{"case":{"id":"OS-0031","title":"A hidden infrared source in NGC 2403 that has flared every 5.3 years since 2007: a dust maker or an extreme pulsating star","hook":"Found blind by Oddsky's mid-infrared difference imaging of nearby galaxies (hunt H-0012 v2): a source invisible to Hubble whose infrared light rises sharply every 5.3 years and then settles. Spitzer saw outbursts in 2007, 2012 and 2018, WISE in 2017-18 and 2023. Either a massive binary forming dust at each close approach or an extremely luminous dusty pulsating star; an infrared spectrum would tell. The next outburst is due around mid-2028.","status":"reproduced","target":{"name":"NGC 2403 periodic IR source","ra":114.22807,"dec":65.59806,"aliases":[]},"summary":"An optically invisible source in NGC 2403 (3.2 Mpc) whose mid-infrared light rises sharply every 5.3 years and then settles: Spitzer caught outbursts in 2007, 2012 and 2018, and WISE in 2017-18 and 2023. Each outburst is about 1e5 L_sun of ~700 K dust. The fast-rise, flat-minimum shape favours a massive binary forming dust at each close approach, but an extremely luminous dusty long-period variable of the kind SPIRITS found is also possible; an infrared spectrum would decide. Next outburst expected around 2028.5. Found blind by Oddsky hunt H-0012 and recovered blind by H-0013.","claims":[{"id":"C1","text":"WISE (unWISE neo11 epochal coadds, difference imaging): two humps in both bands and both visit seasons, peaking 2017.8 (W1 +170, W2 +355 uJy) and 2023.2 (W2 +300 uJy), with a flat minimum between; compact PSF at 8-9 sigma in the per-epoch difference stamps. 21\" from the nucleus of NGC 2403 (hunts/H-0012/reviews/claude-referee-h0012b-2026-11-02.md, section 1).","tag":"observed","calls":[],"author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00"},{"id":"C2","text":"Spitzer/IRAC (about 40 epochs, 2004-2019; aperture photometry on resampled mosaics): [4.5] peaks of +235, +232 and +243 uJy at 2007.25, 2012.45 and 2018.03, with flat minima between; fast rise (2006.2 to 2006.8), slower 1-1.5 yr decline; [3.6] tracks at about 60% of the amplitude. The Spitzer variable is 0.23-0.25\" from the WISE position.","tag":"observed","calls":[],"author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00"},{"id":"C3","text":"Four peaks (2007.25, 2012.45, 2017.5-2018.0, 2023.2) give a period of 5.3 +- 0.1 yr (about 1,940 d); the ephemeris predicts the next peak at about 2028.5. Hump light W1-W2 ~ 1.4 (about 700 K dust), peak nu L_nu about 5-7e4 L_sun per band at 3.2 Mpc.","tag":"computed","calls":[],"author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00"},{"id":"C4","text":"No optical counterpart: HST (ACS 2004/2005/2019, WFC3 2018, WFC3/IR 2010) shows a dust lane, with the nearest faint sources 0.68\" and 0.96\" away (M_I fainter than about -4.5); no Gaia source within 4.6\"; no ZTF alert object within 5\".","tag":"observed","calls":[],"author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00"},{"id":"C5","text":"Not reported (2026-11-02): ADS full text for the coordinates; not SPIRITS 19q (2.7' away) or SPIRITS 16df (1.5' away), the known dust-forming WC binaries in NGC 2403; not [KSK2013] N 2403 5 (5.6\" away); nothing within 3\" in the SPIRITS, Khan et al. 2013/2015, Gaia DR3 or AllWISE catalogues; nothing in TNS within 50\" or in the SPIRITS long-period-variable catalogue within 10\".","tag":"observed","calls":[],"author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00"},{"id":"C6","text":"Correction (2026-11-02, hunt H-0013): luminous dusty long-period variables reach this regime. Karambelkar et al. 2019 (2019ApJ...877..110K, SPIRITS) catalogue 58 variables brighter than M_[4.5] = -12, most with quasi-periods above 1,000 d and four above 2,000 d (brightest M_[4.5] = -15.5, nu L_nu(4.5 um) of order 5e5 L_sun), interpreted as massive or super-AGB stars, red supergiants with extreme mass loss, or interacting binaries. So the earlier statements that periods above 1,900 d are extremely rare and that 7e4 L_sun in one band exceeds an AGB star's output do not hold. What still favours outbursts over pulsation is the light-curve shape: H-0013's template test gives Z = +25 for a flat-minimum outburst train over a sinusoid (a known SPIRITS long-period variable in the same galaxy, SPIRITS 14dd, gives Z = -27), and the joint WISE + Spitzer fit gives P = 5.16 yr over four cycles (hunts/H-0013/README.md).","tag":"observed","calls":[],"author":"claude-opus-5.5","at":"2026-11-02T21:28:13+00:00"}],"hypotheses":[{"id":"H1","klass":"dust_formation","statement":"A colliding-wind massive binary (WC + O, or an LBV binary) forming dust at each periastron every 5.3 yr, about 10x more luminous in the infrared than WR 140.","supports":["C1","C2","C3","C4"],"conflicts":[],"would_falsify":"No mid-infrared brightening between 2027.8 and 2029.3, or an infrared spectrum with silicate or SiC features (AGB dust) instead of a featureless carbon-dust continuum.","status":"live","confidence":0.5,"author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00"},{"id":"H2","klass":"pulsation","statement":"An extreme AGB / OH-IR star with a 1,940-day pulsation.","supports":["C1"],"conflicts":["C2","C3"],"would_falsify":"Already strained: periods above 1,900 d are extremely rare, Miras do not show a fast rise and flat minimum, and 7e4 L_sun in one band exceeds an AGB star's total output.","status":"live","confidence":0.4,"author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00"},{"id":"H3","klass":"artifact_instrumental","statement":"A WISE or Spitzer artefact.","supports":[],"conflicts":["C1","C2"],"would_falsify":"Two observatories see the same periodic source at one position, and the 2017-18 peak is seen by both.","status":"live","confidence":0.02,"author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00"}],"predictions":[],"literature":[],"hunt":"H-0012","featured":false,"created_at":"2026-11-02T19:15:15+00:00"},"series":{},"context":{},"provenance":{},"skeptic":null,"ledger":[{"id":"K1","tool":"add_claim","args":{"id":"C1","tag":"observed"},"digest":"47420e4be8ce39a8","digest10":"47420e4be8ce39a8","author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00","oddsky":"0.3.1","platform":"darwin-arm64"},{"id":"K2","tool":"add_claim","args":{"id":"C2","tag":"observed"},"digest":"810fe66681869023","digest10":"810fe66681869023","author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00","oddsky":"0.3.1","platform":"darwin-arm64"},{"id":"K3","tool":"add_claim","args":{"id":"C3","tag":"computed"},"digest":"a8e3dde17ac83cf1","digest10":"a8e3dde17ac83cf1","author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00","oddsky":"0.3.1","platform":"darwin-arm64"},{"id":"K4","tool":"add_claim","args":{"id":"C4","tag":"observed"},"digest":"a75fcb10446e9d88","digest10":"a75fcb10446e9d88","author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00","oddsky":"0.3.1","platform":"darwin-arm64"},{"id":"K5","tool":"add_claim","args":{"id":"C5","tag":"observed"},"digest":"0bcef7dd6aaf715f","digest10":"0bcef7dd6aaf715f","author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00","oddsky":"0.3.1","platform":"darwin-arm64"},{"id":"K6","tool":"add_hypothesis","args":{"id":"H1","klass":"dust_formation"},"digest":"a3afdee2a580e9e9","digest10":"a3afdee2a580e9e9","author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00","oddsky":"0.3.1","platform":"darwin-arm64"},{"id":"K7","tool":"add_hypothesis","args":{"id":"H2","klass":"pulsation"},"digest":"3c8ab29249da19db","digest10":"3c8ab29249da19db","author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00","oddsky":"0.3.1","platform":"darwin-arm64"},{"id":"K8","tool":"add_hypothesis","args":{"id":"H3","klass":"artifact_instrumental"},"digest":"98cd99168b0a91ec","digest10":"98cd99168b0a91ec","author":"claude-opus-5.5","at":"2026-11-02T19:15:15+00:00","oddsky":"0.3.1","platform":"darwin-arm64"},{"id":"K9","tool":"add_claim","args":{"id":"C6","tag":"observed"},"digest":"82fd9a970193830f","digest10":"82fd9a970193830f","author":"claude-opus-5.5","at":"2026-11-02T21:28:13+00:00","oddsky":"0.3.1","platform":"darwin-arm64"},{"id":"K10","tool":"update_hypothesis","args":{"hypothesis":"H1","status":"open","confidence":0.5,"reason":"Karambelkar et al. 2019 show luminous dusty LPVs with periods of 1,000-2,500 d at this luminosity; only the outburst-like light-curve shape (H-0013 Z = +25) now favours a dust maker."},"digest":"6bc0da1f42f96fc3","digest10":"6bc0da1f42f96fc3","author":"claude-opus-5.5","at":"2026-11-02T21:28:13+00:00","oddsky":"0.3.1","platform":"darwin-arm64"},{"id":"K11","tool":"update_hypothesis","args":{"hypothesis":"H1","status":"live","confidence":0.5,"reason":"Status correction of K10 (an invalid status 'open' was written; update_hypothesis now validates it). Confidence 0.5: Karambelkar et al. 2019 show luminous dusty LPVs with periods of 1,000-2,500 d at this luminosity; only the outburst-like light-curve shape (H-0013 Z = +25) now favours a dust maker."},"digest":"6bc0da1f42f96fc3","digest10":"6bc0da1f42f96fc3","author":"claude-opus-5.5","at":"2026-11-02T21:28:36+00:00","oddsky":"0.3.1","platform":"darwin-arm64"},{"id":"K12","tool":"update_hypothesis","args":{"hypothesis":"H2","status":"live","confidence":0.4,"reason":"Correction C6: extremely luminous long-period variables with quasi-periods above 2,000 d exist (Karambelkar et al. 2019); disfavoured only by the fast-rise, flat-minimum shape."},"digest":"6bc0da1f42f96fc3","digest10":"6bc0da1f42f96fc3","author":"claude-opus-5.5","at":"2026-11-02T21:28:36+00:00","oddsky":"0.3.1","platform":"darwin-arm64"}],"investigations":[],"reviews":[]}