LRG-BEASTS: Sodium absorption and Rayleigh scattering in the atmosphere of WASP-94A b using NTT/EFOSC2

We present an optical transmission spectrum for WASP-94A b, the first atmospheric characterization of this highly-inflated hot Jupiter. The planet has a reported radius of 1.72(-0.05)(+0.06) RJ(up), a mass of only 0.456(-0.036)(+0) (.032) MJ(up), and an equilibrium temperature of 1508 +/- 75 K. We observed the planet transit spectroscopically with the EFOSC2 instrument on the ESO New Technology Telescope (NTT) at La Silla, Chile: the first use of NTT/EFOSC2 for transmission spectroscopy. We achieved an average transit-depth precision of 128 ppm for bin widths of similar to 200 angstrom. This high precision was achieved in part by linking Gaussian Process hyperparameters across all wavelength bins. The resulting transmission spectrum, spanning a wavelength range of 3800-7140 A, exhibits a sodium absorption with a significance of 4.9 sigma, suggesting a relatively cloud-free atmosphere. The sodium signal may be broadened, with a best-fitting width of 78(-32)(+67) angstrom in contrast to the instrumental resolution of 27.2 +/- 0.2 angstrom. We also detect a steep slope in the blue end of the transmission spectrum, indicating the presence of Rayleigh scattering in the atmosphere of WASP-94A b. Retrieval models show evidence for the observed slope to be super-Rayleigh and potential causes are discussed. Finally, we find narrow absorption cores in the Call H&K lines of WASP-94A, suggesting the star is enshrouded in gas escaping the hot Jupiter.