intensity-to-polarization leakage upper limits (below 0.19 percent for TGI pixels and below 0.36 percent for the FGI pixel) support the assumption that the deep polarization maps will not be dominated by leaked total intensity."],"supporting_citations":[{"why":"Supplies the MFI pipeline inherited by TFGI, the Tau A flux model and polarization reference values used for gain and angle calibration.","marker":"J. A. Rubiño-Martín et al. 2023"},{"why":"Provides the laboratory TFGI characterization and the polarization-angle calibration method applied to on-sky data.","marker":"A. Fasano et al. 2026"},{"why":"Describes the TGI phase-switch polarimeter design that gives TFGI its fast polarization modulation.","marker":"R. Hoyland et al. 2014"},{"why":"Describes the FGI receiver and orthomode transducer design used at 41 GHz.","marker":"E. Artal et al. 2020"},{"why":"Provides the PICASSO destriper code adapted for TFGI intensity map-making.","marker":"F. Guidi et al. 2021"},{"why":"Supplies the WMAP 9-year maps used as the sensitivity benchmark and as comparison data for Cygnus and W44 SEDs.","marker":"C. L. Bennett et al. 2013"},{"why":"Provides the Tau A secular decrease correction that transfers the calibration model to the TFGI epoch.","marker":"J. L. Weiland et al. 2011"},{"why":"Supplies the Planck PR3 maps used in the Cygnus comparison and in the sensitivity table.","marker":"Planck Collaboration et al. 2020a"}],"fun_headline_variants":["Two TFGI detectors rival WMAP's polarization sensitivity","Full TFGI array projected to reach 1 µK/deg sensitivity","QUIJOTE TFGI commissioning shows WMAP-level polarization","31 GHz QUIJOTE detectors nearly match WMAP's depth","New CMB polarimeter passes early sensitivity test"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The 5.7-year forecast assumes the remaining detectors will perform like the three good pixels analysed here, and that the new calibration diode will fix the unstable polarization angles that degraded three of the seven commissioned pixels.","fun_headline_variants_meta":{"raw":{"variants":["Two TFGI detectors rival WMAP's polarization sensitivity","Full TFGI array projected to reach 1 µK/deg sensitivity","QUIJOTE TFGI commissioning shows WMAP-level polarization","31 GHz QUIJOTE detectors nearly match WMAP's depth","New CMB polarimeter passes early sensitivity test"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000323,"raw_usage":{"total_tokens":1944,"prompt_tokens":1201,"completion_tokens":743,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":817,"completion_tokens_details":{"reasoning_tokens":660}},"tokens_in":817,"tokens_out":743,"duration_ms":7857,"temperature":1.0,"reasoning_tokens":660,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T04:30:12.456288+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"After one year with the 10 TGI pixels now being installed, construct the half-mission difference maps of a $3^\\circ$-radius aperture in the Galactic-plane field and measure the RMS: if the $Q/U$ sensitivity is not close to the projected $\\sim 2.9\\,\\mu\\mathrm{K}\\,\\mathrm{deg}^{-1}$, the detector-count scaling that underlies the 5.7-year forecast is wrong. The same maps will show whether the calibration diode has reduced polarization-angle scatter to the sub-degree level needed to keep the additional detectors from cancelling in $Q$ and $U$.","supporting_citations":[],"review_version":1}