On the geometrical origin of periodicity in blazar-type sources

Periodicities in blazar light curves may be related to helical trajectories in extragalactic radio jets by differential Doppler boosting effects. We consider ballistic and nonballistic (i.e., radial) trajectories and discuss three possible periodic driving mechanisms for the origin of helical jet paths, namely, orbital motion in a binary black hole system, jet precession, and intrinsic jet rotation. It is shown that precessional-driven ballistic motion is unlikely to result in observable periods of less than several tens of years. We demonstrate that for nonballistic helical motion the observed period is generally strongly shortened relative to the real physical driving period because of light-travel time effects. Internal jet rotation may thus account for observed periods P-obs less than or similar to 10 days. Periodicity due to orbital- driven ( nonballistic) helical motion, on the other hand, is usually constrained to periods of P-obs greater than or similar to 10 days, while Newtonian-driven precession is unlikely to be responsible for periodicity on a timescale P-obs less than or similar to 100 days but may well be associated with periods of P-obs greater than or similar to 1 yr.