The WEBT campaign on the BL Lac object PG 1553+113 in 2013. An analysis
  of the enigmatic synchrotron emission
  A multifrequency campaign on the BL Lac object PG 1553+113 was organized by
the Whole Earth Blazar Telescope (WEBT) in 2013 April-August, involving 19
optical, two near-IR, and three radio telescopes. The aim was to study the
source behaviour at low energies during and around the high-energy observations
by the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes in
April-July. We also analyse the UV and X-ray data acquired by the Swift and
XMM-Newton satellites in the same period. The WEBT and satellite observations
allow us to detail the synchrotron emission bump in the source spectral energy
distribution (SED). In the optical we found a general bluer-when-brighter
trend. The X-ray spectrum remained stable during 2013, but a comparison with
previous observations suggests that it becomes harder when the X-ray flux
increases. The long XMM-Newton exposure reveals a curved X-ray spectrum. In the
SED, the XMM-Newton data show a hard near-UV spectrum, while Swift data display
a softer shape that is confirmed by previous HST-COS and IUE observations.
Polynomial fits to the optical-X-ray SED show that the synchrotron peak likely
lies in the 4-30 eV energy range, with a general shift towards higher
frequencies for increasing X-ray brightness. However, the UV and X-ray spectra
do not connect smoothly. Possible interpretations include: i) orientation
effects, ii) additional absorption, iii) multiple emission components, and iv)
a peculiar energy distribution of relativistic electrons. We discuss the first
possibility in terms of an inhomogeneous helical jet model.
