The peculiar optical spectrum of 4C+22.25: Imprint of a massive black
  hole binary?
  We report the discovery of peculiar features in the optical spectrum of
4C+22.25, a flat spectrum radio quasar at z=0.4183 observed in the SDSS and in
a dedicated spectroscopic follow-up from the Nordic Optical Telescope. The
Hbeta and Halpha lines show broad profiles (FWHM~12,000 km/s), faint fluxes and
extreme offsets (Delta v=8,700+/-1,300 km/s) with respect to the narrow
emission lines. These features show no significant variation in a time lag of
~3.1 yr (rest frame). We rule out possible interpretations based on the
superposition of two sources or on recoiling black holes, and we discuss the
virtues and limitations of a massive black hole binary scenario.
