Characterizing stellar flares in Ariel targets: activity analysis and transit contamination

Stellar flares are sudden releases of magnetic energy that can distort exoplanet transit photometry and transmission spectroscopy, biasing planet radius estimates, transit timings, and atmospheric characterization. Understanding flare activity in Ariel targets is therefore essential to identify stars where flares may compromise observations and to characterize the radiation environment affecting atmospheric escape and photochemistry. We analysed 290 Ariel target stars using TESS light curves. Flares were identified via iterative Gaussian process detrending, and their energy distributions were modelled with two-segment power laws. We performed injection–recovery tests by adding synthetic flares to detrended light curves and running the full pipeline to quantify completeness and detection biases. We detected 15 857 flares across 1638 TESS sectors, with 2–86 events per sector. We defined a normalized flare index GF.01 to compare activity across stellar luminosities. Near 3% of the sample exhibits enhanced flare activity (GF.01 > 1). AU Mic and HD 28109 show a high likelihood of flare contamination during transit observations. GF.01 correlates negatively with stellar bolometric luminosity, indicating higher relative flare output in lower-luminosity stars. AU Mic is an extreme case: four of five observed transits of AU Mic b are affected by flares, consistent with statistical expectations. We validate the framework by comparing predicted flare-contamination probabilities with observed flare occurrences in a representative subset of transits, finding agreement within uncertainties. These results confirm that energetic flares can significantly impact transit observations and provide quantitative guidance for Ariel target selection and analysis strategies.