LRG-BEASTS: Ground-based Detection of Sodium and a Steep Optical Slope in the Atmosphere of the Highly Inflated Hot-Saturn WASP-21b

We present the optical transmission spectrum of the highly inflated Saturn-mass exoplanet WASP-21b, using three transits obtained with the ACAM instrument on the William Herschel Telescope through the LRG-BEASTS survey (Low Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission Spectroscopy). Our transmission spectrum covers a wavelength range of 4635-9000Å, achieving an average transit depth precision of 197ppm compared to one atmospheric scale height at 246ppm. Whilst we detect sodium absorption in a bin width of 30Å, at >4 sigma confidence, we see no evidence of absorption from potassium. Atmospheric retrieval analysis of the scattering slope indicates that it is too steep for Rayleigh scattering from H2, but is very similar to that of HD189733b. The features observed in our transmission spectrum cannot be caused by stellar activity alone, with photometric monitoring and Ca H&K analysis of WASP-21 showing it to be an inactive star. Our work demonstrates that WASP-21b is an excellent target for future infra-red observations.