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

## TL;DR

This study presents an optical transmission spectrum of WASP-4b, revealing no significant atmospheric features and highlighting the importance of stellar spot effects in interpreting exoplanet transmission spectra.

## Contribution

First analysis of stellar spot effects on WASP-4b's transmission spectrum using Bayesian atmospheric retrieval with multiple transits.

## Key findings

- No evidence for scattering or absorption features in the spectrum.
- Stellar spots can significantly impact the observed transmission spectrum.
- Detected small, cool and large, warm spot-like features on the star's photosphere.

## Abstract

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