The WEBT campaign on the BL Lac object PG 1553+113 in 2013. An analysis of the enigmatic synchrotron emissionстатья
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Аннотация: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 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 Hubble Space
Telescope/Cosmic Origins Spectrograph and International Ultraviolet Explorer
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.