On the multifrequency data analysis of the innerjet regions

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On the multi-frequency data analysis of the
inner-jet regions
On the multi-frequency (de)polarization analysis

• Bong Won Sohn
• Korean VLBI Network
• Korea Astronomy Space Science Institute

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Motivation

• Polarimetry is an important tool for AGN study
• Highlighted by Jorstads talk
• Diversity in polarization/depolarization
• Also in many talks yesterday today
• Polarization information is essential
• Rotation Measure, RM
• Faraday conversion circular polarization
• Fractional linear polarization, m(?)
• Distribution of Faraday medium
• RM, m(?), CP Burn (1966), Gardner Whiteoak
  (1966), and many authors

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High freq. VLBI in few years

• Improved phase referencing
• More accurate relative position (Agudo,
  yesterday)
• Higher resolution VSOP-2
• Higher sensitivity - KVN (multi-freq.), VERA
  (dual-beam), big dishes (Yebes, SRT, GBT,
  Effelsberg, Nobeyama, ...)
• Full polarization mode (VSOP-2, KVN, ...)
• Of course, VLBA should be there!
• Faint objects can be imaged
• e.g. Middelberg et al. (2005) VLBA with freq.
  switching phase ref. 0.78mJy/beam NGC4261 at
  86GHz
• (much more) sensitive (polarization) imaging of
  Blazars at high frequency
• Recurrent activities in GRGs, Young radio
  galaxies
• Many other sources...

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KVN phase referencing

• Multi-frequency simultaneous observation
• 22, 43, 86, 129GHz ? longer integration time

More about KVN project Rohs talk
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Motivation

• Polarimetry is an important tool for AGN study
• Highlighted by Jorstads talk
• Diversity in polarization/depolarization
• Also in many talks yesterday
• Polarization information is essential
• Rotation Measure, RM
• Faraday conversion circular polarization
• Fractional linear polarization, m(?)
• Distribution of Faraday medium

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Fractional Polarization

• Polarized flux Ip (Q2U2)0.5
• Fractional polarization m Ip/I
• Depolarization measure (DP) mlow/mhigh
• DP lt 1 depolarized
• DP gt 1 re-polarized
• DP 1 fully resolved Faraday screen

Together with RM, the information of magnetic
field thermal electron distribution
Gardner Whiteoak(1966)
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Positivity of Ip (Q2U2)0.5

• Rice distribution distribution of absolute
  value of a Gaussian distribution (V,s) (Vinokur
  1965)
• For , it becomes a Rayleigh
  distribution.
• For , it becomes a Gaussian
  distribution.

Positive noise added to Ip Overestimation of Ip
at low Ip/s
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Corrections for the Ricean bias

• MLH solution (Killeen et al., 1986) better at gt
  6s
• the mode solution (Wardle Kronberg, 1974)
  better at lt6s

High SNR needed ? By integration By sensitive
observation
Killeen et al. (1986)
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Integrated fractional pol.two methods of
estimation
Simple model source

• Flux weighted
• (in the most papers)
• Projected area weighted ( of pixel)
• Sohn et al. in prep.

mIp/I0.09, I0.9Jy 10 projected area
m0.01, I0.1Jy 90 of projected area
mF0.082, mP 0.018
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Testing the two methodsLaing-Garrington effect

• Higher polarization toward jet-side lobes
• Shorter path length though Faraday screen
• Mostly, if not all, flux weighted measure

Depolarization Measure DP mlow / mhigh Path
length asymmetry leads to DPj gt DPcj
Garrington Conway (1991)
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B2 sample revisitedcomparison between mF mP
Laing-Garrington effect seen clearly in both
cases but less Depol.ed in mP ? Jet/hotspot more
depol.ed than lobes 2 of 14 sources cross the
line Fat double with one-sided jet (to wrong
side) Symmetric double lobes with weak jet (to
right side) What if jets/hot spot removed? (in
work)
14 Radio Galaxies from Morganti et al.(1997) Sohn
et al. (in preparation)
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Summary

• RM DP ? better understanding of Faraday medium
  distribution
• Caution for DP mF ? mP
• Clear projection effect in DP(mP) as well
• Jets/Hot spots are more depol-ed than lobes
• Jets/Hot spots reduce ironically the
  Laing-Garrington effect...
• On going works
• Application to VLBI polarimetry
• (with T. Krichbaum, S.S. Lee, A. Witzel, A.
  Zensus)
• Further test on projection effect
• (with U. Klein, K.-H. Mack, R. Morganti)

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Thank you
Byun and KVN members (2.12.2007) On the first day
of the test observation KVN Ulsan telescope Orion
KL SiO maser at 86GHz