VSOP-2 Detection of Faraday screen? Inoue M., Asada K.*, and Nagai H. National Astronomical Obs. of Japan * Institute of Space and Astronautical Science Extragalactic Jets, May 2007, Girdwood, AK VSOP-2

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

VSOP-2 3 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.

VSOP-2 4 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.

VSOP-2 5 Gradient of RM in 3C273 jet RM (rad/m 2 ) 20 pc mas0 5 First and second epoch observations of the RM gradient (Asada et al. to be submitted).

VSOP-2 Survey program Date: 2002, Oct March Frequency: 5, 8, 15 GHz Station: VLBA 10 stations P.I.: Asada, K. Asada+ to be submitted 14 Sources: NGC C NGC 4261 WCOMAE CTA 102 BL Lac

VSOP-2 Some results of the Survey

VSOP-2 8 Question to be answered RM is a function of thermal electron density (n e ), line of sight component of magnetic field (B // ), along the propagation path of length L, RM  ∫n e B // dL. Question: How is the Faraday screen (sheath) distributed around jets? How can we observe it?

VSOP-2 9 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. Gap here

VSOP-2 10 Sheath? (cont’d) 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

VSOP-2 11 Sheath? (cont’d) If the jet trajectory is spiral in shape, we could see the sheath against jet behind it.

VSOP-2 12 Sheath? (cont’d) Following Sudou et al. (2000), we estimate the optical depth for the free- free absorption; and then τ~ 1.4.

VSOP-2 13 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!

VSOP-2 14 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 τ~1.4. 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)

VSOP-2 15

VSOP-2 16 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.