Hardness ratio and coronal temperature. Top panel: time series of the hardness ratio (HR) calculated in the bands 0.3−1.0 keV (soft) and 1.0−2.0 keV (hard) as a function of the exposure time (bins of 600 s). Vertical lines indicate the durations of each XMM-Newton observation. The average uncertainty of HR is about 0.09 (quantile range 25%–75%: 0.08–0.11). Middle panel: relationship between HR and temperature (kT) derived with a library of synthetic spectra and XSPEC software for the case of Thin1 filter (solid line) and Medium filter (dashed line). Bottom panel: Time series of kT expressed in keV (average uncertainty is 0.09 keV, range 25%–75%: 0.07–0.12 keV).