First light of engineered diffusers at the Nordic Optical Telescope reveal time variability of the optical eclipse depth of WASP-12b^★

¹ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
e-mail: cessen@phys.au.dk
² Astronomical Observatory, Institute of Theoretical Physics and Astronomy, Vilnius University, Sauletekio av. 3, 10257 Vilnius, Lithuania
³ Department of Astronomy & Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
⁴ Center for Exoplanets & Habitable Worlds, University Park, PA 16802, USA
⁵ Nordic Optical Telescope, Apartado 474E-38700 Santa Cruz de La Palma, Santa Cruz de Tenerife, Spain
⁶ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁷ Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz, 1, 37077 Göttingen, Germany

Received: 20 February 2019
Accepted: 11 April 2019

Abstract

We present the characterization of two engineered diffusers mounted on the 2.5-meter Nordic Optical Telescope, located at Roque de Los Muchachos, Spain. To assess the reliability and the efficiency of the diffusers, we carried out several test observations of two photometric standard stars, along with observations of one primary transit observation of TrES-3b in the red (R band), one of CoRoT-1b in the blue (B band), and three secondary eclipses of WASP-12b (V band). The achieved photometric precision is in all cases within the submillimagnitude level for exposures between 25 and 180 s. Along with a detailed analysis of the functionality of the diffusers, we add a new transit depth measurement in the blue (B band) to the already observed transmission spectrum of CoRoT-1b, disfavoring a Rayleigh slope. We also report variability of the eclipse depth of WASP-12b in the V band. For the WASP-12b secondary eclipses, we observe a secondary depth deviation of about 5σ, and a difference of 6σ and 2.5σ when compared to the values reported by other authors in a similar wavelength range determined from Hubble Space Telescope data. We further speculate about the potential physical processes or causes responsible for this observed variability.