Faraday Rotation Measure Synthesis

We extend the rotation measure work of Burn (1966) to the cases of limited sampling of λ2 space and non-constant emission spectra. We introduce the rotation measure transfer function (RMTF), which is an excellent predictor of nπ ambiguity problems with the λ coverage. Rotation measure synthesis can be implemented very efficiently on modern computers. Because the analysis is easily applied to wide fields, one can conduct very fast RM surveys of weak spatially extended sources. Difficult situations, for example multiple sources along the line of sight, are easily detected and transparently handled. Under certain conditions, it is even possible to recover the emission as a function of Faraday depth within a single cloud of ionized gas. Rotation measure synthesis has already been successful in discovering widespread, weak, polarized emission associated with the Perseus cluster (de Bruyn & Brentjens 2005). In simple, high signal to noise situations it is as good as traditional linear fits to χ versus λ plots. However, when the situation is more complex or very weak polarized emission at high rotation measures is expected, it is the only viable option.