Correcting for the impact of starspot-crossing events on the exoplanet transit depth with multiwavelength transit observations of CoRoT-2 b

The photospheric manifestations of stellar magnetic activity, such as starspots, introduce contaminations in exoplanet transit observations. For instance, occulted starspots cause 'bumps' in the transit lightcurve that can change the shape of the ingress and egress phases, potentially mimicking transit timing variations (TTVs), or cause apparent transit depth variations, when closer to mid-transit. Conversely, unnoculted starspots can cause the so called "Transit Light Source Effect" (TLSE), impacting the estimated transit depth and, consequently, introducing contaminations in the transmission spectra. Differently from the usual single band observations, where the differentiation between stellar contamination and the transit signal is challenging, simultaneous multiwavelength transit observations offers the advantage of being able to take advantage from the chromaticity dependency of the stellar contamination to help disentangle it from the transit of the exoplanet. Our target, CoRoT-2 b, is a hot Jupiter exoplanet orbiting a young and moderately active G-type star CoRoT-2, known for displaying starspot-crossing events in its lightcurves during transit observations. In this work we evaluate possible mitigation scenarios for the impacts of occulted starspots on the transit depth, from simultaneous multiwavelength transit observations. We employ the transit mapping method to probe for starspot-crossing events in simultaneous multiwavelength observations of transits of the exoplanet CoRoT-2 b obtained from archival observations from the CoRoT space mission, using its three colors (blue, green, and red) from the exoplanet channel. For the transits identified as clearly containing starspot-crossing events, we compare the transit depths obtained from ignoring, masking, or fitting the spot contaminations on the lightcurves.