ACCESS: Ground-based Optical Transmission Spectroscopy of the Hot Jupiter WASP-4b. (arXiv:1812.07177v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Bixel_A/0/1/0/all/0/1">Alex Bixel</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rackham_B/0/1/0/all/0/1">Benjamin V. Rackham</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Apai_D/0/1/0/all/0/1">D&#xe1;niel Apai</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Espinoza_N/0/1/0/all/0/1">N&#xe9;stor Espinoza</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lopez_Morales_M/0/1/0/all/0/1">Mercedes L&#xf3;pez-Morales</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Osip_D/0/1/0/all/0/1">David J. Osip</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jordan_A/0/1/0/all/0/1">Andr&#xe9;s Jord&#xe1;n</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+McGruder_C/0/1/0/all/0/1">Chima McGruder</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Weaver_I/0/1/0/all/0/1">Ian Weaver</a>

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.

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.

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