{"id":"9a845dc9-2452-46e0-859e-544230bd47ab","arxiv_id":"2606.10655","paper_version":1,"verdict":"UNVERDICTED","confidence":"LOW","novelty_score":4.0,"correctness_risk":"unknown","formal_verification":"none","parameter_count":0,"one_line_summary":"A multi-survey composite morphology score for Gaia-CRF3 sources reveals a -43.7 μas parallax zero-point shift from extended AGNs and a 15.8% reduction in frame spin for the highest-purity point-source subsample.","lead":"The paper trains an XGBoost model on morphology data from DESI, SDSS, SkyMapper and PS1 to assign a point-source score to 1.6 million Gaia-CRF3 quasars. It reports that extended sources produce a -43.7 μas parallax bias not removed by existing corrections and that selecting the cleanest subsample reduces reference-frame spin by 15.8%.","discovery_kind":"unclear","skeptic_critique":{"model":"grok-4.3","headline":"XGBoost morphology score may inherit survey-matching biases or fail to isolate true optical extent from other source properties.","rationale":"The reader's weakest assumption directly identifies the load-bearing step; the abstract-only review correctly flags the absence of validation details needed to confirm the score's independence and accuracy. No other internal inconsistency is visible from the given material.","tokens_in":1893,"tokens_out":314,"duration_ms":11571,"concrete_test":"Split the training sources into 5-fold CV; report mean AUC and calibration slope for point-source vs. extended labels. Separately, match 500 Gaia-CRF3 sources to HST or high-resolution ground-based catalogs with independent extent flags and compute agreement fraction plus Spearman correlation between predicted score and measured FWHM; if AUC < 0.85 or agreement < 75%, the indicator is insufficiently reliable.","verdict_should_be":"UNVERDICTED","load_bearing_attack":"The central claim requires that the composite point_score (XGBoost fusion of DESI/SDSS/SkyMapper morphological parameters plus PS1-PSC) is a clean, Gaia-independent proxy for optical extent. If the training features correlate with magnitude, color, or redshift in ways that also affect Gaia astrometric solutions (or if cross-survey matching introduces selection), the reported -43.7 μas parallax offset and 15.8% spin reduction could arise from those confounders rather than morphology. No validation metrics, feature-importance breakdown, or external morphology catalog comparison appear in the provided text.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.3","summary":"The paper develops a Gaia-independent morphological indicator for sources in the Gaia-CRF3 catalogue using an XGBoost model trained on parameters from DESI, SDSS, SkyMapper, and the PS1-PSC point-source score. A composite point-source score is constructed via multi-survey fusion, achieving 99.59% completeness. The authors report that AGNs with non-point-like morphology induce a parallax zero-point shift of approximately -43.7 μas that persists after standard corrections, and that selecting sources with point_score > 0.95 reduces the all-sky proper motion frame spin amplitude by 15.8% compared to the full sample.","tokens_in":2052,"tokens_out":601,"duration_ms":20140,"significance":"If the composite morphology score is shown to be a clean, independent proxy for optical extent, the work would offer a practical external tool for vetting Gaia-CRF3 reference sources and mitigating morphology-driven systematics in the celestial reference frame. The reported 99.59% completeness and the quantitative impact on parallax zero-point and frame spin are potentially useful for future Gaia data releases and reference-frame construction.","major_comments":[{"comment":"Abstract: the reported parallax zero-point shift of -43.7 μas and the 15.8% spin-amplitude reduction are stated as point values with no accompanying uncertainties, error bars, or statistical significance tests, which is load-bearing for the claim that morphology induces uncorrectable systematics.","section":"Abstract"},{"comment":"Methods (XGBoost training and multi-survey fusion): no description is given of training/validation splits, cross-validation procedure, performance metrics, or feature-importance analysis for the XGBoost model that produces the point-source scores; this directly affects the reliability of the composite score used for all subsample selections and bias measurements.","section":"Methods (XGBoost training and multi-survey fusion)"},{"comment":"Results (parallax and proper-motion analysis): the assertion that the -43.7 μas shift cannot be removed by the current zero-point correction model, and that the spin reduction is morphology-driven, lacks explicit tests for independence from the morphology-model training itself or for selection biases correlated with magnitude, color, or redshift across the input surveys.","section":"Results (parallax and proper-motion analysis)"}],"minor_comments":[{"comment":"Abstract: the G < 20.85 mag reliability limit is stated without indicating how this threshold was determined or whether it was validated against an external morphology catalog.","section":"Abstract"},{"comment":"The notation 'point_score' is introduced without a clear definition of its exact construction from the four survey scores in the abstract, which would aid readability.","section":null}],"recommendation":"major_revision","confidential_remarks":null},"author_rebuttal":{"model":"grok-4.3","summary":"We thank the referee for the constructive comments on our manuscript. We address each major comment point-by-point below, indicating where revisions will be made to strengthen the paper.","responses":[{"response":"We agree that uncertainties and significance tests are important for these key results. In the revised manuscript we will report bootstrap-derived 1σ uncertainties on both the -43.7 μas shift and the 15.8% spin reduction, together with p-values from two-sample tests comparing the relevant subsamples. These additions will appear in the abstract and the corresponding results section.","revision_made":"yes","referee_comment":"[Abstract] Abstract: the reported parallax zero-point shift of -43.7 μas and the 15.8% spin-amplitude reduction are stated as point values with no accompanying uncertainties, error bars, or statistical significance tests, which is load-bearing for the claim that morphology induces uncorrectable systematics."},{"response":"The current Methods section provides only a high-level description of the XGBoost model. We will expand it to include the 80/20 training/validation split, 5-fold cross-validation details, performance metrics (AUC-ROC, precision-recall, accuracy), and SHAP-based feature-importance rankings. This will directly address the reliability of the composite point-source score.","revision_made":"yes","referee_comment":"[Methods (XGBoost training and multi-survey fusion)] Methods (XGBoost training and multi-survey fusion): no description is given of training/validation splits, cross-validation procedure, performance metrics, or feature-importance analysis for the XGBoost model that produces the point-source scores; this directly affects the reliability of the composite score used for all subsample selections and bias measurements."},{"response":"We will add explicit checks in the revised Results section: (i) magnitude-, color-, and redshift-matched subsample comparisons to demonstrate that the observed differences are not driven by selection biases, and (ii) verification that the morphology score shows no significant correlation with Gaia astrometric parameters outside the external-survey training data. These tests will support the claim that the shift persists after the standard correction and that the spin reduction is morphology-driven.","revision_made":"partial","referee_comment":"[Results (parallax and proper-motion analysis)] Results (parallax and proper-motion analysis): the assertion that the -43.7 μas shift cannot be removed by the current zero-point correction model, and that the spin reduction is morphology-driven, lacks explicit tests for independence from the morphology-model training itself or for selection biases correlated with magnitude, color, or redshift across the input surveys."}],"tokens_in":1618,"tokens_out":573,"duration_ms":14383,"standing_objections":[]},"desk_editor":{"model":"grok-4.3","letter":"The main point is that non-pointlike AGNs pull the Gaia-CRF3 parallax zero point by about -43.7 μas in a way the current correction does not remove, and that keeping only the cleanest point sources (point_score > 0.95) lowers the overall frame spin amplitude by 15.8%.\n\nThey reach this by training XGBoost on morphological parameters from DESI, SDSS, and SkyMapper, anchored to the PS1-PSC score, then fusing the outputs into one composite indicator. They apply it to 1.6 million sources at 99.59% completeness and limit reliability to G < 20.85. The result is a practical, Gaia-independent way to flag extended sources and measure their effect on the reference frame.\n\nThis is new in the specific numbers and the multi-survey fusion applied at this scale. It does a solid job of turning an existing idea into concrete selection criteria that people maintaining the CRF could actually use.\n\nThe soft spots sit in the missing pieces. The abstract reports no error bars on either the -43.7 μas shift or the 15.8% reduction, no training or validation metrics for the XGBoost model, and no test that the score is free of magnitude or color correlations that could also affect Gaia solutions. If those checks are absent from the full text, the central claims rest on unverified assumptions about what the composite score actually isolates.\n\nThe stress-test worry about survey-matching biases or hidden confounders is reasonable to raise until the methods show otherwise. The use of independent surveys reduces circularity, but that alone does not prove the score is clean.\n\nThis paper is for people who work on the Gaia reference frame or need stable AGN subsets for astrometry. A reader looking for a ready-made morphological cut will find usable numbers here.\n\nIt deserves peer review because the question matters and the approach is straightforward enough to check.","headline":"The paper quantifies a -43.7 μas parallax offset and 15.8% spin reduction in Gaia-CRF3 by applying an external XGBoost morphology score, but the abstract leaves the validation details thin.","tokens_in":2530,"tokens_out":485,"would_cite":false,"duration_ms":18888,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.3","headline":"AGNs with non-point-like morphology in Gaia-CRF3 produce a -43.7 μas parallax zero-point shift that standard corrections fail to remove.","keywords":["Gaia-CRF3","quasars","astrometric systematics","morphological score","parallax zero-point","celestial reference frame","proper motion","point-source selection"],"falsifier":"Repeating the parallax zero-point measurement for the same strongly non-point-like AGN subsample using an independent astrometric catalog or high-resolution imaging would test whether the -43.7 μas offset persists.","tokens_in":2812,"feed_emoji":"","tokens_out":715,"duration_ms":17936,"temperature":0.7,"pith_summary":"The paper constructs a Gaia-independent morphological score for 1.6 million Gaia-CRF3 sources by training an XGBoost model on parameters from DESI, SDSS, SkyMapper, and the PS1 point-source catalog, then fusing the outputs into a single 0-to-1 composite indicator. It reports that sources with strongly extended morphology induce a parallax zero-point offset of approximately -43.7 microarcseconds. The same indicator reveals that subsamples with different morphological purity produce measurably different all-sky proper-motion fields. Selecting only sources with point_score above 0.95 reduces the total frame spin amplitude by 15.8 percent compared with the full catalog.","feed_headline":"Non-point quasars shift Gaia parallax zero point by 43.7 μas","feed_subtitle":"Morphology cuts reduce frame spin amplitude 15.8 percent in the celestial reference frame.","key_machinery":"The composite point-source score, a 0-to-1 Gaia-independent indicator of departure from ideal point-source morphology obtained by fusing XGBoost predictions trained on external survey morphological parameters.","core_discovery":"Using a multi-survey fusion of morphological scores derived via XGBoost, the authors demonstrate that AGNs with strongly non-point-like morphology induce a parallax zero-point shift of about -43.7 μas in Gaia-CRF3, which cannot be effectively removed by the current correction model. Reference-source subsamples selected in different score ranges exhibit significantly different all-sky proper motion fields; the high-purity point-source subsample with point_score > 0.95 reduces the total frame spin amplitude by 15.8 percent relative to the full Gaia-CRF3 sample.","pith_inferences":["Future reference-frame constructions may benefit from routine morphological filtering before spin or zero-point fitting.","The same external-score approach could be applied to other large astrometric catalogs to diagnose similar biases.","If extended morphology correlates with host-galaxy properties, the effect may also appear in studies of AGN proper motions over longer baselines."],"forward_implications":["Morphology-aware selection can reduce systematic errors in the Gaia celestial reference frame.","The current parallax zero-point model requires additional terms that depend on source extent.","Proper-motion fields derived from reference sources vary with the morphological purity of the sample.","High-purity point-source subsamples improve the rotational stability of the reference frame by 15.8 percent."],"fun_headline_variants":["Gaia-CRF3 non-point quasars shift parallax 43.7 μas","Morphology scores flag 43.7 μas Gaia-CRF3 parallax bias","Point-score >0.95 trims Gaia frame spin amplitude 15.8%","Extended AGNs bias Gaia-CRF3 zero point 43.7 μas"],"cache_read_input_tokens":2112,"weakest_assumption_plain":"The XGBoost model trained on morphological parameters from other surveys produces scores that accurately measure true optical extent without inheriting or masking Gaia-specific systematics.","fun_headline_variants_meta":{"raw":{"variants":["Gaia-CRF3 non-point quasars shift parallax 43.7 μas","Morphology scores flag 43.7 μas Gaia-CRF3 parallax bias","Point-score >0.95 trims Gaia frame spin amplitude 15.8%","Extended AGNs bias Gaia-CRF3 zero point 43.7 μas"]},"model":"grok-4.3","cost_usd":0.004494,"raw_usage":{"total_tokens":2316,"prompt_tokens":822,"num_sources_used":0,"completion_tokens":86,"cost_in_usd_ticks":44937000,"prompt_tokens_details":{"text_tokens":822,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":1408,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":822,"tokens_out":86,"duration_ms":9232,"temperature":1.0,"reasoning_tokens":1408,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-06-27T12:52:12.040533+00:00","model_set":{"reader":"grok-4.3"},"falsifier":"Repeating the parallax zero-point measurement for the same strongly non-point-like AGN subsample using an independent astrometric catalog or high-resolution imaging would test whether the -43.7 μas offset persists.","supporting_citations":[],"review_version":1}