{"id":"c091b4cf-f1f1-4256-9252-4f459ba5a23b","arxiv_id":"2504.17007","paper_version":2,"verdict":"ACCEPT","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"low","formal_verification":"none","parameter_count":0,"one_line_summary":"A source in a z=5.274 galaxy dimmed by 0.19 magnitudes in JWST infrared images, making it the first photometric transient or variable candidate at z greater than 5.","lead":"Using two JWST images taken one year apart, astronomers found a faint source in a distant galaxy that dimmed by nearly 0.2 magnitudes in infrared light. It is the first photometric evidence of a transient or variable object beyond redshift 5, pushing time-domain astronomy toward the epoch of reionization.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"F356W variability may be a PSF/astrometric artifact between two independent surveys; bootstrapping does not test this, so the central z>5 transient claim needs a PSF-matched subtraction.","rationale":"The reader's verdict ACCEPT is reasonable but relies on the weakest assumption being only 'moderate risk.' I argue the risk is higher: the two cross-checks in Section 3.2 share the same two input mosaics and therefore cannot distinguish an intrinsic brightness change from a PSF or astrometric systematic. The absence of a PSF-matched difference image or a star-based validation is a concrete omission. However, the paper has genuine independent support: the z_spec=5.274 redshift from grism spectroscopy, the absence of F090W/F115W variability, the consistency between difference-image and science-image photometry, and the exposure-bootstrap all point toward a real change. My recommendation is CONDITIONAL rather than REJECT because the concern is a missing validation, not a demonstrated flaw. If the PSF-matched subtraction test described above confirms the residual, the claim stands and the verdict should be ACCEPT. The reader and I agree on the weakest link; I only weight it more heavily because the entire discovery hinges on ruling out this systematic.","tokens_in":18022,"tokens_out":6989,"duration_ms":69948,"concrete_test":"Take the public JADES and CONGRESS F356W mosaics; select ~50 isolated unsaturated stars across the overlap region; measure their FWHM and centroid positions in both epochs. Build an empirical PSF or a spatially varying convolution kernel that maps the Epoch2 PSF onto the Epoch1 PSF (or vice versa), apply it, and re-run the difference-image and aperture photometry at AT 2023adya's position. If the residual flux at the host center is no longer significant (SN<5) or the measured 0.19 mag change shifts by >0.1 mag, the variability claim is not robust. Independently, shift one epoch by ±0.5 pixel and recompute; large sensitivity would indicate the alignment is not sufficient.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The central claim rests on the F356W brightness decrease of AT 2023adya being intrinsic to the z=5.274 host. Because the transient is coincident with the host center (Section 3.3) and the two epochs come from different surveys (JADES vs CONGRESS) reduced with 'similar' but not identical pipelines (Section 2.1), the 0.19 mag change could in principle be produced by a small PSF FWHM difference, a sub-pixel astrometric offset, or background-subtraction residuals in the resampled 0.03 arcsecond mosaics. Section 3.2's tests do not close this gap: the bootstrap only randomizes which individual exposures enter each epoch's mosaic and therefore cannot detect a systematic PSF/WCS difference between the two surveys; the agreement between difference-image and science-image photometry is not independent because both use the same two mosaics. The result is a ~6-sigma detection at the worst possible location for subtraction residuals (the galaxy center), with no PSF-matched subtraction, no star-based PSF kernel, and no astrometric residual map reported. If a broader Epoch1 PSF or a ~0.02 arcsecond shift were present, the measured Epoch1-minus-Epoch2 flux at the galaxy center could mimic the observed fading.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports the discovery of AT 2023adya, a transient/variable source candidate in GOODS-North detected by comparing two epochs of JWST/NIRCam imaging taken one observer-frame year apart by the JADES and CONGRESS programs. The source is coincident with the center of a z_spec = 5.274 galaxy and exhibits a 0.19 mag fading in F356W (from 26.05 to 26.24 mag), while showing no significant change in F090W or F115W. The authors support the variability with difference-image photometry, science-image aperture photometry, bootstrap-resampled mosaics, and an SN_var of 6.09. They discuss possible origins including core-collapse supernovae, Type Ia supernovae, variable AGNs, and tidal disruption events, and conclude that although the nature cannot be determined, the source is the first photometric evidence of a z > 5 transient/variable source with JWST.","tokens_in":18233,"tokens_out":4553,"duration_ms":43799,"significance":"If the measured F356W variability is intrinsic, this is a milestone: it pushes JWST transient science beyond z = 5 for the first time, with implications for high-redshift supernova rates, AGN variability, and tidal disruption event rates. The paper is appropriately cautious in not over-claiming the classification and makes useful quantitative comparisons to published supernova luminosity functions, AGN spectral signatures, and TDE models. The data are public and the analysis pipeline is described in enough detail to be reproduced. However, the central claim rests entirely on the robustness of the two-epoch photometric difference, and the current analysis does not fully exclude systematic differences between the two independent survey reductions.","major_comments":[{"comment":"The agreement between the difference-image photometry (m_F356W = 28.01 ± 0.17 mag) and the difference of the science-image photometry (28.04 ± 0.14 mag) does not constitute an independent verification of the variability, because both measurements are derived from the same pair of mosaics. Any systematic difference in PSF shape, astrometric registration, or background subtraction between the Epoch1 (JADES) and Epoch2 (CONGRESS) reductions would produce a residual in the difference image and a corresponding offset in the aperture photometry. The bootstrap test in this section only randomizes which individual exposures are used to construct each epoch's mosaic; it cannot reveal a coherent PSF or WCS offset between the two surveys. I request a PSF-matched image subtraction using empirical PSFs from isolated stars in each mosaic, together with an explicit test of the sensitivity of the measured F356W flux change to plausible PSF FWHM differences (e.g., a few percent) and sub-pixel astrometric shifts. This is the load-bearing test for the central claim that AT 2023adya is a genuine transient/variable source.","section":"Section 3.2"},{"comment":"The source is located within ~0.02 arcsec of the host center, i.e., about one third of a native long-wavelength pixel. At this position the surface brightness gradient of the host is maximal, so small PSF mismatches or registration errors produce the largest possible spurious residuals. The current paper does not quantify the expected residual from such systematics. The F090W and F115W non-detections are encouraging, but they are not conclusive because the PSF sizes and the host morphology differ between filters, and a systematic artifact could in principle affect the long-wavelength band differently. Please add an estimate of the expected spurious signal at the transient position from a PSF mismatch or astrometric error, and show that the observed F356W residual exceeds it by a comfortable margin.","section":"Section 3.3"}],"minor_comments":[{"comment":"There is a typo: 'spectral energy distrubtion' should be 'spectral energy distribution'.","section":"Section 3.3"},{"comment":"The text refers to 'AT 2024adya's host' in the sentence about Sun et al. (2024); this should be 'AT 2023adya's host'.","section":"Section 3.3"},{"comment":"In the RGB difference image, consider adding a clearer label or marker for the residual signal, as the current crosshairs may blend with galaxy light at the printed scale.","section":"Figure 1"},{"comment":"For clarity, the table note could state explicitly that the SNvar values in row 4 are computed from the science-image photometry (Eq. 1), since the difference-image magnitudes in row 3 are not used for SNvar.","section":"Table 1"}],"recommendation":"major_revision","confidential_remarks":"The paper is well written and the scientific case is interesting, but the central variability claim needs to be secured against the systematic differences between two independent survey reductions. The requested PSF-matched subtraction and robustness tests are feasible with the existing public data, so this is a fixable issue rather than a fatal flaw. I recommend major revision rather than rejection or acceptance."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Bottom line: the paper presents a plausible first—a z>5 variable/transient found with JWST—but the core evidence has a PSF-matching gap that the authors' own checks do not close. I'd send it to peer review with a strong request for a re-reduction.\n\nWhat's genuinely new: AT 2023adya, in a host with a spectroscopic redshift of 5.274, faded by ~0.19 mag in F356W over one year, with no F090W/F115W change. The search is systematic and the photometry is carefully done: difference-image and science-image measurements agree, and the bootstrap test rules out the signal being driven by a single bad exposure. The authors also deserve credit for not overclaiming—they explicitly note that the nature remains unknown and walk through SN, AGN, and TDE interpretations with appropriate caveats.\n\nThe soft spot: the two epochs come from different surveys (JADES vs CONGRESS) reduced with 'similar' pipelines. The difference image is created without PSF matching, and the source sits at the host center, which is exactly where a small PSF FWHM difference or sub-pixel astrometric shift would produce a spurious residual. The bootstrap test only randomizes within each epoch, so it cannot catch a systematic difference between the two reductions. The 0.19 mag change is ~16% of the host flux; that's too large for a typical PSF mismatch, but not impossible if the host is compact and the PSFs differ by a few percent. The agreement between the two photometric methods is reassuring, but those methods share the same two mosaics, so they are not independent. A referee needs to see a PSF-matched subtraction using field stars and an astrometric residual map before the variability is considered secure.\n\nMinor: the physical discussion leans on low-redshift SN luminosity distributions, which is fine as a rough guide; the SN Ia rate argument is qualitative; the NIRCam grism non-detection of broad Halpha constrains but does not exclude a star-formation-dominated Type 1 AGN. These are handled honestly.\n\nRecommendation: this deserves a serious referee because the claim is important and the fix is doable with existing data. I would not desk-reject. I'd accept and require the re-reduction in revision.","headline":"A real first-look at z>5 variability, but the two-survey, two-epoch design leaves a PSF-matching question a referee must press.","tokens_in":18934,"tokens_out":4140,"would_cite":true,"duration_ms":40873,"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":"AT 2023adya, a source in a z=5.274 galaxy, is the first JWST photometric transient beyond z=5, fading 0.19 mag in F356W.","keywords":["transients","variable sources","high-redshift supernovae","JWST","image differencing","active galactic nuclei","tidal disruption events","GOODS-North"],"falsifier":"Re-reduce both epochs with matched point-spread functions, a single astrometric solution, and identical background treatment, then re-measure the F356W flux at the host position; if the 0.19 magnitude drop disappears or falls below a signal-to-noise of about 6, the central claim fails. A third JWST epoch showing continued fading or complete disappearance would confirm the change was intrinsic rather than a reduction artifact.","tokens_in":17824,"feed_emoji":"🔭","tokens_out":11664,"duration_ms":94941,"temperature":0.7,"pith_summary":"AT 2023adya is a source near the center of a spectroscopically confirmed $z_{\\rm spec}=5.274$ galaxy in GOODS-N. In the F356W filter, the galaxy plus source faded from $m_{\\rm F356W}=26.05\\pm0.02$ mag to $m_{\\rm F356W}=26.24\\pm0.02$ mag between two JWST epochs one observer-frame year apart, about two rest-frame months; difference imaging yields a residual of $m_{\\rm F356W}=28.01\\pm0.17$ mag with $\\mathrm{SN}_{\\mathrm{var}}=6.09$, and the same change is recovered from direct science-image photometry. F090W and F115W show no rest-frame ultraviolet change, so the variation is red. The paper argues that this is the first JWST photometric evidence of a transient/variable source at $z>5$, extending transient science past the previous frontier of $z\\sim4.8$ and toward the epoch of reionization. The physical nature of AT 2023adya is not settled: any bright core-collapse supernova subtype, and possibly an SN Ia, can match its rest-frame V-band magnitude $M_{\\rm V}=-18.48$ mag, while a variable AGN or TDE is disfavored but not excluded.","feed_headline":"JWST catches the first transient source beyond z=5","feed_subtitle":"A galaxy at z=5.274 dimmed by 0.19 mag in two rest-frame months; if real, JWST can hunt transients near reionization.","key_machinery":"The central mechanism is a two-epoch, single-filter image-differencing search: F356W NIRCam mosaics from JADES (Epoch1, 2023) and CONGRESS (Epoch2, 2024), resampled onto a common $0.03\\arcsec$ pixel grid, are subtracted to expose variable flux. Difference-image photometry in an $r=0.2\\arcsec$ aperture measures the residual at the host position, and the result is cross-checked in two ways: direct aperture photometry on the two science images gives nearly the same residual magnitude ($28.04\\pm0.14$ mag), and bootstrapped mosaics built from randomly dropped exposures keep the per-epoch fluxes consistent with the full mosaics. This combination is what lets a 0.19 mag fade in a galaxy's total light be attributed to a point-like transient/variable source at $z=5.274$.","core_discovery":"On the paper's own terms, the discovery is AT 2023adya: a point-like variable source coincident with the center of the $z_{\\rm spec}=5.274$ host galaxy JADES-GN+189.12004+62.23867 in GOODS-N. The host plus source was 19% brighter in the F356W band in the JADES Epoch1 image (2023) than in the CONGRESS Epoch2 image (2024), fading from $26.05\\pm0.02$ to $26.24\\pm0.02$ mag over roughly two rest-frame months; the Epoch1-minus-Epoch2 difference image shows a residual of $m_{\\rm F356W}=28.01\\pm0.17$ mag at the galaxy center, while F090W and F115W difference images show no significant emission. The authors interpret the brightness change as intrinsic to a source at $z=5.274$, making it the highest-redshift transient/variable source with a photometric brightness change found so far with JWST. They test the detection with bootstrapped mosaics and with independent science-image photometry, then use the implied absolute magnitude, the narrow H$\\alpha$ line, and event-rate arguments to argue that a bright core-collapse supernova is the most plausible explanation, with SN Ia, variable AGN, and TDE possibilities disfavored to varying degrees but not fully excluded.","pith_inferences":["A third epoch would discriminate cleanly between a fading supernova and a flickering AGN, because a supernova should continue to fade or disappear while an AGN may vary again in either direction.","The red-only variability and the absence of AGN signatures in the host's spectral energy distribution suggest that searches requiring multi-band variability may systematically miss the most common $z>5$ transients.","With one event in roughly 38 square arcminutes, the implied rate has large Poisson uncertainty; simply counting additional $z>5$ variables in comparable JWST fields would test whether the discovery is representative.","A moderate-depth NIRSpec spectrum targeting rest-frame optical lines could settle the AGN question that the NIRCam grism data leave open, because the narrow H$\\alpha$ detection only rules out an AGN-dominated continuum."],"forward_implications":["If the detection holds, JWST can find $z>5$ transients and variables using just two epochs separated by one observer-frame year, so existing two-epoch deep fields can be mined for a high-redshift sample.","The absence of $z>5$ transients in the earlier JADES Transient Survey becomes evidence that such events are rare on a one-year baseline, not that JWST is blind to them.","If AT 2023adya is a core-collapse supernova, it adds to the hint that high-redshift CCSNe are more luminous, with the lack of rest-frame UV change pointing to a dusty or intrinsically red explosion.","The follow-up Cycle-4 program with multi-epoch, multi-filter JWST observations can separate supernovae from AGN variability and may catch exotic objects such as Population III supernova candidates."],"supporting_citations":[{"why":"Provides the JADES survey design and the Epoch1 NIRCam imaging used as the first epoch.","marker":"Eisenstein et al. 2023"},{"why":"Defines the CONGRESS program whose Epoch2 F090W/F115W/F356W images form the second epoch.","marker":"Egami et al. 2023"},{"why":"Supplies the $z_{\\rm spec}=5.274$ host redshift measured from the H$\\alpha$ and [O III] grism lines and the host centroid.","marker":"Sun et al. 2024"},{"why":"Documents the JADES Transient Survey's 79 transients with none beyond $z\\sim4.8$, the record this paper extends.","marker":"DeCoursey et al. 2025"},{"why":"Provides the peak absolute B-band magnitude distributions for SN subtypes used to match $M_{\\rm V}=-18.48$ mag.","marker":"Richardson et al. 2014"},{"why":"Supplies dusty TDE spectral-energy-distribution models and event-rate estimates used to argue a luminous $z=5$ TDE is unlikely.","marker":"Inayoshi et al. 2024"},{"why":"Gives the relation used to estimate expected H$\\alpha$ flux from continuum flux to test detectability of a broad line.","marker":"Greene & Ho 2005"},{"why":"Constrains optical variability rates of Type 2 AGN, used to argue a true or naked Type 2 AGN is a rare explanation.","marker":"Barth et al. 2014"}],"fun_headline_variants":["JWST finds first variable source past z=5","Record-redshift transient: JWST sees a galaxy flicker at z=5.274","JWST spots a supernova candidate at z=5.3","First JWST transient beyond z=5: a fading galaxy","AT 2023adya: JWST's first z>5 variable"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The claim stands or falls on the assumption that the 0.19 magnitude drop in F356W light is happening in the distant galaxy itself, not on small differences in how the two survey images were aligned, smoothed, or background-subtracted.","fun_headline_variants_meta":{"raw":{"variants":["JWST finds first variable source past z=5","Record-redshift transient: JWST sees a galaxy flicker at z=5.274","JWST spots a supernova candidate at z=5.3","First JWST transient beyond z=5: a fading galaxy","AT 2023adya: JWST's first z>5 variable"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001371,"raw_usage":{"total_tokens":5723,"prompt_tokens":1277,"completion_tokens":4446,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":893,"completion_tokens_details":{"reasoning_tokens":4352}},"tokens_in":893,"tokens_out":4446,"duration_ms":27745,"temperature":1.0,"reasoning_tokens":4352,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-16T10:50:50.119791+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Re-reduce both epochs with matched point-spread functions, a single astrometric solution, and identical background treatment, then re-measure the F356W flux at the host position; if the 0.19 magnitude drop disappears or falls below a signal-to-noise of about 6, the central claim fails. A third JWST epoch showing continued fading or complete disappearance would confirm the change was intrinsic rather than a reduction artifact.","supporting_citations":[],"review_version":1}