Long-term X-ray variability of the multiple-planet host L 98-59: Hints of an activity cycle

High-energy irradiation in X-rays and UV (XUV) can transform the planetary atmospheres through photoevaporation and photochemistry. This is more crucial for M stars, whose habitable zones for Earth-like planets are located within a few percent of an AU. Transiting exoplanets around M dwarfs offer the opportunity to study their characteristics and habitability conditions. L 98-59 is an M3 dwarf hosting six Earth-like planets, with two of them in the habitable zone of the star. X-ray observations made in 2020 and 2021 detected significant flares above a quiescent luminosity of 4 − 10 × 1026 erg s−1. We present the results from two short XMM-Newton observations of L 98-59, which are part of a monitoring survey to detect long-term X-ray variability and activity cycles. In October 2024 the X-ray quiescent luminosity of the star was ∼5.9 × 1025 erg s−1, and it was about 6.3 × 1026 erg s−1 in February 2025. We speculate that in late 2024 the star had a minimum of activity; in 2021 the star was near a maximum of an activity cycle, and in 2025 it was at the middle of the cycle. We suggest a coarse estimate of the period of ≈2 years and a peak-to-peak amplitude of about ≃10, which is the highest among the stars with a known X-ray cycle other than the Sun. We also infer that even the outer planet in the habitable zone, L 98-59, is exposed to an X-ray dose between 100 and 1600 times the X-ray irradiation of the Earth in the XUV band.