Astron. Astrophys. 364, 732-740 (2000)

4. Polarimetric observations

We have examined the 1.40- and 2.42-GHz polarimetric data in the field surrounding RX J0852.0-4622. These data have been used to estimate the Faraday rotation measures (RMs) across the field, and to calculate the polarisation position-angles at zero wavelength (thereby estimating the orientations of the tangential components of the magnetic fields within the regions of polarised emission). Note that these estimates assume the polarisation position-angles to vary linearly with .

The polarised intensities from the 2.42-GHz data are shown in Fig. 4, at the lower resolution of . Superposed upon this grey-scale image are the orientations of the tangential component of the magnetic field. The RMs derived across the field are shown in Fig. 5.

Fig. 4. Grey-scale image showing the polarised intensities across the field at 2.42 GHz, with a resolution of . The 2.42-GHz Stokes-Q and U data have rms variations of  mJy beam-1 at this resolution. The grey-scale image is blanked wherever the polarised intensity falls below 45 mJy beam-1. Orientations of the tangential components of the magnetic fields are also shown. These angles have been calculated from the 1.40- and 2.42-GHz data, assuming that the vector angles vary linearly with . Errors in the derived angles are generally . A vector is plotted every , wherever both the 1.40- and 2.42-GHz polarised intensities (at resoluti on) exceed 0.1 Jy beam-1. The grey-scale wedge is labelled in units of Jy beam-1.

Fig. 5. The derived rotation measure (RM) from the 2.42- and 1.40-GHz data, assuming that the vector angles vary linearly with . Filled squares represent positive RM values, while empty squares represent negative values. The size of each square is proportional to the magnitude of the RM, with the maximum size corresponding to  rad m-2. The errors in RM values are  rad m-2. A square is plotted every . Data have been blanked wherever the 1.40- or 2.42-GHz polarised intensity (at resolution) falls below 0.1 Jy beam-1.

The large circle shows the boundary of the SNR RX J0852.0-4622. Much of the polarised emission visible within the circle does not appear to be associated with the new SNR. For example, the polarisation detected from the eastern half of RX J0852.0-4622 is clearly associated with the prominent, eastern arc of total-power emission. The only region of RX J0852.0-4622 to potentially exhibit polarised emission is the northern section of the shell. If associated with the new remnant, the northern arc appears polarised to a level of approximately 20% at 2.42 GHz. Other regions of the SNR exhibit no polarised emission (such as the southern section of the shell), or are confused with polarised structure from Vela.

The magnetic field vectors on the north side of the shell appear jumbled, with no clear orientation evident. This is in contrast to the field orientations in other young, shell-type supernova remnants, which are predominantly radial; e.g. Tycho (Wood et al. 1992), Kepler (Matsui et al. 1984) and SN1006 (Reynolds & Gilmore 1993).

However, it is questionable whether the detected polarised emission in this northern section of the shell is attributable to RX J0852.0-4622. The appearance of much of this RM structure is similar to that seen from the Vela emission, beyond the RX J0852.0-4622 shell's northwestern perimeter. Furthermore, there is no discontinuity in either the RM values or the magnetic field orientations near the boundary of the shell. We suggest, therefore, that the polarised emission seen throughout Fig. 4 originates entirely from the Vela remnant. We note that this interpretation is also consistent with the lack of polarisation observed from the southern arc of the RX J0852.0-4622 shell.

The fractional polarisation of the northern arc must then be % at 2.42 GHz. As noted above, we detect no polarisation in the vicinity of the bright, southern section of the shell, to a () limit of approximately 5% at 2.42 GHz. We note that these low fractional polarisations are not inconsistent with the polarimetric properties of other young shells, which exhibit fractional polarisations of % when the emission is well resolved.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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