Transcript PPTX
Time Variable Linear Polarization as
a Probe of the Physical Conditions in
the Compact Jets of Blazars
光の恋人よりも高速
Alan Marscher 会議のおじいさん
Institute for Astrophysical Research, Boston University
Research Web Page: www.bu.edu/blazars
Partial List of Observational Collaborators
Svetlana Jorstad, Manasvita Joshi, & students (Boston U.)
Iván Agudo (JIVE) José Luis Gómez & students (IAA, Spain)
Valeri Larionov (St. Petersburg State U., Russia)
Margo & Hugh Aller (U. Michigan) Paul Smith (Steward Obs.)
Anne Lähteenmäki (Metsähovi Radio Obs.)
Mark Gurwell (CfA) Ann Wehrle (SSI)
+ many others
Telescopes: VLBA, GMVA, EVLA, Fermi, RXTE, Swift, Herschel,
IRAM, UMRAO, Lowell Obs., Crimea, St. Petersburg U.,
VERITAS, Abastumani, Calar Alto, Steward, + many others
Funded by NASA & NSF
Goal: Probe jets as close to black hole as possible
Questions we want to answer:
How are jets accelerated to near the speed of light & focused to within <1°?
- Test theory that helical magnetic fields propel & confine the jets
Where and how do extremely luminous outbursts of radiation occur?
How are relativistic particles accelerated: in shocks, reconnection, turbulence?
Quasar 0836+710 (4C +71.07), γ-ray Blazar, z=2.17
black hole
Our VLBA images reveal a new bright knot (seen best in polarized emission, in
color on images) that moves away from the black hole at apparent speed of 20
4C +71.07
times the speed of light (an illusion)
• Knot appeared in April 2011, just as the blazar became bright in γ-rays
5000 ly from
• Polarization direction of knot rotated with time
black hole
VLBA 15 GHz radio
image
VLB
A
Apparent Speed = 20c
Flares in γ-ray & optical are associated with knot
Direction of optical polarization rotates along with
direction of polarization of radio knot.
Therefore, optical emission comes from radio knot during
flare
γ-ray & optical flares
occur simultaneously.
Therefore, they are
produced in the same
location
We can conclude that the γ-ray, optical, and radio flares all come
from the moving knot, which is located 20 pc from the black hole
during the late-2011 flares
Not in central parsec, as previously thought
Basics of Linear Polarization: Uniform Magnetic Field
Polarization vector p _ Bproj , p = pmax = 3(α+1)/(3α+5) (0.75 for α=1)
α = (s-1)/2, N(E)=NoE-s
If magnetic field is uniform & ν > νSSA, ν > νFR (generally OK if ν > 200 GHz)
(e.g., Pacholczyk 1970, Radio Astrophysics)
If ν < νSSA, p ||Bproj , p = 3/(12α+19), nearly 8 times lower
Since field is uniform, no significant variations occur unless the direction
of B changes or there is a transition between optically thick & thin
Basics of Linear Polarization: Case of Magnetic Field that Is
Random on Small Scales unless Compressed
No compression: p ≈ 0
If compression by a shock with η = npost-shock/npre-shock
p ≈ pmax(η2-1)sin2θ’/[2η2-(η2-1)sin2θ’]
(ν > νSSA, θ’>>0)
(e.g., Hughes & Miller 1991, Beams & Jets in Astrophysics)
θ’ = viewing angle, measured in plasma frame; because of relativistic
aberration:
sinθ’ = sinθ/[Γ(1-βcosθ)], so θ’=90° when cosθ=β (tanθ=1/Γ)
which is the viewing angle at which apparent velocity is maximized
p ||shock normal as projected on sky
Basics of Linear Polarization: Cells with Random Field Directions
Case of N cells, each with a uniform but randomly directed magnetic
field of same magnitude
Mean polarization: <p> = pmax/N1/2 σp ≈ <p>/2 (Burn 1966, MNRAS)
Electric-vector position angle χ can have any value
If such cells pass in & out of emission region as time passes,
p fluctuates about <p>
χ varies randomly, often executing apparent rotations that can be
> 180°, usually not very smooth, but sometimes quite smooth
(T.W. Jones 1988, ApJ)
Basics of Linear Polarization: Helical Magnetic Field
Assume that helical field propagates down the jet with the plasma (as in
MHD models for jet acceleration & collimation)
B’ = Bt’ cosϕ i ’ + Bt’ sinϕ j ’ + Bz’ k ’
Degree of polarization depends on viewing angle & Γ
(see Lyutikov, Pariev, & Gabuzda 2005, MNRAS)
Face-on (θ = θ’ = 0): p = 0 (from symmetry) if Iν is uniform across jet
Side-on (θ’ = 90°): χ = 0° if Bz’ < Bt’ &
p depends on Bt’/Bz’
χ = 90° if Bz’ > Bt’
Other angles: qualitatively similar to side-on case
BL Lac:
Sketch
Helical Magnetic Field with
Non-uniform
Intensity across Jet
Face-on case
Red area: higher intensity than blue area
Centroid is off-center
Net B, & therefore net p depends on location in cross-section
1
3
Bnet
Smaller, more
intense off-center
region gives higher p
P vector
2
4
Rotation of Optical Polarization in PKS 1510-089
Optical
Rotation starts when major optical activity
begins, ends when major optical activity ends
& superluminal blob passes through core
Flux
Polarization
- Non-random timing argues against rotation
resulting from random walk caused by
turbulence implies single blob did all
-
Direction of
optical
polarization
Model curve: blob following a helical path
down helical field in accelerating flow
(model by Vlahakis 2006)
Time when
blob passes
through core
2009.0
Also, later polarization rotation similar to end
of earlier rotation, as expected if caused by
geometry of B; 2nd event occurs as a weaker
blob approaches core
2009.5
increases from 8 to 24, from 15 to 38
Blob moves 0.3 pc/day as it nears core
Core lies 17 pc from black hole
Quasar PKS 1510-089 (z=0.361) in 2009
VLBA images at 43 GHz
Contours: intensity Colors: polarization
Bright superluminal blob passed “core” in early May 2009
“core”
Marscher et al. (2010)
Quasar PKS 1510-089: first 140 days of 2009
High gamma-ray to
synchrotron luminosity ratio:
knot passes local source of
seed photons that get
scattered to gamma-ray
energies
-ray
Lower ratio: gamma-rays
could come mainly from
inverse Compton scattering of
synchrotron photons
produced in same region of
jet
optical
Superluminal knot passes
standing shock in “core”
2009.0
2009.4
Marscher et al. (2010,
Astrophysical Journal
Letters, 710, L126)
Sites of -ray Flares in PKS 1510-089
Quasar PKS 1510-089: Repeated Outbursts
Brightness
As we observe longer with Fermi, etc., we can look for repeated
patterns to discern between transient phenomena and effects
caused by long-lived structure in the jet
γ-ray
X-ray
visible light
microwaves
If this interpretation is correct, later
outbursts in PKS 1510-089 should show
similar rotation of polarization in same
direction as before
Quasar PKS 1510-089: Repeated Outbursts
As we observe longer with Fermi,
etc., we can look for repeated
patterns to discern between
transient phenomena and effects
caused by long-lived structure in
the jet
Outburst in 2012 shows similar rotation of
polarization in same direction as before,
contemporaneous with the passage of a new
superluminal knot through the core at 43 GHz
(Aleksic et al. 2014, ApJ, submitted)
Rotations of Polarization Vector Are Common
Can be helical magnetic field, random walk of turbulence, or twisted jet
0716+714
Larionov et al. 2013, ApJ
3C 454.3
Jorstad et al. 2013, ApJ
Rotation continues after peak of
γ-ray outburst; consistent with
turbulence
3C 279
Kiehlmann et al. 2013, EPJ
Web of Conf., vol. 62
Quasar 0420-014
Optical pol.
flare + χ
rotation
before γ-ray
flare
Knot ejections
2 superluminal knots
ejected
(22c, 13c)
Quasar OJ248 (0827+243)
2 optical polarization outbursts at starts of rotations during
γ-ray outburst, contemporaneous with ejection of
superluminal (13c) knot
The TeV-emitting Quasar 1222+216
This quasar’s optical emission is usually dominated by the big blue
bump, so p > 2% is high; note that long rotation is after ejection of B1
Movie of 1222+216
During most extreme γ-ray activity, core
brightens but only weak knots emerge
During less extreme but active periods,
bright knots do emerge
Perhaps inverse Compton losses
suppress emission from the most
energetic knots after they pass through
core
Quasar CTA102: Looks like turbulence
Polarization varies erratically, as expected if it results from
turbulence
Blazar BL Lacertae in 2011: Looks like turbulence
γ-rays become bright as new superluminal knots
pass through “core” & through 2 other stationary
emission features on the VLBA image
Degree of
linear
polarization
& variations
in degree &
position
angle
suggest
turbulence
at work
Possible Blazar Model
- Helical magnetic field out to parsec scales, then turbulence (+ maybe reconnections)
dominates
- Flares from moving shocks and denser-than-average plasma flowing across standing shock or
region where reconnections occur
Turbulence in Blazar Jets
Possible source of turbulence: current-driven instabilities at
end of acceleration/collimation zone (e.g., Nalewajko &
Begelman 2012, MNRAS)
Note: turbulence can set up conditions for magnetic
reconnections to occur
Cawthorne (2006, MNRAS), Cawthorne et al. (2013, ApJ):
“Core” seen on 43 GHz VLBA images has polarization pattern
similar to that of turbulent plasma flowing through a
standing, conical-shaped shock
In Support of Turbulence: Power-law PSDs Noise process
X-ray
- Rapidly changing brightness
across the electromagnetic
spectrum
-Power spectrum of flux
changes follows a power law
random fluctuations
dominate
Chatterjee et al. 2008 ApJ
Turbulent Extreme Multi-zone (TEMZ) Model: Turbulent Plasma
Crossing Standing “Recollimation” Shock (Marscher 2014, ApJ)
Many turbulent cells across jet cross-section, each followed after crossing
shock, where e-s are energized; seed photons from dusty torus & Mach disk
Each cell has a random turbulent velocity relative to systemic flow
Published version: each column of cells
has unrelated field direction, every 10th
cell along column has new, random
field direction, with smooth rotation in
between
Mach disk (optional)
Looking at the jet from the side
Conical standing shock
Revised TEMZ Code
Cells are nested in 4 zones of sizes 1, 23, 43, & 83 cells, with contribution
of each zone’s B to total B proportional to (zone size)7/4
(Kolmogorov-Kraichnan spectrum; T.W. Jones 1988, ApJ)
Direction of B is selected randomly at zone boundaries and rotated
smoothly in between
Next step: add Kolmogorov spectrum of magnetic field strength &
electron density (current version: no variation in field strength, electron
density varies randomly according to observed power spectrum of flux
variations)
Electron Energy Distribution in TEMZ Code
Power-law (slope= –s) injection into cell that is crossing the shock front
-Synchrotron & external Compton energy losses downstream of shock
-Maximum injected electron energy depends on angle between
magnetic field & shock normal
- This restricts optical & γ-ray emission to a small fraction of cells near
shock front
Spectral index steeper than s/2 (radiative loss value), as observed
Mean polarization is higher & fluctuations greater at higher
frequencies, as observed
Optical & γ-ray flux variability more pronounced than in mm-IR &
X-ray
Observed Polarization Decreases with Wavelength
3C 454.3 during brightest state (Jorstad et al. 2013)
- Expected if fewer turbulent cells are involved in emission
at shorter wavelengths
Sample Simulated Light Curve Similar to BL Lac
Outbursts & quiescent periods arise from
variations in injected energy density
- Random with probability distribution
determined by red-noise power spectrum
- Next slide magnifies 50-day outburst
Polarization is stronger at higher frequencies
Position angle fluctuates, but is usually within 20°
of jet direction (as observed in BL Lac)
Sample Simulated Light Curves during 50-day Outburst
Note general correlation but frequent deviations from one-to-one correspondence, smoother variations at
lower frequencies similar to actual data
*** Intra-day variations are reproduced, since cells are small and turbulent relative velocities increase the
Doppler beaming factor of some cells
Further Development of TEMZ Code
Next step: add organized magnetic field component: helical, ||jet
Longer-term:
Add other sources of seed photons: emission-line clouds alongside jet,
jet sheath, synchrotron emission from other cells (true SSC)
Adapt code to calculate emission from MHD simulations
- Relate physical conditions to geometry of standing shock, presence &
size of Mach disk
Incorporate more refined shock acceleration schemes
Simulate magnetic reconnections (need more development of
relativistic reconnections by others)
CONCLUSIONS
The combined international effort is now producing optical
polarization data with sufficient time coverage to follow variations in
dozens of blazars
- The work of the group in Hiroshima & ROBOPOL in Crete are
welcome additions to this effort
- We are identifying patterns – some apparently systematic, others
apparently random – that we can interpret in terms of physical
properties of the jets
- It would be highly beneficial to combine the datasets, perhaps by
setting up a central website
- Why not? The ratio of interested astronomers to number of
monitored blazars is quite low, so there are many potential papers &
PhD dissertations that would have little or no overlap
- Better theoretical modeling (to compete with TEMZ!) is needed