Detection of Faraday screen?

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Detection ofFaraday screen?

• Inoue M., Asada K., and Nagai H.
• National Astronomical Obs. of Japan
• Institute of Space and Astronautical Science
• Extragalactic Jets, 21-24 May 2007, Girdwood, AK

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Key words

• Faraday rotation and helical B field
• Faraday screen and sheath
• Absorption feature in precessing jet
• Sheath seen as absorption

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Introduction

• Polarimetric observations have been revealing the
  existence of thermal plasma around AGN jets by
  measuring Faraday rotation measure (RM).
• Gradient of RM across the jet of 3C273 was first
  observed, and interpreted to show a helical
  magnetic field (Asada et al. 2002).
• However, it is not clear the nature of the
  thermal plasma (Faraday screen) responsible for
  the Faraday rotation.
• Although the Faraday screen is suggested to
  surround the jets like sheath (Inoue et al. 2003,
  see poster 15 by Asada et al.), its detection is
  difficult.
• Here, we report a possible detection of the
  sheath around a jet, and propose one method to
  detect it.

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Schematic model of helical B field

• Helical B field explains well both the
  acceleration and collimation mechanisms of jet.
• Asada et al (2002) observed a gradient of RM to
  explain it. They explained also the bimodality
  of B direction (parallel or perpendicular) in
  jets and rotational direction of the accretion
  disk.

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Gradient of RMin 3C273 jet
RM (rad/m2)
20 pc
mas
0 5

• First and second epoch observations of the RM
  gradient (Asada et al. to be submitted).

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Survey program
14 Sources NGC 1052 0212735
0333321 3C 120 0736017 0836710
1055018 1150-002 NGC 4261 WCOMAE
CTA 102 BL Lac 2251158 2355498

• Date 2002, Oct - 2003 March
• Frequency 5, 8, 15 GHz
• Station VLBA 10 stations
• P.I. Asada, K.
• Asada to be submitted

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Some results of the Survey
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Question to be answered

• RM is a function of thermal electron density
  (ne), line of sight component of magnetic field
  (B//), along the propagation path of length L,
• RM ? ?ne B// dL.
• Question
• How is the Faraday screen (sheath) distributed
  around jets?
• How can we observe it?

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Do we see the sheath?

• VSOP images of CTA 102 at 4.8 (left) and 15
  (right) GHz with VLBA. A gap shown by the arrow
  is seen more sharply and clearly at 4.8 GHz,
  while it seems rather continuous at 15 GHz. This
  suggests the absorption feature by the sheath, or
  Faraday screen, in front of the jet.

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Sheath? (contd)
-1.0 0.0 1.0

• Distribution of the spectral index.
• It is, in fact, sharply inverted at the gap. We
  suggest free-free absorption working at the gap
  by the sheath around the jet.

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Sheath? (contd)

• If the jet trajectory is spiral in shape, we
  could see the sheath against jet behind it.

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Sheath? (contd)

• Following Sudou et al. (2000), we estimate the
  optical depth for the free-free absorption
• and then t 1.4.

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Jet configuration

• High spatial resolution at rather low frequency
  is desirable to figure out this type of
  absorption. Space VLBI should be a good tool to
  study this, and VSOP-2 will fit with its very
  high spatial resolution. The project has just
  started!

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Summary

• We find a possible absorption feature produced by
  the thermal plasma (Faraday screen, or sheath)
  surrounding an AGN jet.
• Assuming free-free absorption, we estimate the
  optical depth t1.4.
• When a jet trajectory is spiral in shape like a
  precessing jet, we could observe absorption
  features against the jet itself, which is
  presumably produced by the sheath surrounding the
  jet.
• In general, spiral jets could provide a new
  opportunity to study thermal plasma surrounding
  AGN jets.
• Space VLBI will be a powerful tool to investigate
  such phenomena high resolution across jets
  absorption features
• (see poster 4 for VSOP-2)

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ABSTRACT

• We pointed out a possible way to investigate a
  plasma sheath, Faraday screen, which has been
  revealed recent Faraday rotation studies of AGN
  jets. A high spatial resolution observation of
  VSOP reveals a sharp absorption feature which
  suggests free-free absorption by the plasma
  sheath. The spiral jet configuration could
  provide, in general, an opportunity to
  investigate the nature of the sheath surrounding
  AGN jets.