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We present an optical transmission spectrum of the atmosphere of WASP-4bobtained through observations of four transits with Magellan/IMACS. Using aBayesian approach to atmospheric retrieval, we find no evidence for scatteringor absorption features in our transit spectrum. Our models include a componentto model the transit light source effect (spectral contamination fromunocculted spots on the stellar photosphere), which we show can have a markedimpact on the observed transmission spectrum for reasonable spot coveringfractions (< 5%); this is the first such analysis for WASP-4b. We are also ableto fit for the size and temperature contrast of spots observed during thesecond and third transits, finding evidence for both small, cool and large,warm spot-like features on the photosphere. Finally, we compare our results tothose published by Huitson et al. (2017) using Gemini/GMOS and May et al.(2018) using IMACS, and find that our data are in agreement.

ACCESS: Ground-based Optical Transmission Spectroscopy of the Hot Jupiter WASP-4b