Parsec-scale jet precession in BL Lacertae (2200+420)

TL;DR

This study models the precession of BL Lacertae's jet, revealing a 12.1-year precession period and suggesting a supermassive binary black hole system as the origin of observed jet and core variations.

Contribution

It introduces a relativistic jet-precession model fitted with a heuristic method, providing new insights into jet dynamics and black hole system characteristics in BL Lacertae.

Findings

Jet precession period of approximately 12.1 years.

Jet velocity estimated at about 0.9824c.

Compatibility of core position angle variations with a nutation phenomenon.

Abstract

BL Lacertae is the prototype of the BL Lac class of active galactic nuclei, exhibiting intensive activity on parsec (pc) scales, such as intense core variability and multiple ejections of jet components. In particular, in previous works the existence of precession motions in the pc-scale jet of BL Lacertae has been suggested. In this work we revisit this issue, investigating temporal changes of the observed right ascension and declination offsets of the jet knots in terms of our relativistic jet-precession model. The seven free parameters of our precession model were optimized via a heuristic cross-entropy method, comparing the projected precession helix with the positions of the jet components on the plane of the sky and imposing constraints on their maximum and minimum superluminal velocities. Our optimized best model is compatible with a jet having a bulk velocity of 0.9824c, which is precessing with a period of about 12.1 yr in the observer's reference frame and changing its orientation in relation to the line of sight between 4 and 5 degrees, approximately. Assuming that the jet precession has its origin in a supermassive binary black hole system, we show that the 2.3-yr periodic variation in the structural position angle of the very-long-baseline interferometry (VLBI) core of BL Lacertae reported by Stirling et al. is compatible with a nutation phenomenon if the secondary black hole has a mass higher than about six times that of the primary black hole.