Helical Magnetic Fields Associated with the Relativistic Jets of Four BL Lac Objects

1. (1/17) Helical Magnetic Fields Associated with the Relativistic Jets of Four BL Lac Objects Gabuzda, Murray & Cronin astro-ph/0405394

2. (2/17) Introduction (1/5) VLBI polarization observations of radio-loud BL Lac objects have shown a tendency : polarization E vectors in parsec-scale jets are parallel to the local jet direction. →Since the jet emission is optically thin, corresponding B vectors are perpendicular to the jet direction (Gabuzda, Pushkarev, & Cawthorne 2000).←Shocks? (Laing 1980; Hughes, Aller, & Aller 1989) The transverse B fields are perpendicular to the jet direction over extensive sections, even in the the presence of appreciable bending. ←The shock model seems contrived.

3. (3/17) Introduction (2/5) The degree of dominance of the toroidal component and the viewing angle will affect strongly the observed B field structure if a jet has a helical B field (cf. Uchida et al. 2004 ←Calculated from the result of a MHD simulation). Theoretical study for the polarization and the structure of relativistic parsec scale AGN jets including the relativistic effect are done by Lyutikov, Pariev, & Gabuzda astro-ph/0406144.

4. (4/17) Introduction (3/5) A number of compact AGN displaying just such “spine+sheath” B-field structures have been observed. 1055+018 : Attridge, Roberts & Wardle 1999 They concluded that this structure can be interpreted in the shock paradigm as representing jets with series of shocks. But concerning the relativistic effect, the hypothesis that some AGN jets have helical B fields can also provide a simple explanation for the “spine+sheath” structure. Click

5. (5/17) Introduction (4/5) It is therefore of interest to identify robust observational tests that can distinguish between transverse B fields due to a toroidal field component and due to shock compression. ↓ The observation of Faraday rotation

6. (6/17) Introduction (5/5) In the case of a toroidal or helical jet B field, we should observe a gradient in the observed Faraday rotation across the jet (e.g., Asada et al. 2002, Asada et al. 2003, private communication).

7. (7/17) Result (1/5) Before making the RM maps, they first removed the contribution of the known integrated (predominantly Galactic, i.e., foreground; Pushkarev 2001) Faraday rotation at each wavelength, so that any remaining non-zero rotation measure should be due to thermal plasma in the vicinity of the AGN.

8. (8/17) Result (2/5)

9. (9/17) Result (3/5)

10. (10/17) Result (4/5)

11. (11/17) Result (5/5)

12. (12/17) Discussion The result (c.f. figure of Result (4/5)) in this paper is consistent with that in Croke et al. (2004) and Croke et al, in preparation (A transverse RM gradient with the same sense and covering roughly the same range of rotation measure). The presence of toroidal or helical B fields suggests that a significant fraction of jet energy may be carried by the ordered component of the magnetic field. How about the relationship between the circular polarization in the core region and RM gradient ? (these may associated with the rotation of the central black hole and accretion disk)

13. (13/17) Radio Jet-Ambient Medium Interactions on Parsec Scales in the Blazar 1055+018 Attridge, Roberts, & Wardle 1999, ApJ, 518, L87

14. (14/17) Introduction In the jet the magnetic fields parallel and perpendicular to the jet direction are observed. B⊥ : the result of transverse shock (Hughes, Aller, & Aller 1985). B|| : interaction between the jet and the ambient medium (Wardle et al. 1994).

15. (15/17) Result “Spine-shear layer” morphology The magnetic field Fractional linear polarization

16. (16/17) Comparison with Uchida et al. (2004)

17. (17/17) Discussion The authors suggest that the spine and the sheath have significantly different Lorentz factors: The spine : fast-moving At the small angle to the line-of-sight, θ<θcrit ,the radiation from the spine would be dominant.  Toroidal B field At θ>θcrit Longitudinal magnetic field