Coronal X-ray Variability on Exoplanet Host Stars and How It Limits the Predictability of Their X-ray Emission

X-ray and UV emission from the host star play important roles in how exoplanet atmospheres are structured and evolve. X-ray and EUV radiation (shortward of 912 Å) play a major role in thermospheric heating and erosion of planetary atmospheres, while FUV/NUV radiation controls the atmospheric chemistry. Accurate calculation of the stellar high-energy spectral energy distribution (SED) and how it varies are important inputs for astronomers trying to model exoplanet atmospheres. We examine the range of X-ray variability shown by host stars during our Chandra and XMM-Newton observations of exoplanetary systems. As an example, Chandra ACIS-S observations of the M3.5 dwarf LTT 1445A are used to illustrate how single snapshot observations might lead to misleading assessments of the X-ray emission. LTT1445A hosts two rocky exoplanets in 3.1 and 5.4 d orbits and has a high JWST transmission spectroscopy metric as the closest transiting M dwarf system. We describe the spread in X-ray luminosity from low-mass stars as a function of Rossby number (the ratio of rotational period to the convective turnover time) and the role that variability plays in generating this spread. This work has been supported by Chandra grants GO1-22005X, GO2-23002X, and GO8-19017X and HST grants to Treasury programs 15071 and 16701.