Periodic structure in the megaparsec-scale jet of pks 0637-752

We present 18GHz Australia Telescope Compact Array imaging of the megaparsec-scale quasar jet PKS 0637-752 with angular resolution ~0.58". We draw attention to a spectacular train of quasi-periodic knots along the inner 11" of the jet, with average separation 1.1 arcsec (7.6kpc projected). We consider two classes of model to explain the periodic knots: those that involve a static pattern through which the jet plasma travels (e.g., stationary shocks) and those that involve modulation of the jet engine. Interpreting the knots as re-confinement shocks implies the jet kinetic power Q_jet ~ 10⁴⁶erg s^-1, but the constant knot separation along the jet is not expected in a realistic external density profile. For models involving modulation of the jet engine, we find that the required modulation period is 2 x 10³ yr < τ < 3 x 10⁵ yr. The lower end of this range is applicable if the jet remains highly relativistic on kiloparsec scales, as implied by the IC/CMB model of jet X-ray emission. We suggest that the periodic jet structure in PKS 0637-752 may be analogous to the quasi-periodic jet modulation seen in the microquasar GRS 1915+105, believed to result from limit cycle behavior in an unstable accretion disk. If variations in the accretion rate are driven by a binary black hole, the predicted orbital radius is 0.7 pc ≲ a ≲ 30 pc, which corresponds to a maximum angular separation of ~0.1-5 mas.