Persistent Periodic Signals from 51 Peg in TESS Lightcurves

Abstract Transiting Exoplanet Survey Satellite photometry of the archetypal main-sequence exoplanet host star 51 Peg in two 27 days sectors separated by 2 yr contains a 4.55 days periodic signal, distinct from the 4.23 orbit of the planet. The signal is associated with the pixels containing the most flux, and there is no evidence for a companion or suitably bright background object within that aperture. No similar signal appears in nearby stars of comparable brightness nor does the signal appear in uncontaminated background, bias, or readout frames. The signal is too large and at the wrong period to be reflected light from the planet. 4.55 days would represent a synodic period (e.g., a forced oscillation of the star by the planet) if the star rotates in 60 days. Such slow rotation seems unlikely but is not conclusively ruled out by available observations.