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A multifrequency campaign on the BL Lac object PG 1553+113 was organized bythe Whole Earth Blazar Telescope (WEBT) in 2013 April-August, involving 19optical, two near-IR, and three radio telescopes. The aim was to study thesource behaviour at low energies during and around the high-energy observationsby the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes inApril-July. We also analyse the UV and X-ray data acquired by the Swift andXMM-Newton satellites in the same period. The WEBT and satellite observationsallow us to detail the synchrotron emission bump in the source spectral energydistribution (SED). In the optical we found a general bluer-when-brightertrend. The X-ray spectrum remained stable during 2013, but a comparison withprevious observations suggests that it becomes harder when the X-ray fluxincreases. The long XMM-Newton exposure reveals a curved X-ray spectrum. In theSED, the XMM-Newton data show a hard near-UV spectrum, while Swift data displaya 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 likelylies in the 4-30 eV energy range, with a general shift towards higherfrequencies for increasing X-ray brightness. However, the UV and X-ray spectrado not connect smoothly. Possible interpretations include: i) orientationeffects, ii) additional absorption, iii) multiple emission components, and iv)a peculiar energy distribution of relativistic electrons. We discuss the firstpossibility in terms of an inhomogeneous helical jet model.

The WEBT campaign on the BL Lac object PG 1553+113 in 2013. An analysis of the enigmatic synchrotron emission