Helical magnetic fields through Faraday rotation and jet - - PowerPoint PPT Presentation

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Helical magnetic fields through Faraday rotation and jet - - PowerPoint PPT Presentation

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Helical magnetic fields through Faraday rotation and jet stratification studies Jos L. Gmez, Carolina Casadio, Mar Roca-Sogorb, Ivn Agudo Instituto de Astrofsica de Andaluca (CSIC) Alan P. Marscher, Svetlana G. Jorstad Boston

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SLIDE 1

Helical magnetic fields through Faraday rotation and jet stratification studies

José L. Gómez, Carolina Casadio, Mar Roca-Sogorb, Iván Agudo

Instituto de Astrofísica de Andalucía (CSIC)

Alan P. Marscher, Svetlana G. Jorstad

Boston University

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SLIDE 2

Jet formation and helical magnetic fields

Recent numerical simulations show that helical magnetic fields should play an important role in the jet formation, providing the initial flow collimation and acceleration.

Animation by W. Steffen (UNAM & Cosmovision)

How can we study the jet formation and observe these helical fields?

  • A. Within the acceleration and collimation

zone (ACZ), through multi-wavelength

  • bservations
  • B. Beyond the VLBI core, through Faraday

r o t a t i o n s t u d i e s a n d e m i s s i o n asymmetries across the jet width

  • Previous talk by Marscher and next talk

by Agudo

  • Posters by Jorstad and Molina
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SLIDE 3

Asada et al. (2002)

3C273

Faraday Rotation and Asymmetries Across the Jet Width

Faraday rotation produces a rotation of the electric vector position angle (χ)

! = !o + RM "2

Where RM is the rotation measure, given by:

RM = 812 ne

!

B! dl (rad m"2)

A helical magnetic field should lead to a jet stratification in RM across the jet width (e.g. Broderick & McKinney 2010) B pointing away B t

  • w

a r d s t h e

  • b

s e r v e r Aligned field Recent numerical simulations show that helical magnetic fields should play an important role in the jet formation, providing the initial flow collimation and acceleration.

  • B. Beyond the VLBI core, through Faraday

r o t a t i o n s t u d i e s a n d e m i s s i o n asymmetries across the jet width

How can we study the jet formation and observe these helical fields?

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SLIDE 4

Faraday Rotation and Asymmetries Across the Jet Width

Lorentz factor Internal energy

Aloy, Gómez, Ibáñez, Martí & Müller 2000, ApJ, 528, L85

Synchrotron emissivity: ε ! sin θ’ where θ’ is the angle between the magnetic field and line of sight in the fluid frame. For a pitch angle of Φ, viewing angle θ, and flow velocity β:

  • Maximum asymmetry θʼ=Φ , θʼ=Π-Φ
  • Asymmetry reverses for β=cosθ

B top B bottom Φ θʼ Polarized (log scale) Total flux (log scale) Different viewing angles

see also Clausen-Brown et al. (2011)

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SLIDE 5

Faraday Rotation and Asymmetries Across the Jet Width

Synchrotron emissivity: ε ! sin θ’ where θ’ is the angle between the magnetic field and line of sight in the fluid frame. For a pitch angle of Φ, viewing angle θ, and flow velocity β:

  • Maximum asymmetry θʼ=Φ , θʼ=Π-Φ
  • Asymmetry reverses for cosθ=β

B top B bottom Φ θʼ

see also Clausen-Brown et al. (2011)

Numerical simulations of the linearly polarized emission at different pitch angles

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SLIDE 6

Faraday Rotation and Stratification in 3C273

Marscher’s Boston University Blazars Group Data

Jet evolution from January 2008 to January 2011 3C273 at 43 GHz

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SLIDE 7

Faraday Rotation and Stratification in 3C273

Marscher’s Boston University Blazars Group Data

Jet evolution from January 2008 to January 2011 3C273 at 43 GHz Progressive stacking of degree of polarization as components sample the jet

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SLIDE 8

Faraday Rotation and Stratification in 3C273

1 1 2 2 3 years mean total flux

0.6 mas from core 3.1 mas from core

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SLIDE 9

Faraday Rotation and Stratification in 3C273

3 years mean total flux Total flux decay along the jet ridge line

S ! r"1

Stationary feature at ~0.8 mas. See Jorstad poster. Southern side is brighter

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SLIDE 10

Faraday Rotation and Stratification in 3C273

3 years mean degree

  • f polarization

Stratification consistent with a helical magnetic field & shear

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SLIDE 11

Faraday Rotation and Stratification in 3C273

Mojave’s data on 3C273

Jet evolution from January 1996 to June 2010: 14 years of monitoring. 3C273 at 15 GHz

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SLIDE 12

Faraday Rotation and Stratification in 3C273

Mojave’s data on 3C273

Jet evolution from January 1996 to June 2010: 14 years of monitoring. Progressive stacking of degree of polarization as components sample the jet 3C273 at 15 GHz

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SLIDE 13

Faraday Rotation and Stratification in 3C273

1 1 2 2 43 GHz 14 years mean total flux

3.1 mas from core 14 mas from core

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SLIDE 14

Faraday Rotation and Stratification in 3C273

1 1 2 2 Change in the brighter side of the jet may result from the measured acceleration of components (Jorstad et al. 2005; Lister et al. 2009). A reversal in the asymmetry is produced for β=cosθ. 14 years mean total flux

3.1 mas from core 14 mas from core

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SLIDE 15

Faraday Rotation and Stratification in 3C273

1 1 2 2 Proper motions measure pattern s p e e d s , w h i l e c h a n g e s i n asymmetries depend on bulk flow

  • velocity. We can relate both and

estimate shock strengths. 14 years mean total flux

3.1 mas from core 14 mas from core

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SLIDE 16

Faraday Rotation and Stratification in 3C273

Consistent with the 43 GHz d e g r e e o f p o l a r i z a t i o n stratifciation. 15 GHz 43 GHz 14 years mean degree pol.

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SLIDE 17

Faraday Rotation and Stratification in 3C273

43 GHz 22 GHz 15 GHz

MOJAVE

RM map between 43 and 15 GHz

0.0 0.5 1.0 1.5 2.0 2.5 Distance (mas)

  • 800
  • 600
  • 400
  • 200

200 400 600 RM (rad/m2)

RM slice at 3 mas February 2009

Consistent with Hovatta et al. results

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SLIDE 18

Faraday Rotation and Stratification in 3C273

43 GHz 22 GHz 15 GHz

MOJAVE

RM map between 43 and 15 GHz

0.0 0.5 1.0 1.5 2.0 2.5 Distance (mas)

  • 800
  • 600
  • 400
  • 200

200 400 600 RM (rad/m2)

RM slice at 3 mas Slice at 6 mas from Zavala & Taylor in 2000

  • A change in velocity would also

explain the change in the RM between the slices at 3 and 6 mas.

  • It also implies that some of the RM is

produced within the emitting jet. February 2009

Consistent with Hovatta et al. results

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SLIDE 19

Rich polarization structure at all three frequencies. Complex polarized structure evolution in the jet. Superluminal components sample the jet polarization as they travel downstream.

15, 22 and 43 GHz VLBA Observations during 2001

  • Three frequency data set to study

the Faraday rotation screen.

  • 12 monthly VLBA observations to
  • btain a detailed monitoring.

Gómez et al. (2008)

Faraday Rotation and Polarization gradients in 3C120

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SLIDE 20

m (%) RM

Gómez et al. (2008)

Faraday Rotation and Polarization gradients in 3C120

Mean Rotation Measure During 2001

Mean Degree Polarization 2001

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SLIDE 21

m (%) RM

Gómez et al. (2008)

Faraday Rotation and Polarization gradients in 3C120

  • Localized region of enhanced RM.

A local process, such as interaction of the jet with the external medium or a cloud, is required to explain this region of enhanced RM.

  • Stratification in RM and degree of

polarization across the jet as expected for the case of a helical magnetic field. Mean Rotation Measure During 2001

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SLIDE 22

On the Stationarity of the RM Sheath in 3C120

New observations taken in November 2007 at all observing VLBA frequencies (1.7 to 43 GHz), to study:

  • How stationary is the Faraday screen
  • Wether similar jet/external medium

interactions take place further downstream

!"#

!"#$%&'()*+'%&",$-).#/$#0"/()123(45$%26

7$'%/"8$)7"9:/);0<$#0"+#)12%06 !$%

20 40 60 80 100 106 107 108 109 1010 1011 1012 Tb (K) Distance from the core (mas)

High brightness temperature in 3C120

Tb along the jet

VLBA 43 GHz 22 GHz 15 GHz

  • Serendipitous discovery of a component at 80

mas from the core, not seen previously, and with a TB about 600 times higher than expected.

  • Component cannot be produced entirely by jet

bending, moving, or standing shock without a significant flow acceleration (Roca-Sogorb et al. 2010, ApJ, 712, L160) Roca-Sogorb et al. (2010)

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SLIDE 23

Uncorrelated changes in the RM and RM-corrected EVPAs

Mean RM 2001 1998 1999 2000

Comparison with observations before 2001

22 and 43 GHz VLBA observations starting in 1997 (Gómez et al. 2000, 2001)

January 1999

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SLIDE 24

Comparison of Observations between 1999 and 2001

We find consistent values of RM and RM- corrected EVPAs between this 2-year interval, however There are two distinct regions in which the RM-corrected EVPAs show a rotation close to 90 degrees. Therefore we find uncorrelated changes in the linear polarization of the underlying jet emission and the Faraday rotation screen. The emitting jet and the source of Faraday rotation are not closely connected physically and have different configurations for the magnetic field. If we also consider the existence of the localized region of high RM, we conclude that a significant fraction of the RM in 3C120 originates in foreground clouds.

RM = 5500 ! = 40o RM = 1700 ! = 46o RM = 5400 ! = -28o RM = 2000 ! = -32o

Mean RM 2001 January 1999

Uncorrelated changes in the RM and RM-corrected EVPAs

Gómez et al. (2011)

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SLIDE 25

Summary

  • Stacking of images proves very useful in

revealing the full jet structure in total and polarized flux, allowing for the search of stratifications.

  • 3C273 shows a stratification across the jet

in total and polarized flux (RM), which flips sides along the jet. This can be interpreted as produced by a helical magnetic field and jet flow acceleration along the jet.

  • Proper motions measure pattern speeds,

while changes in asymmetries depend on bulk flow velocity. We can relate both and estimate shock strengths.

  • Polarization structure in 3C120 is

compatible with the existence of a helical magnetic field, but a significant fraction of the RM appears to be originated in foreground clouds.