Redshifted Sodium Transient Near Exoplanet Transit

Neutral sodium (Na I) is an alkali metal with a favorable absorption cross-section such that tenuous gases are easily illuminated at select transiting exoplanet systems. We examine both the canonical time-averaged alkali spectrum and time-series alkali flux individually, over 4 nights at a hot Saturn system on a $\sim$ 2.8 day orbit about a Sun-like star WASP-49 A. VLT/ESPRESSO observations on UT 2020-12-15 are analyzed, providing new constraints. We recover the previously confirmed sodium density uniquely when averaged, whereas night-to-night Na I varies by more than an order of magnitude. On UT 2016-01-01 (HARPS/3.6-m) we report a Doppler redshift at $v_{ \Gamma, \mathrm{NaD}} =$ +9.7 $\pm$ 1.6 km/s with respect to the planet's rest frame. Upon examining the lightcurves we confirm stark variability on the order of $\sim$ 1-4\% in NaD. Given that sodium flux variations rarely coincide with planet transit, we explore alternatives to a planetary origin since variations are also not readily explained by stellar activity, star spots, telluric lines, nor the interstellar medium. In the UT 2016-01-01 lightcurve coincident with the Doppler redshift at dF$_{\mathrm{NaD}}$/F$_{\star}$ = 3.6 $\pm$ 1 \%, we detect a transient sodium absorption event enduring $\Delta t_{\mathrm{NaD}} \sim$ 40 minutes. Since most exoplanetary metal signatures are blueshifted due to the natural vector of radiation pressure, estimated here at roughly $\sim$ -5.7 km/s, the radial velocity is rather at +15.4 km/s, far larger than any known exoplanet system. Given that the redshift magnitude v$_{\Gamma}$ is in between the Roche limit and allowed satellite orbits, the transient sodium may be a putative indication of a natural satellite.