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We report the discovery of peculiar features in the optical spectrum of4C+22.25, a flat spectrum radio quasar at z=0.4183 observed in the SDSS and ina dedicated spectroscopic follow-up from the Nordic Optical Telescope. TheHbeta and Halpha lines show broad profiles (FWHM~12,000 km/s), faint fluxes andextreme offsets (Delta v=8,700+/-1,300 km/s) with respect to the narrowemission lines. These features show no significant variation in a time lag of~3.1 yr (rest frame). We rule out possible interpretations based on thesuperposition of two sources or on recoiling black holes, and we discuss thevirtues and limitations of a massive black hole binary scenario.

The peculiar optical spectrum of 4C+22.25: Imprint of a massive black hole binary?