The jet collimation profile at high resolution in BL Lacertae

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 × 105RS).

Dual-high-frequency VLBI study of blazar-jet brightness-temperature gradients and collimation profiles

“…A.25), the s-values are consistent with the canonical value expected for a simple Blandford & Königl (1979) jet model. At ∼1.5 mas from the respective cores, we find a region where the jet locally shows a dip in its diameter and excess T b values which is also consistent with the findings of Casadio et al (2021). They associate this region with a recollimation shock which is connected to the changing external pressure gradient close to the Bondi radius (Kovalev et al 2020).…”

“…2200+420 (BL Lacertae). The inner region of this prototypical BL Lac object features an overall jet shape which is consistent with a cone which is also confirmed by Casadio et al (2021) by additionally taking GMVA data at 86 GHz into account. If fit with a simple power-law model (Fig .…”

“…In result, a diameter gradient fit with the modelfit method will put much stronger weight on the (typically many) components from the compact jet and little weight on the single extended Gaussian component representing the diffuse emission. The extended jet regions in the image-stacking method are also weighted less because the method is sensitive to the signal to noise ratio (e.g., Kovalev et al 2020;Casadio et al 2021). However, since the jet is split into equidistant sections along the jet axis, the larger area of extended emission will yield more data points to be fit in these regions as the modelfit method.…”

“…There are different ways this function can be fit to data. Baczko et al (2022) deploy the orthogonal distance regression while Casadio et al (2021) use a gnuplot implementation of the Levenberg-Marquardt algorithm. In this work we use scipy (Virtanen et al 2020) in order to fit the smoothly broken power law with a classical least square method.…”

Kinematics of Parsec-scale Jets of Gamma-Ray Blazars at 43 GHz during 10 yr of the VLBA-BU-BLAZAR Program

“…This could be an indication of recollimation of the jet flow, such that the jet width narrows or remains constant downstream of this point. Stacking of the 43 GHz images of jets can be used to investigate the jet width (e.g., Pushkarev et al 2017;Kovalev et al 2020;Casadio et al 2021) and test whether the acceleration trends are actually related to the jet width.…”