1

Recollimation Shock, Transverse Waves and the Whip in BL Lacertae

M.H. Cohen Caltech

Granada 13 vi 2013

2

• M. Lister (P.I.), J. Richards (Purdue)

• T. Arshakian (Univ. of Cologne, Germany)

• M. and H. Aller (Michigan)

• M. Cohen, T. Hovatta, (Caltech)

• D. Homan (Denison)

• M. Kadler, M. Böck (U. Wurzburg, Germany)

• K. Kellermann (NRAO)

• Y. Kovalev (ASC Lebedev, Russia)

• A. Lobanov, T. Savolainen, J. A. Zensus (MPIfR, Germany)

• D. Meier (JPL)

• A. Pushkarev (Crimean Observatory, Ukraine)

• E. Ros (Valencia, Spain)

MOJAVE CollaboratorsMonitoring

Of

Jets in

Active Galaxies with

VLBA

Experiments

Very Long Baseline Array

The MOJAVE Program is supported NASA Fermi Grant NNX08AV67G

2Granada 13 vi 2013

3

BL Lac Topics

3

• Components, Ridge Line• Recollimation Shock• PA Variations: Wobble• Transverse Waves• Wave Speeds• Relaxation in 2010: Wiggle• Conclusions

Granada 13 vi 2013

4

Components

Granada 13 vi 2013 4

• Components are a small number of elliptical Gaussians that sum to the image. They usually are circular.

• A component is often a bright spot in the image.

• Components are tracked in time, if the cadence of observations is fast enough.

5

Ridge Line

Granada 13 vi 2013 5

• BL Lac is elongated and has a ridge.

• Most components move downstream• along the ridge (±0.1 mas)

Granada 10 vi 2013Granada 13 vi 2013 6

Ridge Line and Components

2006.862005.71

7

Several New Components per YearMax Speed 10.6c

Granada 13 vi 2013

BL Lac Component Tracks

8Granada 13 vi 2013

quasi-stationaryrecollimation shock

99

Recollimation Shock

• Component 7 = recollimation shock

• Analogy to M87, 3C120

• Simulation (Lind et al 1999, ...) shows a‘magnetic chamber’ and fast cpts ejected into a ‘nose cone’ (not full 3D RMHD) (Meier p715)

• Need strong toroidal component in the magnetic field

Granada 13 vi 2013

10

z vmax dist from core

• 3C 120 0.033 5.3c ~ 5 mas 3.1 * 80 mas ~ 3x107 rg

• M 87 .00436 4.3 ** >= 106 rg • BL Lac .0686 10.6 # ~ 106 rg

* downstream from C1 Agudo et al 2012** downstream from HST1 Cheung et al 2007# downstream from cpt 7 MOJAVE

10

Recollimation Shock II

Granada 13 vi 2013

Granada 10 vi 2013Granada 13 vi 2013 11

BL LacPosition Angle vs Epoch

Granada 10 vi 2013Granada 13 vi 2013 12

V ≈ 1.15 mas yr-1

≈ 4.8 c

Transverse Wave 2004 - 2007

A

B

A

B

Shifted Ridge Lines

Granada 15 vi 2013 13

Transverse Waves 1999-2000

14Granada 15 vi 2013

V ≈ 0.80 mas yr-1

≈ 3.3 c

15

Transverse Wave 2000-2001

Granada 13 vi 2013 15

16

Component 16 Advected with the Transverse Wave

Granada 13 vi 2013 16

17

Several New Components per YearMax Speed 10.6c

Granada 13 vi 2013

BL Lac Component Tracks

18Granada 13 vi 2013

quasi-stationaryrecollimation shock

CenterlinePA=166o

BL Lac 2010.8-2012.8wiggle

19Granada 15 vi 2013

PA ≈ 171o

Granada 10 vi 2013Granada 13 vi 2013 20

• Strong quasi-stationary component is identified as a recollimation shock.

• Distance from core ~ 106 rg

• Superluminal components appear to come from or through the recollimation shock.

BL LacConclusions I

BL LacConclusions II

Granada 13 vi 2013 21

• Jet supports transverse waves.

• Waves have superluminal speed.

• Waves are correlated with swings in the inner PA.

• Components are advected with the transverse motion.

• Jet acts more like a rope than a water hose.

• When wave activity dies down, a stable wiggle appears.

Granada 13 vi 2013 22

These observations support a model in which the jet contains a strong toroidal magnetic field. Transverse waves (Alfven?) are excited by PA swings of the nozzle, and propagate superluminally downstream, as large-scale wiggles on the ridge line. The superluminal components (fast MHD waves?) stay on the ridge, and can be advected transversely. In 2009 the waves died down, and a small-scale stationary wiggle appeared on the jet.

Conclusions III