Planck intermediate results. XLV. Radio spectra of northern
  extragalactic radio sources
  Continuum spectra covering centimetre to submillimetre wavelengths are
presented for a northern sample of 104 extragalactic radio sources, mainly
active galactic nuclei, based on four-epoch Planck data. The nine Planck
frequencies, from 30 to 857 GHz, are complemented by a set of simultaneous
ground-based radio observations between 1.1 and 37 GHz. The single-survey
Planck data confirm that the flattest high-frequency radio spectral indices are
close to zero, indicating that the original accelerated electron energy
spectrum is much harder than commonly thought, with power-law index around 1.5
instead of the canonical 2.5. The radio spectra peak at high frequencies and
exhibit a variety of shapes. For a small set of low-z sources, we find a
spectral upturn at high frequencies, indicating the presence of intrinsic cold
dust. Variability can generally be approximated by achromatic variations, while
sources with clear signatures of evolving shocks appear to be limited to the
strongest outbursts.
