Extending TESS Flare Frequency Distributions with CHEOPS: power-law or lognormal?

Abstract: Stellar flares are intense bursts of radiation caused by magnetic reconnection events on active stars. They are especially frequent on M dwarfs, where they can strongly influence planetary habitability. Flare frequency distributions (FFDs) are usually modeled as power laws, but recent studies have proposed alternatives such as lognormal distributions, implying different flare mechanisms and planetary impacts. This work investigates which statistical distribution best describes flare occurrences on M dwarfs, considering both equivalent duration (ED), the quantity directly measured from photometry, and bolometric energy, which is more relevant for habitability assesments. We analyzed 110 M dwarfs observed with TESS and CHEOPS, detecting 5,620 flares. Complex events were decomposed, detection biases corrected, and FFDs from both missions scaled to build a combined distribution spanning nearly 10 orders of magnitude in bolometric energy. ED-based FFDs follow a power law, reflecting intrinsic photometric flare occurrence. However, bolometric energy-based FFDs deviate from a pure power law, being better described by a lognormal distribution or, more accurately, by a truncated power law with a break near $10^{33}$ erg, the typical superflare threshold. This truncation suggests a change in flare-generation physics between regular flares and superflares, with implications for the cumulative impact on exoplanetary atmospheres. The apparent low-energy flattening previously attributed to lognormal behavior arises from observational biases, while the drop in flare frequency above $10^{35}$ erg remains unexplained, possibly reflecting an intrinsic cutoff or current observational limits. The upcoming PLATO mission will be well suited to probe both regimes.