We present 18 GHz Australia Telescope Compact Array imaging of the Mpc-scale quasar jet PKS 0637–752 with angular resolution \sim 0 \farcs 58 .
We draw attention to a spectacular train of quasi-periodic knots along the inner 11 \arcsec of the jet , with average separation \sim 1.1 arcsec ( 7.6 kpc 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 } \sim 10 ^ { 46 } ergs 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 \times 10 ^ { 3 } ~ { } { yr } < \tau < 3 \times 10 ^ { 5 } ~ { } { yr } .
The lower end of this range is applicable if the jet remains highly relativistic on kpc-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 behaviour 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 \lesssim a \lesssim 30 pc , which corresponds to a maximum angular separation of \sim 0.1–5 mas .