Polarimetric observations at low radio frequencies
Magnetic fields play a fundamental role in the evolution of astrophysical systems. These fields can be studied through wide-field spectropolarimetry, which allows for faint polarised signals to be detected at relatively low radio frequencies. An interferometric polarisation mode has recently become available at the Giant Metrewave Radio Telescope (GMRT). A detailed analysis of the GMRT’s instrumental response is presented. The findings are used to create a polarisation pipeline, which in combination with rotation measure (RM) Synthesis is used for the detection of extended linearly polarised emission at 610 MHz. A number of compact sources are detected and their Faraday depth and polarisation fraction are reported for the first time. New holography observations of the GMRT’s primary beam are presented. Instantaneous off-axis polarisation is substantial and scales with the Stokes I beam. The developed beam models are used to reduce direction-dependent instrumental polarisation, and the Stokes I beam is shown to deviate from circular symmetry. A new technique for electric vector polarisation angle calibration is developed that removes the need for known sources on the sky, eliminates ionospheric effects, and avoids a flaw in current methods which could erroneously yield multiple Faraday components for sources that are well-parameterised by a single RM. A sample of nine galaxies from two Southern Compact Groups are then presented, with constraints being placed on the polarised fraction, RM, spectral index, star formation rate, companion sources, and hydrodynamical state. One galaxy has a displaced peak of radio emission that is extended beyond the disk in comparison to the near-IR disk – suggesting the radio disturbance may be a consequence of ram pressure stripping. Linear polarisation is detected from the core of NGC 7552 at 610 MHz, while another three galaxies ESO 0353–G036, NGC 7590, and NGC 7599 are found to be unpolarised. An analysis of additional extended sources allows for an FR-I and an FR-II radio source to be morphologically classified. Finally, spatial spectral variations are identified in the youngest known Galactic supernova remnant G1.9+0.3, with flatter spectra in the NW and SE. Models of cosmic ray acceleration at oblique shocks suggest the variation is most consistent with an ambient B field perpendicular to the axis of bilateral symmetry. For the first time, the presence of polarised emission is detected. There is increased ordering of the B field in the NW and strong Faraday depolarisation must also be present. An intrinsically radially-oriented field could be provided by a systematic gradient in RM of 140 rad m-2 from N to S and can also explain the depolarisation. Such a gradient may be caused by an anisotropic regular magnetic field within the remnant or in an intervening Faraday screen. The lack of strong constraints on the RM, and the remnant’s current evolutionary stage, leave open the possibility that Rayleigh–Taylor instability formation has not yet fully taken place.
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