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

Alex Bixel, Benjamin V. Rackham, Dániel Apai, Néstor Espinoza, Mercedes López-Morales, David Osip, Andrés Jordán, Chima McGruder, Ian Weaver

Research output: Contribution to journalArticlepeer-review

18 Scopus citations


We present an optical transmission spectrum of the atmosphere of WASP-4b obtained through observations of four transits with Magellan/IMACS, as part of the Arizona-CfA-Católica-Carnegie Exoplanet Spectroscopy Survey (ACCESS). 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. using Gemini/GMOS and May et al. using IMACS, and we find that our data are in agreement.

Original languageEnglish
Article number68
JournalAstronomical Journal
Issue number2
StatePublished - Feb 2019
Externally publishedYes


  • planets and satellites: atmospheres
  • planets and satellites: individual (WASP-4b)
  • stars: activity
  • starspots
  • techniques: spectroscopic


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