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Recollimation Shock, Transverse Waves and the Whip in BL Lacertae

M.H. Cohen Caltech

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• 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 Collaborators Monitoring

Of

Jets in

Active Galaxies with

VLBA

Experiments

Very Long Baseline Array

The MOJAVE Program is supported NASA Fermi Grant NNX08AV67G

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BL Lac Topics

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

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Components

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• 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.

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Ridge Line

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• BL Lac is elongated and has a ridge.

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

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Ridge Line and Components

2006.862005.71

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Several New Components per YearMax Speed 10.6c

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BL Lac Component Tracks

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quasi-stationaryrecollimation shock

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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

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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

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Recollimation Shock II

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BL LacPosition Angle vs Epoch

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V ≈ 1.15 mas yr-1

≈ 4.8 c

Transverse Wave 2004 - 2007

A

B

A

B

Shifted Ridge Lines

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Transverse Waves 1999-2000

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V ≈ 0.80 mas yr-1

≈ 3.3 c

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Transverse Wave 2000-2001

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Component 16 Advected with the Transverse Wave

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Several New Components per YearMax Speed 10.6c

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BL Lac Component Tracks

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quasi-stationaryrecollimation shock

CenterlinePA=166o

BL Lac 2010.8-2012.8wiggle

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PA ≈ 171o

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• 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

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• 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.

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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