The jet collimation profile at high resolution in BL Lacertae^⋆

¹ Foundation for Research and Technology – Hellas, IESL & Institute of Astrophysics, Voutes, 7110 Heraklion, Greece
e-mail: ccasadio@ia.forth.gr
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel, 69, 53121 Bonn, Germany
³ Institute for Astrophysical Research, Boston University, Boston, MA 02215, USA
⁴ Astronomical Institute, St. Petersburg State University, St. Petersburg 199034, Russia
⁵ Department of Physics and Astronomy, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul, South Korea
⁶ Korea Astronomy and Space Science Institute, Daedeok-daero 776, Yuseong-gu, Daejeon 34055, Republic of Korea
⁷ Centro de Desarrollos Tecnologicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
⁸ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹⁰ Instituto de Radio Astronomía Milimetrica, Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain

Received: 8 October 2020
Accepted: 3 March 2021

Abstract

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 × 10⁵R_S).