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arXiv · 2512.04817

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

I. Pillitteri·S. Bellotti·S. Benatti·S. Boro Saikia·A. García Muñoz·K. G. Kislyakova·A. Maggio·G. Micela·K. Vida·A. A. Vidotto
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Abstract

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 x 10^26 erg/s. 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 about 5.9 x 10^25 erg/s, and it was about 6.3 x 10^26 erg/s 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 about 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 98-59f, is exposed to an X-ray dose between 100 and 1600 times the X-ray irradiation of the Earth in the XMM band.

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I. Pillitteri, S. Bellotti, S. Benatti, S. Boro Saikia, A. García Muñoz, K. G. Kislyakova, A. Maggio, G. Micela, K. Vida, A. A. Vidotto. 2025-12-04. Long-term X-ray variability of the multiple-planet host L 98-59: Hints of an activity cycle. https://doi.org/10.1051/0004-6361%2F202555853

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