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3.1. The case of Centaurus A (NGC 5128)

The interesting case of Centaurus A has been much discussed because of its proximity at 3.8 Mpc, much less than a GZK radius, and the statistical likelihood of its coincidence with a slight excess of AUGER events within a few degrees - see Figure 6. Also it belongs to a category of AGN-powered radio sources that are plausible candidates as UHECR acceleration sites.

Figure 6

Figure 6. A plot in (l, b) of the CR events above 5 × 1019 eV as of 2007 [12]. Dashed lines show the supergalactic plane, and the inset shows a lambda6.3 cm image of the giant jet-lobe radio source Centaurus A [13], in a version kindly provided by Patricia Reich.

If this association is real, the scatter of arrival directions of a few degrees around the Cen A position is entirely plausible, given the expectation of the propagation path - which includes the environment of Cen A itself, ~ 3.5 Mpc of intergalactic pathlength from Cen A, and finally the local Milky Way environment.

For small angle deflections theta(E) over a path l containing B fluctuations on a scale of l0(<< l) and a UHECR nucleus of charge Z, the deflection is

Equation 2 (2)

Inserting l = 3.8 Mpc, B of 10-7 - 10-8 G, E = 1020 Z = 1, and a guess of l0 ~ 0.3 Mpc brings us to a likely deflection range of a few degrees. Adding Fe into the mix of CR nuclei would greatly increase theta for the iron events, given the other assumed parameter values adopted above. Therefore, concentration of events within 10° - 20° of Cen A's direction would seem to rule out a 100% Fe contribution.

Recent Faraday RM data for the Milky Way suggest a very small RM in the Galactic halo above our disk location of 8kpc from the Galactic center (e.g. [3] and Mao et al. [14]). A recent RM study of the angular vicinity of Cen A (Feain et al. [15]), in which Cen A's outer radio contours will be recognized from Fig. 6), finds at most a very small RM contribution from the immediate intergalactic environment of Cen A (Figure 7). The fact of this small RM perturbation around the Cen A direction may be encouraging for the interpretation of UHECR arrival directions, given that there are no strong deflections beyond what we might expect from the Milky Way. The MW component probably comes from a propagation path mostly within ~ a disk z-height of approx 3 kpc [3]. All of this assumes that Cen A (and other AGN BH jet-lobe sources) are indeed sources of UHECR's. The next section briefly examines evidence for this possibility.

Figure 7

Locations and RMs of the 281 sources in Table 3. To better highlight the variations, the diameter of the sources represent the amplitude of their residual RM after the mean RM of the whole distribution (-57 rad m-2) has been subtracted. Black and white sources are those with positive and negative residuals from the mean, respectively. Overlaid are Parkes 1.4 GHz radio continuum contours of Centaurus A. Contour levels are 1.5, 2, 3, 4, 5, 6, 10, 100 Jy beam-1. The legend on the right hand side of the figure shows the relation between the source diameter and the absolute value of the mean-subtracted RM in units of rad m-2.

Figure 7. Background RM's in the direction of Cen A in rad m-2, from [15], including their original caption. Positive and negative RM values are shown black and white, respectively.

3.2. 3C 303 - a possible UHECR acceleration site

The magneto-plasma parameters of the radio and X-ray jet emission knots of 3C 303, a radio galaxy at z = 0.141, have been diagnosed from multi-frequency observations [16]. Figure 8 shows three radio (and X-ray) emitting "knots" over a total distance of 28 kpc. Each knot's projected magnetic field structure is remarkably coherent, with an average projected magnetic field direction close to the jet axis. The visible "cocoon" around the jet is ~ 1 kpc thick, and contains a Faraday-rotating, non-relativistic electron component whose plasma beta is ~ 10-5 T8 (T8 in units of 108 K), so that its Alfvén speed is in the relativistic régime. |B| in the cocoons is ~ 3 × 10-3 G at ~ 0.5 kpc from the jet axis. Combining this with the 2 kpc knot length on a Hillas plot suggests that 3C 303's jet knots are potential acceleration sites for CR nuclei up to ~ 1019-20 eV. That is,

Equation 3 (3)

Figure 8

Figure 8. A 5 GHz VLA image of the main jet in 3C 303 at 0.3" resolution. Lines show the Faraday rotation-corrected intrinsic polarization direction (i.e. normal to the projected B-orientation).

The energy flow along the jet is ~ 3 × 1043 tau7-1 ergs s-1 (tau7 is the total outflow time in units of 107 yr) [16]. The jet current has been determined at ~ 1018 ampères along one of the knots [17]. This is the first direct observational estimate of extragalactic jet current, and is probably the largest current yet measured. Although the detailed mechanism for accelerating the CR's is not specified by the above, it would probably be in a j|| configuration in the highly aligned field by a current carrier starvation process [18], rather than a "conventional" Fermi-type shock mechanism. The expected natural fall-off of field strength B(r) --- propto r-1 --- with transverse distance from the invisible, thin energy "pipe" in Figure 8 implies that the above reference value of ~ 1 mG is location-variable within the jet cocoon.

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