Planck early results. XV. Spectral energy distributions and radio
  continuum spectra of northern extragalactic radio sources
  Spectral energy distributions (SEDs) and radio continuum spectra are
presented for a northern sample of 104 extragalactic radio sources, based on
the Planck Early Release Compact Source Catalogue (ERCSC) and simultaneous
multifrequency data. The nine Planck frequencies, from 30 to 857 GHz, are
complemented by a set of simultaneous observations ranging from radio to
gamma-rays. This is the first extensive frequency coverage in the radio and
millimetre domains for an essentially complete sample of extragalactic radio
sources, and it shows how the individual shocks, each in their own phase of
development, shape the radio spectra as they move in the relativistic jet. The
SEDs presented in this paper were fitted with second and third degree
polynomials to estimate the frequencies of the synchrotron and inverse Compton
(IC) peaks, and the spectral indices of low and high frequency radio data,
including the Planck ERCSC data, were calculated. SED modelling methods are
discussed, with an emphasis on proper, physical modelling of the synchrotron
bump using multiplecomponents. Planck ERCSC data also suggest that the original
accelerated electron energy spectrum could be much harder than commonly
thought, with power-law index around 1.5 instead of the canonical 2.5. The
implications of this are discussed for the acceleration mechanisms effective in
blazar shock. Furthermore in many cases the Planck data indicate that gamma-ray
emission must originate in the same shocks that produce the radio emission.
