The jet collimation profile at high resolution in BL Lacertae
Journal article, 2021

Context. Controversial studies on the jet collimation profile of BL Lacertae (BL Lac), the eponymous blazar of the BL Lac objects class, complicate the scenario in this already puzzling class of objects. Understanding the jet geometry in connection with the jet kinematics and the physical conditions in the surrounding medium is fundamental for better constraining the formation, acceleration, and collimation mechanisms in extragalactic jets. Aims. With the aim of investigating the jet geometry in the innermost regions of the BL Lac jet, and resolving the controversy, we explore the radio jet in this source using high-resolution millimeter-wave VLBI data. Methods. We collect 86 GHz GMVA and 43 GHz VLBA data to obtain stacked images that we use to infer the jet collimation profile by means of two comparable methods. We analyze the kinematics at 86 GHz, and we discuss it in the context of the jet expansion. Finally, we consider a possible implication of the Bondi sphere in shaping the jet of BL Lac. Results. The jet in BL Lac expands with an overall conical geometry. A higher expanding rate region is observed between ∼5 and 10 pc (de-projected) from the black hole. Such a region is associated with the decrease in brightness usually observed in high-frequency VLBI images of BL Lac. The jet retrieves the original jet expansion around 17 pc, where the presence of a recollimation shock is supported by both the jet profile and the 15 GHz kinematics (MOJAVE survey). The change in the jet expansion profile occurring at ∼5 pc could be associated with a change in the external pressure at the location of the Bondi radius (∼3.3 × 105RS).

Instrumentation: high angular resolution

Instrumentation: interferometers

Galaxies: jets

BL Lacertae objects: individual: BL Lacertae

Galaxies: active


C. Casadio

Foundation for Research and Technology Hellas (FORTH)

Max Planck Society

N. R. Macdonald

Max Planck Society

B. Boccardi

Max Planck Society

S.G. Jorstad

Saint Petersburg State University - Spsu

Boston University

A. Marscher

Boston University

T.P. Krichbaum

Max Planck Society

J. Hodgson

Sejong University

Jae-Young Kim

Korea Astronomy and Space Science Institute

Max Planck Society

E. Traianou

Max Planck Society

Z. R. Weaver

Boston University

M. Gómez Garrido

Centro de Desarrollos Tecnológicos

J. González García

Centro de Desarrollos Tecnológicos

J. Kallunki

Aalto University

Michael Lindqvist

Chalmers, Space, Earth and Environment, Onsala Space Observatory, Observation Support

S. Sánchez

Instituto de Radio Astronomía Millimétrica

Jun Yang

Chalmers, Space, Earth and Environment, Onsala Space Observatory, Observation Support

A.J. Zensus

Max Planck Society

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 649 A153

Subject Categories

Astronomy, Astrophysics and Cosmology


Fusion, Plasma and Space Physics



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