Megaparsec-scale Radio Structure Associated with a Hybrid Blazar SBS B1646+499: Episodic Jet Activity with Precessing Axis

TL;DR

This paper presents GMRT observations of the hybrid blazar SBS B1646+499, revealing complex megaparsec-scale radio structures, episodic jet activity, and a precessing jet axis, advancing understanding of blazar jet dynamics and morphology.

Contribution

It provides new high-resolution radio data showing episodic jet activity and precession in a hybrid blazar, linking large-scale structures with jet evolution.

Findings

Detection of a steep-spectrum giant halo indicating aged electron populations.

Spectral gradient along the jet suggests spine-boundary shear layer morphology.

Observation of superluminal knots in the nuclear jet.

Abstract

Here we report on the total-intensity 610 MHz GMRT observations of the peculiar hybrid blazar SBS B1646+499, which merges the properties of BL Lacertae objects and flat-spectrum radio quasars. The complex radio structure of SBS B1646+499, emerging from the archival radio data and our new GMRT observations, consists of the Mpc-scale, elongated halo, the unilateral kpc-scale jet, and the nuclear jet extending up to 20pc from the compact core. The giant halo is characterized by a steep radio spectrum, indicative of the advanced ageing of the electron population within the lobes. For the large-scale jet, we detected a spectral gradient along and across the outflow, and in particular spectral flattening of the radio continuum toward the jet edges, suggestive of the spine--boundary shear layer morphology. The nuclear jet displays superluminal knots emerging from the self-absorbed and variable radio core. We interpret all these findings in the framework of the model of an episodic jet activity with a precessing jet axis.