J/ApJ/787/151 Fitted jet components in BL Lacertae (Cohen+, 2014)
Studies of the jet in BL Lacertae.
I. Recollimation shock and moving emission features.
Cohen M.H., Meier D.L., Arshakian T.G., Homan D.C., Hovatta T.,
Kovalev Y.Y., Lister M.L., Pushkarev A.B., Richards J.L., Savolainen T.
<Astrophys. J., 787, 151 (2014)>
=2014ApJ...787..151C 2014ApJ...787..151C (SIMBAD/NED BibCode)
ADC_Keywords: BL Lac objects ; Active gal. nuclei
Keywords: BL Lacertae objects: individual: BL Lacertae - galaxies: active -
galaxies: jets - magnetohydrodynamics, MHD - waves
Abstract:
Parsec-scale VLBA images of BL Lac at 15 GHz show that the jet
contains a permanent quasi-stationary emission feature 0.26 mas
(0.34 pc projected) from the core, along with numerous moving
features. In projection, the tracks of the moving features cluster
around an axis at a position angle of -166.6° that connects the
core with the standing feature. The moving features appear to emanate
from the standing feature in a manner strikingly similar to the
results of numerical two-dimensional relativistic magneto-hydrodynamic
(RMHD) simulations in which moving shocks are generated at a
recollimation shock (RCS). Because of this, and the close analogy to
the jet feature HST-1 in M87, we identify the standing feature in BL Lac
as an RCS. We assume that the magnetic field dominates the dynamics in
the jet, and that the field is predominantly toroidal. From this we
suggest that the moving features are compressions established by slow
and fast mode magneto-acoustic MHD waves. We illustrate the situation
with a simple model in which the slowest moving feature is a slow-mode
wave, and the fastest feature is a fast-mode wave. In the model, the
beam has Lorentz factor Γbeamgal∼3.5 in the frame of the
host galaxy and the fast mode wave has Lorentz factor
ΓFwavebeam∼1.6 in the frame of the beam. This gives a
maximum apparent speed for the moving features, βapp=vapp/c=10.
In this model the Lorentz factor of the pattern in the galaxy frame is
approximately three times larger than that of the beam itself.
Description:
The MOJAVE program (Lister & Homan 2005, J/AJ/130/1389, Monitoring of
Jets in Active Galactic Nuclei with VLBA Experiments) is conducting
regular observations of about 300 sources with the VLBA at 15 GHz
(Lister et al. 2013, J/AJ/146/120). The observing cadence varies from
a few weeks for the most variable sources to about three years for
slowly changing ones. BL Lac varies rapidly and we are currently
observing it every one to two months. Here we present results from
55 epochs between 1996.2 and 2013.0. For this paper we have also added
results from other programs whose results are in the VLBA archives,
for a total of 110 epochs, all observed on the VLBA at a frequency
near 15.3 GHz.
Objects:
----------------------------------------------------------
RA (ICRS) DE Designation(s)
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22 02 43.29 +42 16 40.0 BL Lacertae = NAME BL Lac
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 49 616 Fitted Jet Components of BL Lac
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See also:
J/AJ/130/1389 : Linear polarization of AGN jets at 15GHz (Lister+, 2005)
J/AJ/137/3718 : 15GHz monitoring of AGN jets with VLBA (Lister+, 2009)
J/ApJ/722/L7 : Fermi/LAT detected MOJAVE AGNs (Pushkarev+, 2010)
J/ApJ/742/27 : 15GHz and jet properties of MOJAVE blazars (Lister+, 2011)
J/AJ/146/120 : MOJAVE. X. Parsec-scale kinematics of AGNs (Lister+, 2013)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 2 I2 --- ID [0/39] Component identifier; zero for core
component
4 A1 --- f_ID [a] Flag on ID (1)
6- 15 A10 "date" Date Date of the observation
17- 20 F4.2 Jy I Flux density at 15 GHz
22- 25 F4.2 mas r Distance from the core
27- 33 F7.2 deg PA [] Position angle with respect to core
35- 38 F4.2 mas MajAxis FWHM major axis of fitted Gaussian
40- 43 F4.2 --- Ratio Axial ratio of fitted Gaussian
45- 49 F5.2 deg MajPA ? Major axis position angle of fitted Gaussian
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Note (1): Flag as follows:
a = Fewer than 5 epochs, or r>4 mas.
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History:
From electronic version of the journal
References:
Cohen et al., Paper II, 2015ApJ...803....3C 2015ApJ...803....3C
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 30-Jun-2017