PHANTOM Candidate TIC 54603687: A 3.87807-Day Transit Signal in TESS Photometry

Status: planet candidate. A periodic transit-like signal on TIC 54603687 found by PHANTOM (Planet Hunting Across Noisy TESS Observations and Measurements), an independent search of NASA TESS photometry. It has not been confirmed as a planet: its source star and nature are not yet established by follow-up observations (see Unresolved). Version 1.0 of this record. TIC: 54603687 Star: Tmag 15.04, Teff 3209 K, radius 0.287 ± 0.009 R☉, distance 110.3 pc (TIC v8) Period: 3.878074 ± 0.000012 d Mid-transit epoch: BJD_TDB 2459534.6508 ± 0.0014 Depth: 22,928 ± 1,745 ppm Duration: 1.50 ± 0.11 h Radius if on the target: 4.54 +0.21/−0.21 R⊕ (the detailed review's physical fit with the star's density as a prior: its Rp/Rs × the TIC stellar radius, both uncertainties); ± 0.4 R⊕ more for the reduction-dependent depth: about half the difference between the radii the two reductions' depths on the same images imply (20,100 ± 1,900 ppm in TGLC's photometry and 27,600 ± 1,900 in PHANTOM's, sectors 43-45; 4.44 and 5.21 R⊕, depth-only illustrative estimates scaling the adopted radius by the square root of each depth over the adopted fit's area ratio, (Rp/Rs)² = 20,984 ppm, not separate physical fits; a half-difference of 0.38; see *Detailed review*): a working allowance, not a calibrated 68% interval. The ± 0.21 R⊕ above does not include it. It assumes the dip is on the target with no dilution beyond the photometry's own Transits meeting the fit's coverage criterion: 27 in TESS sectors 43, 44, 45, 71, 72 Detection S/N: 15.9 (PHANTOM's search) Open questions (README.md, Unresolved): A possible secondary eclipse, after phase 0.5; The depth depends on the reduction; The transit implies a stellar density modestly below the TIC's; No single-transit pixel views; No radial-velocity measurement; Not statistically validated. The full record, with the evidence, uncertainties, data sources, cross-matches, unresolved questions and publication history, is README.md in this deposit.