Quantifying the Impact of Starspot-Crossing Events on Retrieved Parameters from Transit Lightcurves

Abstract: Starspot-crossing events (SCEs) in exoplanet transit lightcurves are becoming increasingly common as we focus on cooler host stars and observe higher precision photometric and spectroscopic lightcurves. In this work we explore how these events affect our retrievals of transit depths, and the accuracy with which we can derive spot properties. We inject and recover synthetic SCEs in photometric lightcurves using starry. We find that for high signal-to-noise SCEs we constrain the spot longitudes tightly (>80% within 1 degree of the true value), but degeneracies complicate retrieving spot contrasts, radii and latitudes (within 17%, 19%, and 9 degrees respectively). On average the difference between injected and recovered transit depths is 0.78% or 78.3ppm. In most (80%) injections we recover the transit depth to within 0.6%. For transit depths inflated >1.3% by the Transit Light Source Effect (TLSE), fitting for a spot-crossing improves the transit depth retrieval over masking the SCE in >95% of cases. However, we find that for spots with small contrasts (<5%) and/or covering fractions (<2%), we are likely to over-correct for the TLSE, recovering a worse transit depth than simply masking. In addition, even when fitted, we find SCEs can inflate the uncertainties on recovered transit depths significantly, especially for JWST-like precisions. Finally, we determine how SCE observables can narrow the degenerate spot parameter space to provide useful priors for MCMC sampling, demonstrating this technique on a real SCE observed in Kepler-51d's lightcurve.