A hidden infrared source in NGC 2403 that has flared every 5.3 years since 2007: a dust maker or an extreme pulsating star

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.

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 Lsun 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

C1 [observed] 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).
C2 [observed] 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.
C3 [computed] 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 Lnu about 5-7e4 Lsun per band at 3.2 Mpc.
C4 [observed] 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 (MI fainter than about -4.5); no Gaia source within 4.6"; no ZTF alert object within 5".
C5 [observed] 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".
C6 [observed] 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 Lnu(4.5 um) of order 5e5 Lsun), 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 Lsun 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).

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