2026AJ....171...66S

We report results of Chandra X-ray observations of CI Tau, a young magnetically active classical T Tauri star for which previous studies have reported periodic variability attributed to a massive planet in a short-period orbit. CI Tau was clearly detected by Chandra in four separate observations acquired in late 2023. The X-ray emission was steady in the first two observations with a characteristic plasma temperature kT ≈ 2 keV (≈23 MK) and X-ray luminosity log L_x = 29.74 erg s^–1. During each of the last two observations obtained two weeks later the count rate increased slowly and the X-ray plasma temperature was much higher but remained nearly steady at kT ≈ 4-5 keV (≈46-58 MK) and peak luminosity log L_x = 30.5 erg s^–1. Such variable X-ray emission in T Tauri stars accompanied by high plasma temperatures is a signature of magnetic activity, consistent with the known presence of a strong magnetic field in CI Tau. We summarize the variable X-ray emission properties of CI Tau within the framework of T Tauri stars of similar mid-K spectral type, identify possible variability mechanisms, and assess the effects of stellar X-ray irradiation on the claimed planet.