Time Variable Linear Polarization as a Probe of the Physical Conditions in the Compact Jets of Blazars

Alan MarscherInstitute for Astrophysical Research, Boston University

Research Web Page: www.bu.edu/blazars

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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

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 times the speed of light (an illusion)• Knot appeared in April 2011, just as the blazar became bright in γ-rays• Polarization direction of knot rotated with time

black hole

VLBA 15 GHz radio image

4C +71.07

VLBA

5000 ly from black hole

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’ & χ = 90° if Bz’ > Bt’ p depends on Bt’/Bz’

Other angles: qualitatively similar to side-on case

BL Lac: Sketch

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

Helical Magnetic Field with Non-uniform Intensity across Jet

P vector

Bnet

1

2

3

4

Smaller, more intense off-center region gives higher p

Rotation of Optical Polarization in PKS 1510-089Rotation starts when major optical activity

begins, ends when major optical activity ends & superluminal blob passes through core

Direction of optical polarization

Time when blob passes through core

FluxPolarizationOptical

2009.0 2009.5

Model curve: blob following a helical path down helical field in accelerating flow(model by Vlahakis 2006)

increases from 8 to 24, from 15 to 38Blob moves 0.3 pc/day as it nears core

Core lies 17 pc from black hole

- Non-random timing argues against rotation resulting from random walk caused by turbulence implies single blob did all

- 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

Bright superluminal blob passed “core” in early May 2009

Quasar PKS 1510-089 (z=0.361) in 2009

Marscher et al. (2010)

VLBA images at 43 GHz Contours: intensity Colors: polarization

“core”

Quasar PKS 1510-089: first 140 days of 2009

Marscher et al. (2010, Astrophysical Journal Letters, 710, L126)

2009.42009.0

-ray

optical

High gamma-ray to synchrotron luminosity ratio: knot passes local source of seed photons that get scattered to gamma-ray energies

Lower ratio: gamma-rays could come mainly from inverse Compton scattering of synchrotron photons produced in same region of jet

Superluminal knot passes standing shock in “core”

Sites of -ray Flares in PKS 1510-089

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

γ-ray

X-ray

visible light

microwaves

B

right

ness

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 CommonCan be helical magnetic field, random walk of turbulence, or twisted jet

0716+714Larionov et al. 2013, ApJ

3C 454.3Jorstad et al. 2013, ApJ

Rotation continues after peak of γ-ray outburst; consistent with turbulence

3C 279Kiehlmann et al. 2013, EPJ Web of Conf., vol. 62

Quasar 0420-014

Optical pol. flare + χ rotation before γ-ray flare

2 superlumi-nal knots ejected(22c, 13c)

Knot ejections

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

Optical polarization

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

- Rapidly changing brightness across the electromagnetic spectrum-Power spectrum of flux changes follows a power law random fluctuations dominate

X-ray

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 diskEach cell has a random turbulent velocity relative to systemic flow

Conical standing shock

Mach disk (optional)

Looking at the jet from the side

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

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