PHAT-11-A1: Discovery of an Extreme Chromatic Microlensing Anomaly in the M31 Outer Disk

We present the discovery of PHAT-11-A1 (internally tracked as PHAT-MICRO-0003), a highly confident, high-significance chromatic microlensing-anomaly candidate located in the outer disk of M31. Identified using multi-epoch HST/PHAT imaging of Brick 23 via fixed-position ePSF/DOLPHOT photometry, the event decisively rejects the flat/no-event baseline hypothesis (ΔBIC = 16,994.33). The defining feature of PHAT-11-A1 is a 195.80-day chromatic temporal split: the ultraviolet F336W band peaks on MJD 55589.010 (SNR = 174.42) while the optical F814W band peaks 195.80 days later on MJD 55784.814 (SNR = 89.2). This significant chromaticity violates standard point-source point-lens (1L1S) geometric microlensing frameworks. Rigorous stress testing confirms the signal survives the removal of data-quality flagged frames, is offset from the nearest resolved stellar source by 66.2 mas, and is independently recovered via raw-frame forced photometry. Standard 1L1S, physically constrained binary-source (1L2S), thermal tidal disruption event (TDE) blackbody cooling, and classical nova dust-echo models fail quantitative tests. The two surviving physical interpretation classes include: (1) a non-thermal or line-dominated tidally disruptive encounter near an intermediate-mass black hole, or (2) compact-object lensing by a primordial black hole or MACHO requiring source-plane chromaticity or blend complexity. We provide the complete manuscript outlining the photometric reduction, model tournament, and systematics scorecard.