The jet collimation profile at high resolution in BL Lacertae
Artikel i vetenskaplig tidskrift, 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

Idryma Technologias kai Erevnas (FORTH)


N. R. Macdonald


B. Boccardi


S.G. Jorstad

Saint Petersburg State University - Spsu

Boston University

A. Marscher

Boston University

T.P. Krichbaum


J. Hodgson

Sejong University

Jae-Young Kim

Korea Astronomy and Space Science Institute


E. Traianou


Z. R. Weaver

Boston University

M. Gómez Garrido

Centro de Desarrollos Tecnológicos

J. González García

Centro de Desarrollos Tecnológicos

J. Kallunki


Michael Lindqvist

Chalmers, Rymd-, geo- och miljövetenskap, Onsala rymdobservatorium

S. Sánchez

Instituto de Radio Astronomía Millimétrica

Jun Yang

Chalmers, Rymd-, geo- och miljövetenskap, Onsala rymdobservatorium

A.J. Zensus


Astronomy and Astrophysics

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

Vol. 649 A153


Astronomi, astrofysik och kosmologi


Fusion, plasma och rymdfysik



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