Synchrotron emission from the blazar PG 1553+113. An analysis of its
  flux and polarization variability
  In 2015 July 29 - September 1 the satellite XMM-Newton pointed at the BL Lac
object PG 1553+133 six times, collecting data for 218 hours. During one of
these epochs, simultaneous observations by the Swift satellite were requested
to compare the results of the X-ray and optical-UV instruments. Optical,
near-infrared and radio monitoring was carried out by the Whole Earth Blazar
Telescope (WEBT) collaboration for the whole observing season. We here present
the results of the analysis of all these data, together with an investigation
of the source photometric and polarimetric behaviour over the last three years.
The 2015 EPIC spectra show slight curvature and the corresponding light curves
display fast X-ray variability with a time scale of the order of 1 hour. In
contrast to previous results, during the brightest X-ray states detected in
2015 the simple log-parabolic model that best-fits the XMM-Newton data also
reproduces reasonably well the whole synchrotron bump, suggesting a peak in the
near-UV band. We found evidence of a wide rotation of the polarization angle in
2014, when the polarization degree was variable, but the flux remained almost
constant. This is difficult to interpret with deterministic jet emission
models, while it can be easily reproduced by assuming some turbulence of the
magnetic field.
