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

TL;DR

This study investigates long-term X-ray variability in L 98-59, a nearby M-dwarf hosting six Earth-like planets, to search for coronal activity cycles and their potential impact on planetary atmospheres. By combining two new XMM-Newton observations with archival X-ray data and HARPS/ESPRESSO spectra, the authors track X-ray luminosity and chromospheric activity, inferring an activity cycle with a period of about $1.95$ years and amplitude around $10$. The outer habitable-zone planet f experiences XUV irradiation of roughly $100$–$1600$ times Earth's X-ray dose, implying strong and variable high-energy forcing on its atmosphere. The findings have implications for atmospheric evaporation and photochemistry, and they provide guidance for observing strategies with Ariel/JWST during low-activity phases.

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.

Figures (4)

• Figure 1: Comparison of X-ray rate images. The red crosses mark the positions of the objects in SIMBAD. L 98-59 has high proper motions toward the bottom left of the image and its current position is coincident with the centroid of the X-ray emission. The images are on the same pixel and color intensity scales (2"/pix, maximum 2.5 counts per pixel) and are smoothed with a Gaussian kernel with $\sigma=1.5$ pixels.
• Figure 2: X-ray luminosity of L 98-59 in the band 0.3-10 keV vs. time. A cycle of about 1.95 years is shown as a sinusoidal dashed line. We made a rough estimate of the period by assuming that the minimum of the sinusoid is the value recorded in October 2024 and that its mean is equal to the value measured in February 2025.
• Figure 3: Index $\rm log\,R^{\prime}_\mathrm{HK}$ for F to M dwarfs from Boro-Saikia2018 and the range of $\rm log\,R^{\prime}_\mathrm{HK}$ observed in L 98-59.
• Figure 4: Light curves of L 98-59 recorded with the pn camera. The shaded areas mark the intervals selected as quiescent phases in the archival observations. For the observations obtained with our program (ObsID 0940540301 and 0940540401) we used the full exposure. For observations 0871800201 and 0871800301 the Y-axis scale is increased to show the entire flares.