A hidden source in NGC 4945 that has flared in the infrared every 5.4 years: three outbursts with a sudden rise

Found blind by Oddsky's search for repeating infrared sources in nearby galaxies (hunt H-0013): WISE sees cycles peaking in 2011, 2016-17 and 2022, and Spitzer caught the 2016 rise, a jump of 25x in under half a year. Nothing is catalogued at the position. Next peak expected around 2027.5.

An infrared source in the dusty edge-on starburst galaxy NGC 4945 (3.8 Mpc) that brightens sharply every 5.4 years: WISE saw cycles in 2011, 2016-17 and 2022, and Spitzer caught the 2016 rise. Each swing is about 3e5 Lsun, with no optical counterpart. Most likely a massive binary forming dust at each close approach; an extreme dusty pulsating star is the alternative. Next peak expected around 2027.5. Found blind by Oddsky hunt H-0013.

Claims

C1 [observed] WISE (unWISE neo11 difference photometry, H-0012/H-0013): three cycles peaking 2011.27, 2016.71 and 2022.14, a period of 5.43 yr (WISE) and 5.37 yr (joint WISE + Spitzer fit), with a sawtooth shape: fast rise (about 8% of the period), slower decline, short minimum (hunts/H-0013/README.md).
C2 [observed] Spitzer/IRAC warm mission 2012-2019 tracks two cycles: from +22 to +505 (3.6 um) / +748 uJy (4.5 um) between 2016.34 and 2016.78, a rise in under 0.45 yr; a single 2007.6 [4.5] epoch fits the decline of a 2005.9 cycle.
C3 [computed] Peak-to-trough W2 amplitude about 3e5 Lsun at 3.8 Mpc. The light-curve shape test favours a flat-minimum outburst train over a sinusoid (Zjoint = +83). Luminous dusty long-period variables reach P = 1,000-2,500 d and nu Lnu(4.5 um) of order 1e5-5e5 Lsun (Karambelkar et al. 2019, SPIRITS), so pulsation is not excluded by luminosity alone.
C4 [observed] No optical or near-infrared counterpart: nothing in Gaia DR3 or HST at the position, in the dust lane of an edge-on starburst; a Galactic foreground star (b = +13 deg) is implausible with no Gaia source and P = 5.4 yr.
C5 [observed] Not reported (2026-11-02): ADS full text for the coordinates and for the host with variability terms finds nothing on this source; nothing in TNS, SIMBAD, Gaia DR3, AllWISE or the Spitzer SEIP catalogue at the position; the SPIRITS catalogue of luminous long-period variables (Karambelkar et al. 2019) does not cover this galaxy.

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