A halo giant brightened at least 2.4 times in March 2020 and faded over six months: a microlensing event

Found in Oddsky hunt H-0007 among faint stars: a one-visit, colour-neutral 2x brightening in NEOWISE (2020 Apr 9-10). ATLAS shows one smooth event, x2.4 on Mar 11 and within 2% of normal by October; ZTF catches the tail. The peak fell in a gap. Not in VSX, TNS, SIMBAD or the Gaia alerts.

A K giant in the Galactic halo, beyond about 6 kpc (l = 18, b = -19), brightened once in 2020: x2.4 in ATLAS on Mar 11, x2.0 in both ATLAS and NEOWISE W1/W2 on Apr 9-10, fading through ZTF's June-July tail, within 2% of normal by October. The extra light has the star's own colour from g to 4.6 microns. Microlensing with annual parallax fits (peak 2020 Mar 17-22 at about x2.7-2.9, tE 40-55 d; the peak itself fell in a gap and is unobserved); a lens without parallax leaves an asymmetry. Two referee passes: P(mundane) = 0.02. Nothing similar in 2014-2026. Not in VSX, TNS, SIMBAD, the Gaia alerts or KMTNet. Gaia DR5 will hold three transits during the event.

Claims

C1 [observed] In the NEOWISE visit of 2020 Apr 9-10 the star was 0.744 mag brighter in W1 (12.309 against 13.053) and 0.731 mag brighter in W2 (12.387 against 13.118): a factor of 1.98, equal in both bands. 2,021 stars within 20' in the same frames are unchanged (referee, hunts/H-0007/reviews/claude-referee-J193353-2026-10-29.md).
C2 [computed] Apart from that visit NEOWISE W1 is flat over 20 visits (2014-2024), robust scatter 0.025 mag.
C3 [observed] ATLAS o contains exactly one excursion in 2015-2026: brighter by 0.82 mag on 2020 Mar 11, the first night of the season, then declining for 57 days above the 0.2-mag threshold. On 2020 Apr 9-10 ATLAS o is 0.70 mag above its 2015-2019 median (22 sigma), and ATLAS c on Apr 22 is brightened by a similar amount, so the change is achromatic.
C4 [observed] ZTF r and g in 2020 Jun 15 - Jul 25 are 0.11 and 0.10 mag brighter than in 2018-2019 (9-10 sigma). After correction with 12 colour-matched stars in the same exposures, the star is 0.17 mag bright on Jun 18 and 0.08-0.10 on Jul 14-21; it is the only one of 84 stars in its CCD quadrant to move.
C5 [observed] No Gaia DR3 source within 9.7"; the brightest within 30" (G = 18.3 at 17.8") supplies under 7% of the target's G flux.
C6 [computed] A point-source point-lens fit to nightly ATLAS c+o (scripts/hunth0007j193353fit.py, outside the Oddsky tools), with no blending or parallax, gives t0 = 2020 Mar 17, tE = 63 d, u0 = 0.43, Amax = 2.5 (chi2/dof 352/190, residual rms 2%). Without fitting them, it predicts the NEOWISE visit at x1.94 (observed x1.98) and the ZTF tail at x1.12 and x1.06 (observed x1.17 and x1.09). ATLAS has no 2020 data before Mar 11 or between Mar 11 and Apr 8, so the peak date and height are poorly constrained.
C7 [inferred] The extra light has the giant's own colour from ZTF g to WISE W2. A flare, accretion outburst or nova would be bluer and would not decline smoothly over six months.
C8 [observed] Not found (2026-10-29) in VSX, the ASAS-SN, ZTF, ATLAS or Gaia DR3 variable catalogues, TNS (60"), SIMBAD (CfA mirror), the Gaia Science Alerts, KMTNet 2020 or a web search. Gaia transited the star on 2020 Feb 28, Mar 12 and Apr 17 without an alert; those epochs will be in DR5.
C9 [inferred] The source is a K giant (BP-RP 1.26; Teff ~4,900 K with map extinction), beyond ~6 kpc from its parallax, and with a proper motion of 12.5 mas/yr moving at vt 350 km/s: probably a halo giant about 2.7 kpc below the Galactic plane.
C10 [computed] The peak fell in ATLAS's 2020 gap (Mar 11 - Apr 8) and is unobserved. A point lens without parallax leaves the same asymmetry in ATLAS, NEOWISE and ZTF: the 2019 rising wing 2-3% below the model and the Jun-Sep 2020 tail 2-5% above. Annual parallax removes it (joint chi2 715 - 460). Parallax point lens, 95% ranges (second referee): t0 = 2020 Mar 17-22, tE = 40-55 d, |u0| = 0.25-0.40, piE = 0.39-0.80, peak x2.66-2.94 observed, source fraction 0.6-1.04 per survey. Exponential and power-law declines fit the decline far worse (chi2 413 and 728 against 79). ATLAS c/o excess ratio 1.05 +/- 0.03: achromatic to ~5% from g to W2. Outside the Oddsky tools: hunts/H-0007/reviews/claude-referee-J193353-atlas-2026-10-29/.

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