An infrared flare in the galaxy NGC 3239 in 2020 that no optical survey saw: an obscured transient

Found blind by Oddsky's mid-infrared difference imaging of 318 nearby galaxies (hunt H-0012): a flare peaking in 2020 November at about 5e5 Lsun, with no optical alert. Hubble caught a faint point source 85 days after the peak that has since vanished, and JWST found cooling dust at the same spot in 2024.

A mid-infrared flare in the irregular galaxy NGC 3239 (about 8-12 Mpc), peaking in 2020 November at roughly 5e5 Lsun from ~960 K dust, that no optical survey reported. Hubble caught a faint, fading point source 85 days after the peak, and JWST found cooling dust at the same spot in 2024. Probably a dust-obscured stellar merger or eruption; a buried low-energy supernova is possible. Found blind by Oddsky hunt H-0012.

Claims

C1 [observed] WISE (unWISE neo11 epochal coadds, difference imaging): flat from 2010 to 2020 May 2; on 2020 Nov 23 W1 +232 +- 8 and W2 +308 +- 20 uJy above the 2010-11 level; 91/119 uJy in 2021 Apr, 38/42 in 2021 Nov, ~27 in 2022 Apr. Position 156.29118 +17.16387 (Gaia-tied, ~0.25"). Positive in both visit seasons, so not an asteroid (hunts/H-0012/reviews/ngc3239-check-2026-11-01.md).
C2 [computed] The transient's W1-W2 is 0.95 +- 0.08 (~960 K dust); a blackbody gives ~5.6e5 Lsun at 7.9 Mpc (1.6-2.4x more at the 10-12 Mpc of SN 2012A's host distance and CosmicFlows-4). The rise is unresolved (< 205 d), so the true peak may be higher.
C3 [observed] HST (2021 Feb 16): a point source at 156.291224 +17.163778, 0.37" from the WISE centroid, F606W 25.93 +- 0.06 and F814W 24.95 +- 0.06 AB, present in all four 2021 exposures; absent in 2022 (F606W 27.1) and in every 2025 filter, with no residual cluster (MV -2.9). The chance of an unrelated vanishing source within 0.37" is ~0.2%.
C4 [observed] JWST (programme 3707, 2024 Apr-May): a new unresolved source 0.01" from the HST position, F335M 5.9 uJy and F300M 3.2-3.6 uJy, nothing in F150W ( 26.5 AB): ~580 K dust at ~3.6e4 Lsun. Spitzer 2008 shows nothing there ( 8 sigma below the 2024 source).
C5 [observed] No optical detection: no TNS object or ZTF alert; ATLAS limits o ~20.4-21.7 around the event; the galaxy was in solar conjunction in 2020 August. VLASS < 0.36 mJy (2021 Nov); Chandra 2025 < 2.6e37 erg/s.
C6 [inferred] Not a buried normal supernova as it stands: the HST colour allows AV <~ 2, while the faintness relative to WISE needs AV 9-20, and an enshrouded Type II SN should be 30-100x brighter in the infrared at this phase. An obscured luminous red nova (stellar merger), an ILRT-like eruption (no progenitor seen), or a low-energy SN behind clumpy dust fit better. Not an AGN: 2.7 kpc from the centre of a galaxy with no nucleus.
C7 [observed] Not reported (2026-11-01): TNS, Gaia alerts, CBAT, ATel (web search), SPIRITS, MIRONG, the literature; no publication on the JWST 3707 data at this position. ADS full-text check of the De et al. WISE transient papers still owed.

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