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 time-averaged and time-series alkali spectral flux individually, over 4 nights at a hot Saturn system on a similar to 2.8 day orbit about a Sun-like star WASP-49 A. Very Large Telescope/ESPRESSO observations are analyzed, providing new constraints. We recover the previously confirmed residual sodium flux uniquely when averaged, whereas night-to-night Na i varies by more than an order of magnitude. On HARPS/3.6 m Epoch II, we report a Doppler redshift at v Gamma,NaD = + 9.7 +/- 1.6 km s-1 with respect to the planet's rest frame. Upon examining the lightcurves, we confirm night-to-night variability, on the order of similar to 1%-4% in NaD, rarely coinciding with exoplanet transit, not readily explained by stellar activity, starspots, tellurics, or the interstellar medium. Coincident with the similar to+10 km s-1 Doppler redshift, we detect a transient sodium absorption event dF NaD/F star = 3.6% +/- 1% at a relative difference of Delta F NaD(t) similar to 4.4% +/- 1%, lasting Delta t NaD greater than or similar to 40 minutes. Since exoplanetary alkali signatures are blueshifted due to the natural vector of radiation pressure, estimated here at roughly similar to-5.7 km s-1, the radial velocity is rather at +15.4 km s-1, far larger than any known exoplanet system. Given that the redshift magnitude v Gamma is in between the Roche limit and dynamically stable satellite orbits, the transient sodium may be a putative indication of a natural satellite orbiting WASP-49 A b.