EHT observations and imaging
of 3C 279 at 20 μas
Jae-Young Kim (MPIfR)
with: T. P. Kricbaum, A. E. Broderick, M. Wielgus, L. Blackburn, J. –L. Gomez, M.
D. Johnson, K. L. Bouman, A. Chael, K. Akiyama, S. G. Jorstad, A. P. Marscher,
S. Issaoun, M. Janssen, C.-K. Chan, T. Savolainen, D. W. Pesce, F. Ozel, et al.

with the EHT collaboration and EHT AGN & MWL WGs

Sendai AGN Jet Workshop, Sendai, Japan, 21 Jan 2020

Layout• Background• EHT observations• Key results – images, rapid variability• Analysis – imaging, dynamic model-fitting• Discussions, interpretations• Conclusions & outlook

Introduction

AGN jet physics on various scales

Key questions to address● Magnetic launching: Blandford & Znajek (black hole) vs. Blandford & Payne (disk)● Magnetic field, velocity stratification, and collimation profile in the acceleration and collimation zone
● Transition between the Poynting (ordered B, accelerating) to kinetically-flux (partially disordered B, conical) regimes.

Polarimetric VLBI imaging at resolution ≲50 μas (≲104 Rs) is requiredSlide from4
EHT AGN WG

High resolution imaging of various AGN jets
Major science motivations:1.

study jet formation for different source classes (FRI/FRII,FSRQ/BLLac), luminosities, BH masses

A simple AGN Unification modela2.

test specific models, BP vs. BZ, magnetic vs. kinetic, etc.
a3.

compare hot accretion, thick disk (SgrA*, RIAF), vs. coldaccretion, thin disk (FSRQs)a4.

location of 𝛄-ray emission, SSC vs ECs; seed photons?
a5.

AGN often highly polarized → B-field in jet launching region
a6.

RM studies → densities and matter composition (internal &ambient)

Padovani+2017 (AARev)

a7.

Address differences in B-field topology and jet speed between
source classes (SgrA*, LLAGN, RGs, BLLacs, FSRQs)

Slide fromEHT AGN WG

3C 279: an archetypal blazar●One of fastest AGN jets observed by VLBI (𝛄 ~ 10-40)●

Many speeds seen in the jet (~10c - 40c) withacceleration, deceleration, and curvatures

Core separation●First AGN jet showing apparent superluminalmotions on monthly timescale (Whitney+71, Cohen+71)Whitney+71(~10c motion)~4 months

Larionov, Jorstad,Marscher, …, Kim JY+ in press

z=0.536
20 μas ~ 1700 RS (for ~109 Msun)

VLBA 43GHz
BU monitoringTime3C 279: an archetypal blazar•

Bright and highly variable gamma-ray emission•

One of first EGRET sources (Fichtel+ 1994),
arguably the most studied gamma-ray AGN•

Rapid gamma-ray variabilities (record holder: 5 min!)•Often requires jet 𝛄 >100 (how to achieve?)•Invoke various models (turbulences, jet-in-jet,hadronic plasma etc)

•Important to identify and determine by VLBI technique:•jet acceleration & collimation zone
•most variable regions•jet composition

Fermi @ >100 MeV

Ackermann+16

Minutes3C 279: an archetypal blazar
Optical polarization●Rapid (optical) polarization variability(e.g., Abdo+10)●

Abdo+10
Kiehlmann+16

EVPA swings over days, often coincident
with fast gamma-ray flares
2 days●●Polarization variability often deterministic(e.g., not random-walk events;
Kiehlmann+16)

High VLBI linear & circular polarization detections●Constrains jet plasma composition; significant
amount of 𝑒 + 𝑒 − pair (e.g., Wardle+99;Homan+09)

Stokes I

Stokes VWardle+98

Past 1mm VLBI observations of 3C279
Pre-ALMA EHT
2011

Lu+13
(CARMA, Hawaii, SMT)

Pre-ALMA EHT
2013

Wagner+15(SMA, SMT, APEX)• Presence of ultracompact jet
features repeatedly found: pilot
1mm VLBI obs. (up to 7Glambda;Lu+13; Wagner+15)• Detailed structure of thosecompact features however
remained elusive (only crude
model-fitting)• Imaging the ultracompactstructure is crucial

Observations& key findings

Kim et al. 2020, submitted

EHT 2017 observationsof 3C 279

EHTC+19a• Full-track observations over four nights in 2017 Apr(as main target and calibrator)• SPT provides maximum N-S baselines: ~8.9 Glambda• Also accompanying MWL VLBI observations by GMVA 86 GHz,VLBA 43/15 GHz• Other dense MWL observations at all wavelengths (radio – TeV)
Comparison: M87 uv-coverage

Kim+20, submitted

Zooming into the core of 3C279~30-40 μaselongation

20 μas = 0.13 pc = 1700Rs●●●

First image of 3C279 at 20 μas resolution (see also EHTC+19c,d)Robust fringe detections up to 8.9 Glambda in all days, mildvariability seen in the amplitudes
The 1mm core structure elongated perpendicular to outer jet –not visible in the low-frequency images

Zooming into the core of 3C279

20 μas = 0.13 pc = 1700Rs●●●

Four days average images over three imaging pipelines (4x3; SMILI, ehtim, Difmap)
The perpendicular core structure persistent over four days
Significant flux variability in the core, and small changes in other details

Substantial time variability!
• CP clearly varies over days; indicating structurechange on inter/intra-day timescales• More pronounced in bigger triangles;
variability mainly comes from small scales• Amplitudes don’t change much
• What is actually varying in images?

Can closure phases vary
just by ~1 μas shift?

Linear motion

Rotation●Yes, they can●

Also by flux changes, butsomewhat weaker effects●High enough SNR make thedata sensitive●

Challenges to traditional
VLBI data analysis

EHT-specific model-fitting analysis tools
Examples fromEHTC+19f• Various MCMC and relevant packages available: Themis (Broderick+20), Dynesty (Pesce+),
GENA (Fromm+19)• Fitting to various EHT measurements possible (closures, baseline visibilities etc)
• “Dynamic” modeling analysis also possible (in progress)

3C279 jet kinematics on daily & μas scales
Themis fully time-variable
Gaussian model-fitting

C0-0

C1-0
(This is a gif animation)

Full posterior distributions from
Themis time-variable model-fitting

Fully dynamical model-fitting -- fluxes, positions, sizes, and their time derivatives

μas scale jet kinematics in detail●

Extremely high S/N → ~1-2 μas levelof positional uncertainty (classically,positional accuracy ~ Beam/SNR).

●

C1 components move southward,
only when C0-0 is used as reference.

●All components move by ~1-2 μas/day(~10-20c) w.r.t. C0-0.

●

App. speeds comparable to VLBA 15/43
GHz monitoring results on mas (pc)scales (e.g., Jorstad+17).

●

C0-1 and C0-2 clearly show non-radial
motions w.r.t. C0-0.

Discussions

Origin of the peculiar subnuclear structure?

3C279 by EHT 230 GHz3C 84, RadioAstron 22 GHz(Giovannini+18)

Broad jet base, perhaps associated with the accretion disk and/oredge-brightened jet, similar to 3C 84 (see Giovannini+18 for more discussions)?

Broad jet base/accretion disk?
AGN jet base/diskBlazars●Viewing angles differ by large amounts:
Radio galaxies (e.g., M87, 3C84, … :~> 20 deg) vs. Blazars (e.g., 3C279:~< 2 deg)

●Accretion disk (or base of a circular jet)would rather appear circular thannarrow elliptical for small viewing angle

RGs

Nearly face-on view of circular
accretion disk (or jet base)edge-on view(thus more elliptical)

Jet cross-sections (Hardee 2000)

Other explanations?
●Many! for example…

●Oblique shocks●

Illuminating patterns (plasma instabilities)
on the jet surface●

Large scale magnetic reconnection (a long,linear string of “plasmoids”)

Magnetic reconnection simulation (Sironi+ 16)

AGN jet ridge lines from RA 22GHz obs.
(Vega-Garcia+20 in press)

Spatially bent jet in 3C279
Jet direction

VLBA 43GHz images of 3C279(Larionov+ in press)• A number of evidence suggestingpresence of curvature in the 3C279 jet(e.g., Homan+03, Jorstad+04,Abdo+10, Aleksic+14, Homan+15, …)• May explain the complex geometry &kinematics without introducing
unnecessary complexity
Fig. from Young 2010

Testing a bent jet model for the “core”

Sketch of bending jet model (from Aleksic+14)

Assuming constant Gamma=20 for
C0-1 and C0-2 (theta ~3 and 1.5 deg).

●

Many suitable families of parameters to fit theapparent speed using a constant Lorentz factor
but varying viewing angles along the jet (by afew degrees.).

●90 deg apparent P.A. change would require~90*sin(1-2 deg) ~ 1.5-3.0 deg jet viewing angle
change; internally consistent values.

Comparison with pre-ALMA EHT obs. of 3C 279
Pre-ALMA EHT
Mar-Apr 2011

VLBA 43 GHz
Apr 2011

VLBA 43 GHz comp. (Jorstad+17)

Core

Observing Year

Lu+13
●In Apr 2011, similar innermost jet feature orientation was
found by pre-ALMA EHT, during very high radio flux state.
a●Later 2011, VLBA 43 GHz saw bright jet component in similar
direction, later aligned to larger-scale jet.
a●2017 EHT observations may be similar.

NB: data point size ~ comp. flux.

What causes jet bending/rotation?
●

Binary SMBHs and/or BH+disk precession (Abraham & Carrara 98; Qian+
19; P~22 yrs for 3C 279) → EHT jet structure similarity between 2011 and2017 would constrain period ~< 6 yrs, ruling out this scenario●

Jet being collimated by interaction with ISM (e.g., Homan+15) →misaligned AGN jet components tend to align with pre-established direction●Excitation of emission going along helical magnetic field in the jet →Inner jet dynamics (e.g., Mertens+16) & polarization (Asada+02;Marscher+08; Hovatta+12; Kiehlmann+16)●

Understanding which one of these works better would require additional
constraints (outlook)How does the EHT 3C279 jet connect to the downstream?
●

EHT jet speeds (~10-20c) arecomparable to historical masscale speed measurements(before 2017)●

Constrains jet acceleration
zone to be potentially< 3000Rs projected(~< 8x10^4 Rs deprojected)●

However in 2017 the 3C279 jet
was exceptionally fast at ~>0.5mas; ~37c speed (Larionov+20in press)

Brightness temperature in the nuclear region• Observed Tb ~ 10^(10-11) K → Intrinsic T’b ~< 10^(9-10) K considering Doppler factors ofat least ~10-20• Tb ~> 10^12 K and T’b ~ 10^11 K at lower frequencies (Kovalev+05; Jorstad+17)• EHT values far below Compton limit (~5x10^11 K) and the equipartition value (~5x10^10 K)

Interpretations of the low T’b• Much depends on the core opacity at 230 GHz• Optically thick (turnover ~ 1mm)• may represent some upstream emission of the jet. Signature of under-accelerated(thus slow) jet underlying the 1mm core.
• Also T’b ~10^(9-10)K << T’b,eq ~ 5x10^10 K: more magnetic energy dominated flow
(cf. kinetic power dominated)• On the other hand, Tb may simply decrease with freq. if 3C279 becomes
optically thin @ 1mm• EHT spectral index measurement between 230 and 345 GHz would be
helpful (also EHT polarimetry).

Conclusions• EHT imaging analysis of the 3C 279 jet at 230 GHz• Peculiar “perpendicular” nuclear structure• Several physical interpretations• Rapid day-to-day closure phase variations; ~1-2 uas/day motions• Also complex kinematics; spatially bent jet provides good explanations• Unexpectedly low intrinsic brightness temperature (~< 10^10 K)• May be a signature of very low jet opacity at 230 GHz• Still some details quite remain elusive• How to improve / what to do more?

Outlook

EHT and MWL
connections
Plot removed●Significant radio flux variability within theEHT subnuclear structure within ~1 week;
factors ~2 or more changes●Also rapid gamma-ray variability; details
under analysis

Constraining the curved jet structure•••Rapid linear polarization variability (both mL and EVPA)
Indicates small variable size and ordered evolution ofstructure (associated with motions?)
Can independently reconstruct the curved jet parameters
Optical polarization of 3C279

Plot removed
Figure from Aleksik+2014;
Detailed model from Nalewajko 2010

Note: lines are from predictions, NOT fittingComparison uv-coverages 2017 - 2020 (for 3C279)
EHT 2017

8 stations

EHT 2020

11 stations(+GLT+KP+NOEMA)50 uasEHT 2017
DR ~ 50

50 uas

EHT 2020
DR ~ 350

50 uasNote: the simulations
assume ideal conditions(no gain errors)

Larger scale

Inner region

Truth

200 uas200 uas200 uasFinal outlook – hope to have soon
• Science side:• MWL / polarization• More sources (many already observed in 2017; all under analysis)• Instrument side:• Expanding EHT in 2020
• e-KVN and EAVN-high 230 GHz• Will provide a lot of insights about future EHT time-variable studies• People, more people!