Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.atnf.csiro.au/people/Enno.Middelberg/pubs/middelberg2005b/middelberg2005b.pdf Дата изменения: Thu Jun 3 03:55:05 2004 Дата индексирования: Wed Dec 26 14:12:09 2007 Кодировка: Поисковые слова: cygnus

Future Directions in High Resolution Astronomy: The 10th Anniversary of the VLBA ASP Conference Series, Vol. ***, 2003 J. D. Romney & M. J. Reid (eds.)

Where has all the p olarization gone?
E. Middelb erg, A. L. Roy, U. Bach Max-Planck-Institut fur Radioastronomie, Bonn, Germany Ё D. C. Gabuzda Physics Department, University Col lege Cork, Ireland T. Beckert Max-Planck-Institut fur Radioastronomie Ё 1. Intro duction

Circumnuclear tori are a central ingredient in the unification of the AGN phenomenon, but the conditions in the tori, the jet collimation, and the accretion mechanisms are still p o orly constrained. Magnetic fields are involved in jet collimation and probably in feeding material into the nucleus, but those that are derived using equipartition are uncertain since equipartition conditions are not known to hold. A more direct measurement of magnetic field strength can b e made using Faraday rotation (FR) and free-free absorption (FFA). FR changes the electric vector p osition angle of a p olarized wave passing through a magnetized plasma by an angle = Rm 2 , where the rotation measure Rm is the path integral over the line-of-sight comp onent of the magnetic field, B , and the density of thermal electrons, ne . FFA dep ends on ne , the path length, L, and the electron temp erature, Te . The value of L can b e estimated by assuming it to b e the same as the width of the region of FFA in the VLBI images. Making reasonable assumptions ab out Te , estimates of ne can also b e derived from the FFA measurements. Thus, a joint analysis of FR and FFA measurements can provide direct diagnostics of the magnetic-field strength B with minimum imp osed assumptions. 2. Sample and Results

We selected all five AGNs that we found in the literature that had steeply rising sp ectra across parts of the jet at p c scales. In all cases, the absorption was most likely FFA due to a p c-scale foreground absorb er, p erhaps the ionized inner edge of an obscuring torus or an accretion flow. The sample comprises NGC 1052 (LINER), NGC 4261 (FR I), Centaurus A (FR I), Hydra A (FR I) and Cygnus A (FR I I). Polarimetric observations were carried out with the VLBA 1 at 15.4 GHz
1

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

1


2
Relative Dec / mas 3 2 1 0 -1 -2 -3

Middelb erg et al.

4

2

0

-2 -4 Relative RA / mas

-6

-8

Figure 1. Uniformly weighted 15 GHz image of Cyg A with sup erimp osed p olarization vectors. with 60 min to 240 min integration time p er source to see whether p olarized emission is present b efore making FR observations. Only Cyg A (Fig. 1) showed significant linear p olarization, having 1.4 mJy b eam p olarized emission at the p osition of the total intensity p eak flux density of 315 mJy b eam-1 (0.4 %). With a detection threshold of 0.3 %, all other sources app eared entirely unp olarized, in the absorb ed gaps as well as in all other locations along their jets. As the emission pro cess is undoubtedly synchrotron emission (given the high brightness temp eratures), the lack of p olarized emission in these sources needs explanation.

-1

3.

Discussion

We suggest several intrinsic and extrinsic mechanisms to dep olarize the emission. Tangled internal magnetic fields: Assume the source is optically thin and its magnetic field is comp osed of a uniform comp onent B 0 and a random comp onent Br . Provided that Br varies on scales much less than the source diameter and that the electrons have a p ower-law energy distribution with index , the intrinsic degree of p olarization p( ) will b e averaged over the source 2 2 2 to pi = p( )(B0 )/(B0 + Br ) (Burn 1966). For = 2, the intrinsic degree of p olarization is 70 %. To dep olarize this b elow our detection threshold, the magnetic field energy would need to b e extremely dominated by the random comp onent, and so the jets would have to b e turbulent, and very little ordering of the magnetic fields by the overall outward motion of the jet flow would b e p ermitted. In case the source is optically thick, the maximum intrinsic degree of p olarization is 10 % to 12 %, and we receive emission only from the surface. For the source to app ear dep olarized, the scale on which the surface magnetic field is tangled must b e much smaller (< 1/10) than the observing b eam. This explanation is unsatisfactory b ecause the magnetic fields inside the jets must b e ordered to confine them, but the surface must b e turbulent, and why the transition o ccurs is unexplained.


APS Conf. Ser. Style

3

Internal Faraday rotation: In the transition region b etween optically thick and optically thin source parts, internal FR could b e significant. Polarized emission from various depths along the line of sight through the source are Faraday rotated by the source itself, the degree of rotation dep ending on the depth of the emitting region. However, internal FR is also unsatisfactory for explaining our observed lack of p olarization b ecause it requires a significant fraction of "cold" (min 1 - 10) electrons in the jet. Internal FR pro duces a characteristic dep endence of p olarization fraction on frequency, but unfortunately, we are not able to test for the exp ected wavelength dep endence b ecause we lack the required measurements of the p olarization fraction at several wavelengths. Bandwidth dep olarization: This dep olarization mechanism requires very high, homogeneous RM (106 rad m-2 ), and such conditions should also pro duce strong FFA which is not seen along most of the dep olarized jets. A p ossible way out is if the Faraday screen/absorb er is very extended or hot, or b oth. Beam dep olarization: If the magnetic field in a foreground Faraday screen is tangled on scales much smaller than the observing b eam, regions with similar degrees of p olarization but opp osite signs will average out and the observed degree of p olarization will b e decreased. Thus, with spatially highly variable RM, one could in principle dep olarize the source, although the changes in RM from region to region still need to b e of the order of 10 4 rad m-2 to dep olarize the 15 GHz band. If one requires at least 10 cells across the b eam, the typical cell sizes in NGC 1052, NGC 4261, Cen A, Hyd A and Cyg A need to b e 0.01 p c, 0.02 p c, 0.002 p c, 0.12 p c and 0.13 p c, resp ectively. Since these are the least exotic conditions required by any of the mechanisms discussed, we feel that b eam dep olarization in an external medium is the most likely mechanism to dep olarize the sources presented here. 4. Summary

Compact core-dominated AGN such as BL Lac ob jects and quasars typically display linear p olarization of a few to a few tens of p ercent on p c scales, with the degree of p olarization o ccasionally approaching the theoretical maximum of 70 %. Thus, some AGN jets have comparatively mo dest dep olarization. In contrast, the p c-scale structures of the five radio sources considered here exhibit strong dep olarization at high frequencies. The weakness of their p c-scale p olarization indicates that most likely tangled jet magnetic fields on sub-p c scales in a foreground screen causes b eam dep olarization, although we lack enough information to conclusively decide among the discussed mechanisms. Why that screen is present in these ob jects but absent in most core-dominated AGNs might b e an orientation-selection effect. Core-dominated ob jects are viewed preferentially p ole-on, and then the lack of dep olarization means that the Faraday screen lies in the equatorial plane. The sample of jets selected here is viewed preferentially face-on, making the Faraday-screen viewed against the source. References Burn, B. J. 1966, MNRAS, 133, 67