A red giant toward the Galactic anticentre brightened sevenfold for a few weeks in 2017: a microlensing event

Found in Oddsky hunt H-0007 as a one-visit, colour-neutral 2.7x brightening in NEOWISE (2017 Aug 27-28); ATLAS shows a single smooth event peaking near 2017 Sep 16. Not in VSX, TNS, the Gaia alerts or Gaia DR3.

A red-clump K giant at about 3 kpc toward the Galactic anticentre (l = 163, b = -15) brightened in a single smooth event in 2017: x2.7 in NEOWISE W1 and W2 on Aug 27-28 (equal in both bands), and in ATLAS from early August to early November, peaking near Sep 13-16 at about x7. Nothing similar in 2014-2026. A symmetric point-lens curve is rejected; annual parallax fits (tE 47-56 d, |piE| 0.6-1.6, a candidate measurement). Two referee passes: P(not microlensing) = 0.05. The lens mass is not measured. Not in VSX, TNS, SIMBAD, the Gaia alerts or Gaia DR3. Gaia DR4 (Dec 2026) should show it.

Claims

C1 [observed] In the NEOWISE visit of 2017 Aug 27-28 (MJD 57992.6-57993.5) the star was 1.087 mag brighter in W1 (10.036 against a baseline median of 11.123) and 1.072 mag brighter in W2 (10.131 against 11.202): factors of 2.72 and 2.68, equal within 0.02 mag. In the 13 single exposures the factors are 2.731 and 2.716 (hunts/H-0007).
C2 [computed] Apart from that visit, NEOWISE W1 is flat over 21 visits (2014-2024) with a robust scatter of 0.007 mag.
C3 [observed] ATLAS o (2015 Oct - 2026 Oct, 2,911 frames) contains exactly one excursion beyond max(5 sigma, 0.2 mag): a brightening from MJD 57970.6 to 58063.5 (2017 Aug 5 - Nov 6) peaking on MJD 58009.6 (2017 Sep 13) at 2.13 mag (x7.1) above baseline. On the night of the NEOWISE visit ATLAS o was 1.13 mag (x2.8) above baseline.
C4 [computed] ZTF r (2018 Jul - 2025 Aug, 869 points) is flat with a robust scatter of 0.011 mag.
C5 [observed] No Gaia DR3 source within 6" contributes more than 0.6% of the target's G flux; the nearest comparable star (G = 15.9) is 15.3" away, outside the NEOWISE beam.
C6 [computed] A symmetric point-lens curve is rejected (chi2 462/69 against 98/67 with annual parallax) by a joint ATLAS + NEOWISE fit (scripts/hunth0007lensfit.py, outside the Oddsky tools). The asymmetry appears in both ATLAS units and survives every systematics variant a referee tried. Parallax model: t0 = MJD 58012.2, tE = 47.7 or 51.8 d (u0 sign), Amax about 7; piE,par = 0.62 is robust, |piE| = 0.6-1.6 (hunts/H-0007/reviews/claude-referee-2-2026-10-29.md).
C7 [inferred] A hot additive source (flare, accretion disk) supplying 63% of the light during the NEOWISE visit would make W1-W2 redder by about 0.06 mag; the observed change is -0.01 to -0.02 mag. A flare of that size on a 33 Lsun giant is not physical.
C8 [observed] Not found (2026-10-29) in VSX (6"), TNS (60"), SIMBAD (CfA mirror, 30"), the Gaia Science Alerts, Gaia DR3 varimicrolensing (which ends 2017 May) or a web search; no ATLAS17 designation. ADS not checked. Gaia observed the star on 2017 Jul 27, Aug 22 and Sep 18 (one field of view) without alerting.
C9 [inferred] The lens mass is not measured: thetaE is unknown. Along the mass-distance relation the lens is 0.1-0.65 Msun at 0.5 kpc or 0.04-0.26 Msun at 1 kpc, most likely an M dwarf within about 1 kpc.

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