2025MNRAS.540L..48U

We carried out the first high-resolution transit observations of the exoplanet WASP-49 Ab with Keck/HIRES. Upon custom wavelength calibration we achieve a Doppler RV precision of $<$ 60 ${\rm m\, s}^{-1}$. This is an improvement in RV stability of roughly 240 ${\rm m\, s}^{-1}$ with respect to the instrument standard. We report an average sodium flux residual of $\Delta \mathcal {F}_{\rm NaD}/ \mathcal {F}_{\star } (\lambda) \sim$ 3.2 $\pm$ 0.4 per cent (8.0$\sigma$) comparable to previous studies. Interestingly, an average Doppler shift of -6.2 $\pm$ 0.5 ${\rm km\, s}^{-1}$ (12.4 $\sigma$) is identified offset from the exoplanet rest frame. The velocity residuals in time trace a blueshift (v$_{\Gamma , \rm ingress} \sim$ -10.3 $\pm$ 1.9 ${\rm km\, s}^{-1}$) to redshift (v$_{\Gamma , \rm egress} \sim$ + 4.1 $\pm$ 1.5 ${\rm km\, s}^{-1}$) suggesting the origin of the observed sodium is unlikely from the atmosphere of the planet. The average Na light curves indicate a depth of $\Delta \mathcal {F}_{\rm NaD} /\mathcal {F}_{\star } (t) \sim$ 0.47 $\pm$ 0.04 per cent (11.7 $\sigma$) enduring $\lesssim$ 90 min with a half-max duration of $\sim$ 40.1 min. Frequent high-resolution spectroscopic observations will be able to characterize the periodicity of the observed Doppler shifts. Considering the origin of the transient sodium gas is of unknown geometry, a co-orbiting natural satellite may be a likely source.