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
Author
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
S. Sánchez
Instituto de Radio Astronomía Millimétrica
Jun Yang
Chalmers, Space, Earth and Environment, Onsala Space Observatory
A.J. Zensus
Max Planck Society
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 649 A153Subject Categories
Astronomy, Astrophysics and Cosmology
Geophysics
Fusion, Plasma and Space Physics
DOI
10.1051/0004-6361/202039616