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In 2015 July 29 - September 1 the satellite XMM-Newton pointed at the BL Lacobject PG 1553+133 six times, collecting data for 218 hours. During one ofthese epochs, simultaneous observations by the Swift satellite were requestedto compare the results of the X-ray and optical-UV instruments. Optical,near-infrared and radio monitoring was carried out by the Whole Earth BlazarTelescope (WEBT) collaboration for the whole observing season. We here presentthe results of the analysis of all these data, together with an investigationof the source photometric and polarimetric behaviour over the last three years.The 2015 EPIC spectra show slight curvature and the corresponding light curvesdisplay fast X-ray variability with a time scale of the order of 1 hour. Incontrast to previous results, during the brightest X-ray states detected in2015 the simple log-parabolic model that best-fits the XMM-Newton data alsoreproduces reasonably well the whole synchrotron bump, suggesting a peak in thenear-UV band. We found evidence of a wide rotation of the polarization angle in2014, when the polarization degree was variable, but the flux remained almostconstant. This is difficult to interpret with deterministic jet emissionmodels, while it can be easily reproduced by assuming some turbulence of themagnetic field.

Synchrotron emission from the blazar PG 1553+113. An analysis of its flux and polarization variability