0.5 − 10 keV pulse profile evolution in time. Black: data and best fitting model. The different harmonics contributing to the best fit model are also shown: blue, green and red curves are the first, second and third harmonic, respectively (see text for details). The background subtracted average count rate is also reported in each panel. The low count rate of the Jan. 23, 2009 observation is due to the presence of the grating in front of the CCD, while the Feb. 03, 2009 observation was performed with XMM-Newton, which has a larger effective area with respect to Chandra.