A star in Serpens brightened fivefold over four months in 2022: an apparently unreported microlensing event

Found blind in Oddsky hunt H-0008 as a colour-neutral 3.5x brightening in NEOWISE (2022 Aug 31). ATLAS shows one smooth, symmetric event peaking near 2022 Sep 19 at about 5x, lasting months. Not in TNS, the Gaia alerts, the KMTNet or OGLE lists, the ZTF microlensing catalogues, SIMBAD, VSX or ADS full text.

A reddened giant 20 degrees from the Galactic centre and 6 degrees off the plane brightened once in 2022: x3.5 in NEOWISE on Aug 31, then x5 in ATLAS at the peak on Sep 19, back to normal by spring 2023. The rise and fall are symmetric and the same from 0.5 to 4.6 microns, and a point lens fits (tE about 115-150 d) better than any flexible bump. Found blind by the H-0008 all-sky NEOWISE search; not in any alert stream, catalogue or paper we could search.

Claims

C1 [observed] In the NEOWISE visit of 2022 Aug 31 the star was 1.37 mag brighter in W1 and 1.38 mag brighter in W2 than its 20-visit median (W1 11.53, robust scatter 0.02 mag): a factor of 3.5, equal in both bands. No other visit in 2014-2024 departs from the median by more than 0.15 mag. Found blind by the H-0008 all-sky search.
C2 [observed] ATLAS (task 5272650, four units, c and o) shows one event in 2015-2026: the star rises from x1.1 in March 2022 to x4.9 (o) and x5.0 (c) on 2022 Sep 18-20, falls to x2.1 by late November and x1.2 by February 2023. The peak is sampled on 24-26 nights. No other 20-day bin in 2015-2026 is above x1.10.
C3 [observed] ZTF r (field 1428, catflags 0) catches the rise in 2022 at the same flux ratio as ATLAS o.
C4 [computed] A point-lens fit to ATLAS o and c (lensfit, baseline 2023 Nov - 2026) gives t0 = 2022 Sep 19 (MJD 59840.7-59842.1), tE 108-139 d (95%), Amax 4.5-5.5, fs 0.87-1.1, chi2 252 for 291 dof. Across 18 reduction choices with ATLAS, ZTF and NEOWISE together (referee), t0 is Sep 19-20 and tE 113-153 d; quote tE = 115-150 d. Separate rise and decline timescales agree (ratio 0.91-0.95).
C5 [computed] The point lens beats a free symmetric bump with lens-like wings (Pearson VII, same per-series freedom) by delta chi2 = 26-88 depending on the reduction, and a fast-rise exponential-decay pulse by about 4,000.
C6 [computed] The source flux changes by the same factor from ATLAS c to WISE W2: fitted alone, c and o peak within 0.3 d of each other with the same timescale. Blend fractions fall from 0.84-0.98 (c) to 0.68-0.73 (W1/W2), as expected from the red neighbours Gaia resolves (G 18.1 at 6.1", G 20.2 at 2.5") inside WISE's 6" beam.
C7 [computed] Before 2019 ATLAS reads the star 10-40% faint, stepping up when each unit's PSF narrows (02a between 2017 and 2018, 01a between 2019 and 2020): flux against PSF width is -0.13 mag/px in o. NEOWISE W1 is flat to 0.001 +- 0.009 mag over the same years. The drift is instrumental and does not change the fit.
C8 [observed] Gaia DR3 4158424169627436800: a reddened K giant or subgiant (GSP-Phot Teff 4770 K, AG 3.6), parallax -0.32 +- 0.13 mas (beyond ~5 kpc), no infrared excess (W1-W2 -0.13, W2-W3 0.10), l = 19.7, b = +5.7.
C9 [inferred] Annual parallax improves the fit by delta chi2 4-36 depending on the reduction, along one direction (piEN about -0.2). That is a hint, not a measurement. Without the angular Einstein radius no lens mass is measured: allowed masses run from about 0.3 to 10 solar masses.
C10 [observed] Not found (2026-10-30) in TNS (valid empty result), the Gaia Science Alerts (60"), the KMTNet and OGLE EWS lists (outside both footprints), the ZTF microlensing catalogues (Zhai+2024, Rodriguez+2022, Medford+2023), ALeRCE/Lasair, SIMBAD, VSX, ADS full text or a web search. ZTF22abjkknd, the ZTF object at this position, has only 2025-26 negative-difference alerts. Not checked: MOA 2022 (page empty), ASAS-SN (server errors).

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