{"id":"37d791ca-b48f-44fd-aaa8-620f375d9f6d","arxiv_id":"2504.15389","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":6,"one_line_summary":"Eclipse mapping of TESS light curves locates starspots on the giant primaries of three eclipsing binaries, with longitudes and sizes broadly consistent with full-light-curve modeling.","lead":"This paper adapts the eclipse-mapping technique, previously used for planet transits, to map starspots on giant stars in eclipsing binaries, and applies it to three systems observed by TESS. It also presents a catalog of 29 candidate systems and finds spots concentrated near the point facing the companion star.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The claimed 'remarkable agreement' is partly built in: for two of the three stars the full-light-curve models fix the spot temperature and latitude to the eclipse-mapping values, so the temperature/radius agreement is not an independent validation.","rationale":"The reader's verdict is already CONDITIONAL, and the strongest reason for that condition is the circularity identified above: for two of the three stars, the full-light-curve spot models adopt the eclipse-mapping spot temperature (and, in at least one case, latitude) as fixed inputs, so the reported agreement in temperatures and radii is not an independent validation. The reader's stated weakest assumption, the P/2 circular-orbit assumption in Section 4.1, is real but less load-bearing: a detectable eccentricity would shift the secondary eclipse midpoint by an amount that high-cadence TESS data would likely reveal, and the systems are plausibly tidally circularized. The circularity concern directly weakens the paper's headline comparison and should force a revised abstract or an explicit statement that the agreement for two targets is a consistency check, not an independent confirmation. The 'always at substellar points' claim should be softened to 'spots are found near the substellar points in the scanned regions of the three systems studied.' Since the reader's conditional verdict already requires such reframing, I do not move the verdict.","tokens_in":25206,"tokens_out":6670,"duration_ms":66893,"concrete_test":"Re-run the time-series spot modeling of TIC 271892852 and TIC 326257590 with spot temperature as a free parameter (e.g., a uniform prior independent of the eclipse-mapping values), and leave latitude free where the data quality allows; then compare the posterior spot temperatures, radii, and longitudes with the eclipse-mapping results. If the free-temperature posteriors are consistent within roughly 100 K and the radii/longitudes remain within the plotted scatter, the agreement claim survives; if the temperatures shift systematically or the longitudes drift outside the quoted uncertainties, the abstract's 'remarkable agreement' must be downgraded to consistency under an assumed temperature.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The abstract's first claim ('Remarkable agreement is found between the starspot temperatures, sizes, and longitudes from the eclipse mapping results and the corresponding full light curve solutions') overstates the evidence because the two methods are not independent for two of the three stars. Section 4.6 states that for one-color TESS data 'we had to suppose constant, typical spot temperatures based on the eclipse mapping results'; Section 5.2 fixes the TIC 271892852 latitude to the eclipse-mapping average of about 40 degrees; and Section 6.2 states that 'we used fixed spot temperatures resulting from the eclipse maps for all three giant stars' (3550 K, 3700 K, and 3500 K for the three targets). Consequently, the agreement in spot temperatures is built in by construction, and the agreement in spot radii is conditional on those temperatures because single-band photometry cannot separately determine spot area and temperature contrast. The only genuinely independent temperature check is the contemporaneous ZTF two-color measurement for TIC 235934420 (3550 +/- 80 K versus 3600/3500/3700 K from eclipse mapping). Thus the headline agreement over the full three-system sample is better described as internal consistency than as independent confirmation. The 'spots are always present at the substellar points' claim is also weaker than stated: it rests on only three systems, and the scanned longitude band is +/-65-77 degrees centered on the substellar point, so the detection is partly a selection effect of where the secondary star happens to scan.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript presents a visually selected catalog of 29 eclipsing binaries with active giant primaries from TESS and then studies three systems in the TESS Continuous Viewing Zones (TIC 235934420, TIC 271892852, TIC 326257590). Starspot signatures during primary eclipses are modeled with a plateau-based eclipse-mapping approach, with the number of spots selected via Bayes factors and posterior sampling checked with the Gelman-Rubin statistic. The full out-of-eclipse light curves are independently modeled with time-series spot modeling, and the two sets of spot parameters are compared. The paper claims remarkable agreement between the two methods in spot temperatures, sizes, and longitudes, and that spots are always present at the substellar points of the tidally locked binaries.","tokens_in":25535,"tokens_out":6543,"duration_ms":60093,"significance":"If the eclipse-mapping results are correct, the paper demonstrates a valuable new application: spatially resolved spot information on giant stars in eclipsing binaries, including a direct high-latitude spot detection on TIC 271892852 and long-term spot tracking in the TESS CVZ. The inclusion of the contemporaneous two-color ZTF data for TIC 235934420 is a genuine strength, as are the explicitly stated MCMC convergence criterion (Gelman-Rubin below 1.05) and the Bayes-factor model comparison. However, the headline cross-method agreement is substantially built in for two of the three targets, because the full-light-curve model adopts the eclipse-mapping temperatures and latitudes as fixed inputs. The significance of the paper is therefore higher as a pilot methodology demonstration and catalog paper than as an independent validation of the eclipse-mapping temperatures and sizes.","major_comments":[{"comment":"The claim of 'remarkable agreement' between eclipse mapping and full-light-curve solutions is not an independent validation for two of the three stars. In Sec. 4.6 the authors state that 'for the one-color TESS data, we had to suppose constant, typical spot temperatures based on the eclipse mapping results,' and Sec. 6.2 states that 'we used fixed spot temperatures resulting from the eclipse maps for all three giant stars.' Sec. 5.2 also fixes the TIC 271892852 latitude to the eclipse-mapping average of about 40 degrees. Because single-band TESS photometry cannot separately determine spot area and temperature contrast, the agreement in temperatures and in the radii that depend on them is partly built in by construction. The only genuinely independent cross-check in the paper is the contemporaneous ZTF two-color analysis for TIC 235934420 (Sec. 5.1 and Appendix B), which does support that one target. I recommend rewording the abstract and Sec. 6.2 to distinguish this internal consistency from the one independent temperature confirmation.","section":"Sec. 4.6 and Sec. 6.2, Table 4"},{"comment":"The statement that 'spots are always present at the substellar points' overstates what the data support. The claim rests on only three binaries, and the scanned longitude ranges listed in Table 3 span roughly +/-65 to +/-77 degrees centered on the substellar point. The observations therefore demonstrate spots within a broad substellar-facing region, not necessarily at the substellar point itself. Please quantify the longitude uncertainty of individual spot detections and soften the claim accordingly, for example to 'spots are found within the scanned region centered near the substellar point in all three systems.'","section":"Abstract; Sec. 6.1; Table 3"},{"comment":"The eclipse-mapping longitude solution assumes circular orbits with primary and secondary eclipses separated by exactly P/2 (Sec. 4.1). For a small eccentricity, the secondary's sky-projected velocity during eclipse differs between the two conjunctions, shifting the time-to-longitude mapping and hence the recovered spot longitudes. This directly affects the association of spots with the substellar point. The paper does not quantify the size of this effect or place bounds on the eccentricity from the observed eclipse timing symmetry. Please add a sensitivity test varying e and the argument of periastron over plausible ranges, or give an explicit upper limit on e based on the approximately P/2 separation of the primary and secondary eclipses.","section":"Sec. 4.1 and Sec. 4.2"}],"minor_comments":[{"comment":"There are typographical object names: 'TIC 23594420' and 'TIC 32627590' in Sec. 6.1 and Fig. 17 should read TIC 235934420 and TIC 326257590, and 'TIC 23934420' in Appendix A.2 should be TIC 235934420.","section":"Fig. 17; Sec. 6.1; Appendix A.2"},{"comment":"The cadence list '200-s, 600s and 1800s' has an inconsistent space and unit format; please make it uniform, e.g., '200-s, 600-s, and 1800-s.'","section":"Sec. 3.1"},{"comment":"The caption states that latitudes are fixed, but the latitude panel in Fig. B.1 shows time-varying values with error bars. Please clarify whether the ZTF modeling used fixed or free latitudes.","section":"Appendix B, Fig. B.1"},{"comment":"The longitude convention should be stated explicitly, in particular that longitude zero corresponds to the substellar point and the direction of increasing longitude; this would make the 'substellar point' claims in Sec. 6.1 easier to interpret.","section":"Sec. 4.2 and Sec. 6.1"},{"comment":"The use of the local light-curve maxima as the unspotted brightness level is a potential systematic bias for spot sizes; the authors acknowledge this issue in Sec. 4.6, but it should also be restated when the average spot radii and coverage values in Table 4 are interpreted as physical spot sizes.","section":"Sec. 4.6 and Sec. 6.2"}],"recommendation":"major_revision","confidential_remarks":"The paper is a competent pilot study, and the independent ZTF temperature check for TIC 235934420 is a real point in its favor. My main concern is that the abstract and discussion present as 'remarkable agreement' what is partly internal consistency because temperatures and latitudes are fed from one method into the other. A revision that reframes the claims, adds the requested eccentricity-sensitivity analysis, and states the substellar-point caveat should make the paper publishable."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: this paper is worth taking seriously, and it is also oversold in its own abstract. The catalog of 29 eclipsing binaries with active giant primaries is new, and the three CVZ analyses are the first real attempt I know of to apply eclipse mapping to stellar companions rather than planets. The method is a clean adaptation of earlier transit-mapping work (Haris et al., Tuomi et al.), and the MCMC setup, convergence checks, and Bayes-factor model comparison are standard and stated plainly. Credit where due: for TIC 235934420, contemporaneous ZTF two-color data give an independent spot temperature of 3550±80 K, agreeing with the eclipse-mapping values of 3600/3500/3700 K. That is a real external validation.\n\nThe soft spots are real, and they are mostly interpretive. Sections 4.6 and 6.2 make clear that for TIC 271892852 and TIC 326257590 the full-light-curve modeling uses spot temperatures and latitudes taken directly from the eclipse-mapping results. So the 'remarkable agreement' in temperatures, sizes, and latitudes for those two stars is partly built in by construction. The agreement in longitudes is independent and meaningful, but the abstract does not say that. The claim that spots are always present at the substellar points is also weaker than it reads: three systems, and the secondary scans a longitude band of roughly ±65–77 degrees centered on the substellar point, so detecting spots there is at least partly a selection effect. The authors actually acknowledge much of this in the text, which is to their credit, but the abstract and summary overstate it.\n\nThe circular-orbit assumption (P/2 between eclipses) is reasonable for these synchronized binaries and is clearly stated, so I do not treat it as a major weakness. Spot sizes as lower limits is also flagged. I do not see a load-bearing flaw in the math or the data; the issue is calibration of the claims.\n\nWho gets value: anyone working on stellar activity in evolved stars, binaries, or TESS photometry. The catalog alone is a useful resource. The paper deserves a serious referee; it is not a desk reject. I would send it out with a request to revise the abstract and discussion so the agreement is described as internal consistency plus one independent temperature check, and to qualify the substellar-point claim. That is doable without new data.","headline":"A solid pilot study and a genuinely useful catalog, but the 'remarkable agreement' in the abstract is partly built in for two of the three stars; it needs a reframing, not new data.","tokens_in":26190,"tokens_out":3340,"would_cite":true,"duration_ms":29086,"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":"Eclipse mapping of starspots on the giant primaries of three TESS binaries recovers spot temperatures, sizes, and longitudes that agree with full-light-curve modeling.","keywords":["starspots","eclipsing binaries","eclipse mapping","giant stars","stellar activity","TESS","differential rotation","spot modeling"],"falsifier":"Measure the radial velocities of the three binaries to determine their eccentricities and re-run the eclipse mapping in an eccentric-orbit model; if the recovered spot longitudes move away from the substellar point by more than the reported uncertainties, the substellar-spot claim fails. Doppler imaging of one of these giants in the same epoch would provide an independent check on the spot longitudes and latitudes.","tokens_in":25007,"feed_emoji":"⭐","tokens_out":8235,"duration_ms":70651,"temperature":0.7,"pith_summary":"This paper establishes that the bumps seen in the light curve while a small companion star crosses the face of a giant primary are starspots, and that those bumps can be turned into spot maps. The authors assembled a catalog of 29 eclipsing binaries with active giant components from TESS photometry and analyzed three in detail, comparing the eclipse-derived spot maps with spot models fit to the full out-of-eclipse light curves. The central claim is that the two independent routes agree on spot temperature, size, and longitude, and that every one of the three binaries shows a spot at the point on the giant that faces the companion. If this holds, single-band photometry can locate spots on evolved stars and track their drift, which bears directly on magnetic activity, differential rotation, and tidal interaction in close binaries.","feed_headline":"Eclipse mapping recovers starspot positions on three giant binaries","feed_subtitle":"Bumps in TESS eclipses give spot temperatures, sizes, and longitudes that match full-light-curve models.","key_machinery":"The load-bearing device is the eclipse-mapping light-curve model: the planetary transit equations modified for two stars, with each spot adding a Gaussian-ramp 'plateau' brightness bump during the eclipse. The timing of a bump inside the eclipse is converted into spot longitude, while the chord of the secondary's path across the stellar disk yields latitude and radius; the number of spots per eclipse is decided by Bayes factors computed from the Bayesian information criterion. Independent time-series spot modeling of the full light curves with analytic circular-spot equations provides the cross-check that anchors the result.","core_discovery":"In the paper's own terms, the eclipse mapping technique previously applied to planets transiting spotted main-sequence stars is adapted to binaries where a small secondary scans a giant primary, and is demonstrated on TIC 235934420, TIC 271892852, and TIC 326257590. The spot bumps during primary eclipses are modeled as occultation features, and the resulting spot temperatures (about 3500-3700 K), angular radii (a few to roughly 25 degrees), and longitudes agree with the time-series spot modeling of the full TESS light curves. For one system the eclipse-mapping temperature is independently confirmed by two-color ZTF photometry, and for another the eclipse chord crosses latitudes near 40 degrees, giving direct evidence of high-latitude spots. All three giants show spots at or very near the substellar point, and the continuous TESS data reveal slow systematic drifts of spot longitudes over about a year.","pith_inferences":["If substellar spots turn out to be a general property of tidally locked giant binaries, spot longitudes could serve as a clock for measuring the rate of tidal synchronization and the offset between orbital and rotational periods.","The technique should transfer to subgiant or giant stars with transiting planets, where the same mapping could show whether active regions on evolved planet hosts concentrate at the subplanetary point.","Because spots are modeled as circular and centrally occulted, the reported sizes are lower limits; joint inversion of several eclipse chords crossing the same spot could test the circular-spot assumption and recover spot shapes.","Separating true differential rotation from spot emergence and decay will require a longer baseline, and the century-scale archival photometry already available for one target offers a direct way to extend the longitude tracking."],"forward_implications":["The same eclipse mapping can be applied to the remaining binaries in the 29-system catalog, yielding spot latitudes, longitudes, and temperatures for a large sample of active giants.","Because the two methods agree, spot temperatures taken from eclipse mapping can be fixed in full-light-curve modeling, reducing the degeneracy of one-color photometry.","The direct high-latitude spot detection on TIC 271892852 shows that eclipse mapping can anchor spot latitudes that full light-curve modeling alone cannot determine.","The slow longitude drifts measured for the three stars are consistent with weak differential rotation and can be followed year by year with TESS continuous-viewing data.","The persistent spot at the substellar point suggests that tidal locking in these binaries systematically favors spot emergence on the hemisphere facing the companion."],"supporting_citations":[{"why":"Supplies the planetary transit light-curve equations that the binary eclipse model adapts to two stars.","marker":"Mandel & Agol (2002)"},{"why":"Gives the equations that convert spot-eclipse timing into spot latitude, longitude, and radius.","marker":"Silva-Valio (2008)"},{"why":"Establishes the plateau spot-signal model and the Bayesian procedure for counting spots that this paper follows.","marker":"Haris et al. (2025)"},{"why":"Provides the analytic circular-spot equations used in the time-series spot modeling of the full light curves.","marker":"Budding (1977)"},{"why":"Supplies the Bayes-factor scale used to decide how many spots are present in each eclipse.","marker":"Kass & Raftery (1995)"},{"why":"Provides the Adaptive Metropolis MCMC sampler used for posterior sampling.","marker":"Haario et al. (2001)"},{"why":"Used to justify the circular-orbit assumption by noting that transit data give weak eccentricity constraints.","marker":"Van Eylen & Albrecht (2015)"}],"fun_headline_variants":["Eclipse mapping reveals spot drifts on tidally locked giants","TESS eclipses map starspots on three giant binaries","Eclipse mapping cross-checks spot temperatures on giants","Spots at substellar points persist in giant binaries","Three giant stars get detailed spot maps from TESS eclipses"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The mapping assumes circular orbits with primary and secondary eclipses separated by exactly half the orbital period, so a small eccentricity would change the companion's speed during the eclipse and shift every recovered spot longitude and the inferred scanned latitude.","fun_headline_variants_meta":{"raw":{"variants":["Eclipse mapping reveals spot drifts on tidally locked giants","TESS eclipses map starspots on three giant binaries","Eclipse mapping cross-checks spot temperatures on giants","Spots at substellar points persist in giant binaries","Three giant stars get detailed spot maps from TESS eclipses"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000751,"raw_usage":{"total_tokens":3390,"prompt_tokens":1039,"completion_tokens":2351,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":655,"completion_tokens_details":{"reasoning_tokens":2270}},"tokens_in":655,"tokens_out":2351,"duration_ms":15778,"temperature":1.0,"reasoning_tokens":2270,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-16T11:28:09.324026+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the radial velocities of the three binaries to determine their eccentricities and re-run the eclipse mapping in an eccentric-orbit model; if the recovered spot longitudes move away from the substellar point by more than the reported uncertainties, the substellar-spot claim fails. Doppler imaging of one of these giants in the same epoch would provide an independent check on the spot longitudes and latitudes.","supporting_citations":[{"cited_title":"& Agol, E","cited_arxiv_id":null,"evidence_quote":"Supplies the planetary transit light-curve equations that the binary eclipse model adapts to two stars."},{"cited_title":"2008, ApJ, 683, L179","cited_arxiv_id":null,"evidence_quote":"Gives the equations that convert spot-eclipse timing into spot latitude, longitude, and radius."},{"cited_title":"2025, arXiv e-prints, arXiv:2502.18129","cited_arxiv_id":null,"evidence_quote":"Establishes the plateau spot-signal model and the Bayesian procedure for counting spots that this paper follows."},{"cited_title":"1977, Ap&SS, 48, 207","cited_arxiv_id":null,"evidence_quote":"Provides the analytic circular-spot equations used in the time-series spot modeling of the full light curves."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the Bayes-factor scale used to decide how many spots are present in each eclipse."}],"review_version":1}