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

TL;DR

This paper presents high-resolution radio imaging of the quasar PKS 0637-752, revealing quasi-periodic knots in its jet, and explores models involving jet shocks and engine modulation, suggesting possible binary black hole influence.

Contribution

It provides detailed observations of the jet structure and evaluates models for the periodic knots, proposing a new interpretation involving jet engine modulation and potential binary black hole effects.

Findings

Quasi-periodic knots separated by ~7.6 kpc along the jet.

Jet modulation periods estimated between 2,000 and 300,000 years.

Possible binary black hole orbital radius between 0.7 and 30 parsecs.

Abstract

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.