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We present 18 GHz Australia Telescope Compact Array imaging of the Mpc-scalequasar jet PKS 0637-752 with angular resolution ~0.58 arcseconds. We drawattention to a spectacular train of quasi-periodic knots along the inner 11arcseconds of the jet, with average separation ~1.1 arcsec (7.6 kpc projected).We consider two classes of model to explain the periodic knots: those thatinvolve a static pattern through which the jet plasma travels (e.g. stationaryshocks); and those that involve modulation of the jet engine. Interpreting theknots as re-confinement shocks implies the jet kinetic power Q ~ 10^{46} erg/s,but the constant knot separation along the jet is not expected in a realisticexternal density profile. For models involving modulation of the jet engine, wefind that the required modulation period is 2 x 10^3 yr < \tau < 3 x 10^5 yr.The lower end of this range is applicable if the jet remains highlyrelativistic on kpc-scales, as implied by the IC/CMB model of jet X-rayemission. We suggest that the quasi-periodic jet structure in PKS 0637-752 maybe analogous to the quasi-periodic jet modulation seen in the microquasar GRS1915+105, believed to result from limit cycle behaviour in an unstableaccretion disk. If variations in the accretion rate are driven by a binaryblack hole, the predicted orbital radius is 0.7 < a < 30 pc, which correspondsto a maximum angular separation of ~0.1 - 5 mas.

Periodic structure in the Mpc-scale jet of PKS 0637-752