Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron. I. Imaging BL Lacertae at 21 microarcsecond resolution

TL;DR

This paper presents the first polarimetric space VLBI imaging of BL Lacertae at 22 GHz with 21 microarcsecond resolution, revealing jet structures, recollimation shocks, and a helical magnetic field in the innermost regions.

Contribution

It demonstrates RadioAstron's polarimetric imaging capabilities at 22 GHz and provides unprecedented high-resolution insights into AGN jet magnetic fields and shock structures.

Findings

Detection of upstream emission possibly indicating a recollimation shock

Observation of polarized emission within 0.5 mas of the core and downstream knots

Evidence of a helical magnetic field from Faraday rotation analysis

Abstract

We present the first polarimetric space VLBI imaging observations at 22 GHz. BL Lacertae was observed in 2013 November 10 with the RadioAstron space VLBI mission, including a ground array of 15 radio telescopes. The instrumental polarization of the space radio telescope is found to be within 9%, demonstrating the polarimetric imaging capabilities of RadioAstron at 22 GHz. Ground-space fringes were obtained up to a projected baseline distance of 7.9 Earth's diameters in length, allowing us to image the jet in BL Lacertae with a maximum angular resolution of 21 $\mu$as, the highest achieved to date. We find evidence for emission upstream of the radio core, which may correspond to a recollimation shock at about 40 $\mu$as from the jet apex, in a pattern that includes other recollimation shocks at approximately 100 $\mu$as and 250 $\mu$as from the jet apex. Polarized emission is detected in two components within the innermost 0.5 mas from the core, as well as in some knots 3 mas downstream. Faraday rotation analysis, obtained from combining RadioAstron 22 GHz and ground-based 15 GHz and 43 GHz images, shows a gradient in rotation measure and Faraday corrected polarization vector as a function of position angle with respect to the core, suggesting that the jet in BL Lacertae is threaded by a helical magnetic field. The intrinsic de-boosted brightness temperature in the unresolved core exceeds $3\!\times\!10^{12}$ K, suggesting at the very least departure from equipartition of energy between the magnetic field and radiating particles.