vlba imaging of the ray emission regions in blazar jets
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VLBA Imaging of the γ -ray Emission Regions in Blazar Jets. or: A Sequence of Images is Worth 1000 Light Curves Alan Marscher Boston University Research Web Page: www.bu.edu/blazars. Main Collaborators in the Study. Svetlana Jorstad, Iván Agudo, & M. Joshi (Boston University)

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vlba imaging of the ray emission regions in blazar jets

VLBA Imaging of the γ-ray Emission Regions in Blazar Jets

or: A Sequence of Images is Worth 1000 Light Curves

Alan Marscher

Boston University

Research Web Page:www.bu.edu/blazars

main collaborators in the study
Main Collaborators in the Study

Svetlana Jorstad, Iván Agudo, & M. Joshi (Boston University)

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) Paul Smith (Steward)

Thomas Krichbaum (MPIfR) + many others

Telescopes: VLBA, GMVA, EVLA, Fermi, RXTE, Swift, Herschel, IRAM, UMRAO, Lowell Obs., Crimean Astrophys. Obs., St. Petersburg U., Pulkovo Obs., Abastumani Obs., Calar Alto Obs., Steward Obs., + many others

Funded by NASA & NSF

diagram of a quasar
Diagram of a Quasar

Gamma-ray emission might occur inside BLR, in 7 mm core, or elsewhere along the jet via scattering of different sources of seed photons by relativistic electrons in the jet

method for locating high frequency emission
Method for Locating High-frequency Emission
  • mm-wave VLBI imaging to follow changes in jet, especially motions of superluminal knots
  • Associate optical, X-ray, or -ray flares with superluminal knot if flare is coincident with passage of knot through 43 GHz “core” (which is parsecs downstream of black hole)
  • Measure optical polarization position angle during flare & match with VLBI feature with similar value of 
  • Determine location of X-ray & gamma-ray emission sites by time lag of high-E variations relative to changes in optical & mm-wave flux or when knot is in core
quasar pks 1510 089 z 0 361 in 2009
Quasar PKS 1510-089 (z=0.361) in 2009

Multiwaveband monitoring: General (but not one-to-one) correspondence between -ray & optical

37 GHz flux starts rising at same time as start of -ray/optical outburst

VLBA images at 43 GHz

Color: linearly polarized intenisty Contours: total intensity

Bright superluminal knot passed “core” at time of extreme optical/-ray flare

Apparent speed = 21c

Time when knot passes through core

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



rotation of optical polarization in pks 1510 089
Rotation of Optical Polarization in PKS 1510-089

Rotation starts when major optical activity begins, ends when major optical activity ends & centroid of superluminal blob passes through core

  • - After rotation, optical pol. angle ~ same as that of superluminal knot
  • Also, later polarization rotation similar to end of earlier rotation, as expected if caused by geometry of B; event occurs as a weaker blob approaches core

Model curve: blob following a spiral path in an accelerating flow

 increases from 8 to 24,  from 15 to 38

Blob moves 0.3 pc/day as it nears core

Core lies > 17 pc from central engine



sites of ray flares in pks 1510 089
Sites of -ray Flares in PKS 1510-089


All flares in early 2009 caused by a single superluminal knot moving down jet

Sharp flares occur as knot passes regions of high photon density or standing shocks that compress the flow or energize high-E electrons

Standing shock system, “core”

Broad-line clouds


Sites of high optical/IR emission in relatively slow sheath of jet

3c 279 in 2008 09
3C 279 in 2008-09

1. Multi-flare outburst in -ray, optical, & X-ray after new superluminal knot appears


2. X-ray dominant flare, peak after new knot appears

3. Simultaneous -ray, optical, & X-ray flare near time when superluminal knot appears



3C 454.3: 2010 super-outburst

Knot ejected in late 2009,

vapp = 10c

RJD=5502, 1 Nov 2010; core: 10.3 Jy

RJD=5507, 6 Nov 2010; core: 14.1 Jy

RJD=5513, 12 Nov 2010; core: 14.2 Jy

RJD=5535, 4 Dec 2010; core: 17.7 Jy

oj287 agudo et al 2011 apjl 726 l13
OJ287 (Agudo et al. 2011, ApJL, 726, L13)

Change in jet direction starting ~ 2005

Core is the more southern compact feature, C0

High-E flare near start of mm-wave flux outburst & ~ coincident with max. in polarization of feature C1, which moves very slowly

Flare B appear to occur as superluminal knot passes through C1

  • Many gamma-ray flares in blazars occur in superluminal knots that move down the jet & are seen in VLBA images
  • Sometimes upstream of 43 GHz core
  • Often in or downstream of 43 GHz core
  • Intra-day γ-ray/optical variability can occur in mm-wave regions
  • The highest-Γ jets are very narrow, < 1°, so at 10 pc from the central engine, jet < 6 lt-months across
  • Doppler factors can be very high, >50 (Jorstad et al. 2005)
  • Volume filling factor of γ-ray/optical emission << 1 if very high-energy electrons are difficult to accelerate (as in turbulent jet model of Marscher & Jorstad 2010)
  • Rotations of polarization & timing of flares agree with magnetic-launching models of jets, where jet flow accelerates over long distances
advantage of wider bandwidth higher recording rate
Advantage of Wider Bandwidth/Higher Recording Rate

Mkn 421

1 Aug 2010


  • High dynamic range (> 100:1) at 43 GHz if core flux > ~0.5 Jy
  • Low sensitivity/dynamic range at 86 GHz
  • Cannot image knots in key TeV blazars such as Mkn 421 with high enough fidelity to measure motion

With upgrade (this year!):

Bandwidth for routine observations ~ 4 times broader

  • High sensitivity  higher dynamic range

24 Oct 2010

4 Dec 2010

emission feature following spiral path down jet rotationn of evpa

Emission feature following spiral path down jet

Emission feature following spiral path down jet - rotationn of EVPA

Feature covers much of jet cross-section, but not all

Centroid is off-center

 Net B rotates as feature moves down jet, P perpendicular to B




P vector