A bulge giant in Sagittarius brightened by half for four months in 2022: a blended microlensing candidate

Found blind in Oddsky hunt H-0008 as a colour-neutral 1.5x brightening in NEOWISE (2022 Sep 8). ATLAS and ZTF show one smooth event peaking in late August 2022, equal from g to 4.6 microns. ALeRCE classifies the ZTF object as LPV/YSO.

An M giant toward the Galactic bulge brightened once, by about 60%, peaking in 2022 mid-August, equally from g to 4.6 microns, and was back to normal by December. NEOWISE, ATLAS and ZTF all see it. It looks like microlensing, but a simple point lens fits only with a blend Gaia does not see, so it stays a candidate. Found blind by the H-0008 all-sky NEOWISE search.

Claims

C1 [observed] In the NEOWISE visit of 2022 Sep 8 the star was 0.47 mag brighter in W1 and 0.46 in W2 than its median (W1 9.70): x1.54, equal in both bands. W1 is otherwise flat to 0.7% rms over 2014-2024. Found blind by the H-0008 all-sky search.
C2 [observed] ATLAS (task 5272651) shows one brightening in 2015-2026, peaking in 2022 Aug: 2022 Aug 10-25 is 0.53 mag brighter in o and 0.49 mag in c than in 2023 Nov - 2026 (79 and 70 sigma). It rises from April and is back near baseline by December 2022. No other 20-day bin is above x1.05 in o.
C3 [observed] ZTF g and r (field 283) show the same brightening at the same flux ratio, 1.55-1.6 at the peak; the ZTF difference-image centroid of ZTF22aalviwh (38 alerts) is 0.09" from the Gaia star.
C4 [computed] The flux ratio is the same from ZTF g to WISE W2 (1.53-1.6 near the peak), and the bands peak together (2022 Aug 15-20 within a few days).
C5 [computed] A point lens fits only if 80-90% of the light in every band from g to W2 comes from an unresolved blend; Gaia sees no companion (RUWE 0.97, no multiple peaks) and resolves neighbours that give 3% of the ATLAS flux. With the blend fixed to Gaia's value the lens fit is worse by delta chi2 27-78, and a free symmetric bump fits better than any lens by 16-48. Robust: peak 2022 Aug 15-20, peak flux ratio 1.55-1.6, effective timescale |u0| tE about 35 d. tE itself is unconstrained.
C6 [observed] Gaia DR3 4090003970570642816 (G 14.3): an M giant (J-K 1.28, W1-W2 -0.17 from CO absorption), no infrared excess, l = 10.9, b = -5.6, toward the bulge.
C7 [inferred] ALeRCE labels ZTF22aalviwh LPV/YSO, but its classifier has no microlensing class. As a pulsation it fails: the only period in the ATLAS baseline is 1.1% at 323 d, W1 varies by 0.7% rms in ten years, and a pulsation would be 3-10 times larger in g than in W1, not equal.
C8 [observed] Not found (2026-10-30) in TNS, the Gaia Science Alerts (60"), KMTNet or OGLE EWS (outside both footprints), the ZTF microlensing catalogues, SIMBAD, VSX, ADS full text or a web search. Not checked: MOA 2022, ASAS-SN.

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