Transcript The Radio Jets of AGN

VLBI Polarisation Varability and
Implications for Jet Models
Denise Gabuzda (University College Cork)
Juan Carlos
Algaba
Redmond
Hallahan
Slava Bezrukovs
(CIT)
Mehreen
Mahmud
Misha Lisakov
(Moscow State)
Brian
Moloney
Vasilii Vitrishchak
(Moscow State)
Shane
O’Sullivan
What basic elements are contributing to the observed
polarization?
1) Now abundant and varied evidence for an intrinsic
underlying helical B field, due to rotation of the central
supermassive black hole and its accretion disc plus the
relativistic jet outflow.
Meier, Koide &
Uchida 2003
2) The effects of relativistic shocks and interaction with
the surrounding medium can be superimposed in some
cases - often invoked to explain variability.
There is now a plethora of observational evidence that
many, possibly all, AGN jets have helical B fields,
coming from:
• Faraday rotation gradients across the jets
• Transverse polarisation and total intensity structures
• Sign of detected circular polarisation
• Core B fields
• Interknot polarisation
Faraday rotation – rotation of the observed plane of LP
when polarised EM wave passes through a magnetised
plasma, due to different propagation velocities of the RCP
and LCP components of the EM wave in the plasma.
The amount of rotation is proportional to the square of the
observing wavelength, and the sign of the rotation is
determined by the direction of the line of sight B field:
= o + RM 2
RM = (constants)  ne B•dl
If jet has a helical B field, observe Faraday-rotation gradient
across the jet – due to systematically changing line-of-sight
component of B field across the jet.

LOS B away
from observer
B
Jet axis
LOS B towards
observer
•
Croke, O’Sullivan, Gabuzda & Katz, in prep.
Gabuzda, Vitrishchak, Mahmud
& O’Sullivan (2008)
Transverse RM
gradients have
been observed in
about a dozen
AGN jets …
Zavala & Taylor 2003
…providing direct
evidence that these
jets have helical
magnetic fields.
“Spine+sheath” structures - come
about naturally if jet has helical B field
(Laing 1981; Lyutikov, Pariev &
Gabuzda 2005)
Lister & Homan 2005
Attridge et al. 1999;
Pushkarev et al. 2005
Pushkarev et al. 2005
Circular polarisation (CP) of synchrotron radiation – very low
(less than 0.1%) for B fields thought to be typical (~100 G)
Prime suspect for mechanism generating CP: Faraday
conversion of LP to CP when EM wave travels through
magnetised plasma.
Charges can move only along B in conversion region:
component of polarisation E field parallel to B is absorbed
& re-emitted by free charges, but component perpendicular
to B is not  delay of “E parallel” relative to “E
perpendicular”
 Manifest as introduction of small amount of CP
Angle between plane of linear polarisation (E) and
conversion B field determines sign of CP produced.
Helical B-field geometry can facilitate conversion –
linear polarisation emitted at “back” of helix is
converted to CP as it passes through “front” of helix.
Sign of CP is determined by pitch angle and helicity of
helical B field.
Left-handed helical B field
0735+178
Gabuzda et al. (2008)
FR gradient across jet
Predominantly  B field
AGN
Jet B
field
Pitch
angle
Helicity
Predicted
CP sign
Obs CP
(%)
Implied
Pole?
0735+178
1156+295
3C273

/SS
||
Large
Large
Small
N
R
R
L
3C279
3C345
1749+096

||
Large
Small
Large
R
R
R
L
L
L
– +
– +
+ –
+ 0.30
+ 0.17
– 0.42
S
S
S
Small
Large
L
L
R
R
+ –
– +
– 0.48
+ 0.23
S
S
2230+114
2251+158

||
 /SS
S
L
L
R
N S
+ –
+ –
+ –
– 0.30
– 0.27
– 0.45
S
S
S
Predictions imply S pole for jet in 8 of 8 cases!?
Probability of this happening by chance is ~ 0.8%.
Not fully understood, but clear connection between CP and
helical B fields!
CP News Flash — First detection of parsec-scale CP
at 43 GHz (Vitrishchak et al. 2008, in process)!
CP detected @ 43 GHz in 5 AGN, in all cases degree of
CP is  degree of CP at 15, 22 GHz.
Sometimes sign change in CP between 22/43 GHz.
So how is presence of shocks in the jets manifest,
what role do they play?
• Individual moving or stationary features
• Strongly affect the local, but not overall (global) jet
B field
• Often invoked to explain (rapid) variability
What types of polarization variability do we observe?
• Changes in linear polarisation angle & degree
• Changes in core Faraday rotation
• Changes in core circular polarisation
• Changes in direction of transverse RM gradient
• Changes in core or jet polarisation correlated with
changes in jet direction
Changes in linear polarisation degree & angle
Simultaneous rapid rotation of 7mm core and optical pol.
angles in 0420-014 (D'Arcangelo et al. 2007)
Polarization flipped by
90o during the 10-hr
observation
Excellent candidate for event
due to a shock — passing
through a turbulent plasma
(D’Arcangelo et al. 2007) or a
region of ordered B field (e.g.
Konigl 1985)?
knot in jet
7mm core (gray)
optical (black)
knot in jet
Rapid changes in core properties on time scale of weeks, during
time of high TeV activity (Charlot et al. 2007) - are these related?
Changes in core Faraday rotation
Core Faraday rotation
known to be time variable
in magnitude, sometimes
dramatically — must be
associated with changes in
electron density and B field
in core region (birth of new
components?), but details
not known.
Also not known how
frequently core RM is
variable and on what range
of time scales.
Zavala & Taylor 2002
Core RM can also be variable in
magnitude and sign with
distance from central engine.
Can be explained by
decrease in ne with
distance, small bends
in jet about viewing
angle of ~90o in jet
rest frame.
Changes in core circular polarisation
CP Measurements of 3C279 VLBI core @ 15 GHz
07 Aug 2002
+0.19 +/- 0.11
Gabuzda et al. 2008
23 Nov 2002
+0.30 +/- 0.08
Homan & Lister 2006
04 Mar 2003
+0.83 +/- 0.10
Gabuzda et al. 2008
15 Mar 2005
+0.26 +/- 0.09
Gabuzda et al. 2008
(More
MOJAVE epochs To come…)
CP of AGN VLBI cores can be variable, but not yet
known with what this is associated — though
probably with birth of new VLBI knots etc.
Changes in direction of transverse RM gradient
4/1997
8/2002
8/2003
6/2000
Transverse RM gradient in
1803+784 changed direction
(Mahmud & Gabuzda 2008)!
RM grad is due to rotation
of central BH + outflow
— Rotation direction can’t
change!
— Magnetic polarity of jet
(probably) can’t change!
— Change in whether
“outer” or “inner” helical
field in magnetic-tower
model is giving rise to
observed RM gradient,
possibly due to change in
jet to LOS?
Winding up of field
lines due to
differential rotation
Direction of transverse RM
gradient sometimes changes
between core and jet —
transition from domination
of inner to domination of
outer helical field?
Changes in core or jet polarisation correlated
with changes in jet direction
Gabuzda et al. 1992, 1994
Example: 0300+470
Epoch 5/1987: VLBI jet to East, core
pol angle at 45o to jet direction
Epoch 3/1989: New jet component
has emerged in direction of core
polarisation observed earlier!
Another example:
0814+425
Gabuzda et al. 2006
O’Sullivan & Gabuzda 2008
Change in direction of inner VLBI jet of BL Lac with time,
pol electric vectors always aligned with jet (B  jet) —
natural if B is toroidal component of helical field
Summary
• Many (all?) AGN jets have helical B fields, formed by
rotation of central BH + jet outflow. Helical jet B field
determines global behaviour, properties of jet, origin of
variability of core RM, core CP, transverse RM gradients,
global polarisation structure.
• Shocks can also be present, and can strongly affect local
properties, likely origin of relatively rapid variability in
degree of linear polarisation, polarisation angle in compact
features.
Question: is any rapid/high-energy variability associated
with underlying helical B field, e.g., kinks, reconnection?
How can we distinguish such “magnetic” variability??
Blazars!
Regions of interknot
polarisation corresponding
to transverse jet B fields interpreted as the toroidal
component of a helical B
field
Gabuzda 1999
Pushkarev, Gabuzda,
Vetukhnovskaya &
Yakimov 2005