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Astron. Astrophys. 356, L13-L16 (2000)

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3. The nature of the `canals' in polarized intensity

The strong and abrupt decrease of polarized intensity in the `canals' suggests that depolarization is responsible. There are several mechanisms that can produce depolarization, but the only plausible type in this case is beam depolarization. This occurs when the polarization angle varies significantly within a beam. Complete depolarization requires that for each line of sight there is a `companion' line of sight within the same beam that has the same polarized intensity but for which the polarization angle differs by 90o. Below we will show that our observations indicate that the polarization angle indeed changes by large amounts across low polarized intensity `canals', and close to 90o across the `canals' of lowest P.

Depolarization can also be caused by `differential Faraday rotation'. This happens when along a line of sight emitting and (Faraday) rotating plasmas coexist (e.g. Burn 1966; Sokoloff et al. 1998). However, the absence of correlated structure in Stokes I and the high degree of polarization suggest that this is not a dominating effect. Significant bandwidth depolarization, which occurs when the polarization angle is rotated by greatly different amounts in different parts of a frequency band could only play a rôle (given our 5 MHz bandwidth) if the RM were of order 80 rad m-2, which is not the case in this region near the galactic anti-centre (see below).

In Fig. 2 we show the polarization vectors around a few of the deepest `canals', superimposed on gray-scale plots of P, in two frequency bands. The area shown is indicated in Fig. 1. The polarization vectors on either side of the `canals' are quite close to perpendicular, demonstrating that the `canals' are produced by beam depolarization. This perpendicularity applies to all `canals', irrespective of frequency band and is very convincing, especially because everywhere else the polarization vectors vary quite smoothly (if significantly!).

[FIGURE] Fig. 2. Polarized intensity P at 349 MHz (left) and 360 MHz (right) of the area inside the box in Fig. 1. Polarization angles and intensities are indicated by the vectors, which are sampled at locations 4´ apart (independent beams). Note [FORMULA] across low-P `canals'.

Beam depolarization creates `canals' that are one beam wide, which is exactly what we observe. This implies that the 90o `jump' must occur on angular scales smaller than the beamwidth. At [FORMULA] resolution (about twice that in Fig. 2), the `canals' indeed seem unresolved, but the decrease in S/N-ratio precludes conclusions on even smaller scales (the original data have [FORMULA] resolution).

Additional evidence that the `canals' are due to beam depolarization is statistical. We defined `canal-like' points from the observed values of P, as follows. For each point in the mosaic we compared the observed value of P with the P-values in pairs of two diametrically opposed neighbouring (adjacent) points. If the value of P in the central point was less than a certain small fraction of the values in both comparison points, the point was defined `canal-like'. This definition mimics the visual detection `algorithm'.

In the top panel of Fig. 3 we show the distribution of the difference between the [FORMULA]'s in the two adjacent points that define the `canal-like' points, for a P-threshold of 30%. The [FORMULA]-distribution peaks at 90o, fully consistent with the beam depolarization hypothesis. This conclusion is reinforced by a comparison with the distribution of [FORMULA] (again for diametrically opposed adjacent neighbours) of all points for which P is between 1.0 and 2.0 times larger than both P-values in the two diametrically opposed neighbouring points, which is shown in the bottom panel of the same figure.

[FIGURE] Fig. 3. Top panel: Polarization angle difference [FORMULA] between two points on opposite sides of a `canal-like' point (see text for definition). A clear preference for [FORMULA] across `canals' is visible.
Bottom panel: [FORMULA] between two points on opposite sides of non-`canal-like' points (see text for definition). For the non-`canal-like' points, [FORMULA], rather than [FORMULA].

Similar `canals' were noted by Uyaniker et al. (1999) and Duncan et al. (1998), who also invoked beam depolarization. Yet, Fig. 3 is the first quantitative proof for this explanation.

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© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000