Long-term evolution of the spectral parameters obtained with a single BB fit in the 0.15−1.2 keV range. Panels a), b), and c), show the count rate, temperature, and BB normalization of a simultaneous fit with the column density fixed to a common value. Panel d) indicates the column density obtained when, instead, the BB parameters are fixed to a common value for all the observations (see text). Diamonds, triangles, and squares represent “soft”, “hard”, and off-axis observations, respectively. The plus symbol is the October 2006 observation, which is at an intermediate detector position (see Fig. 3). Error bars correspond to 3σ confidence intervals.