Periodic structure in the Mpc-scale jet of PKS 0637-752
  We present 18 GHz Australia Telescope Compact Array imaging of the Mpc-scale
quasar jet PKS 0637-752 with angular resolution ~0.58 arcseconds. We draw
attention to a spectacular train of quasi-periodic knots along the inner 11
arcseconds 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 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 ~ 10^{46} erg/s,
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 10^3 yr < \tau < 3 x 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 quasi-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 < a < 30 pc, which corresponds
to a maximum angular separation of ~0.1 - 5 mas.
