Optimum frequency band for radio polarisation observations
  Polarised radio synchrotron emission from interstellar, intracluster and
intergalactic magnetic fields is affected by frequency-dependent Faraday
depolarisation. The maximum polarised intensity depends on the physical
properties of the depolarising medium. New-generation radio telescopes like
LOFAR, SKA and its precursors need a wide range of frequencies to cover the
full range of objects. The optimum frequency of maximum polarised intensity
(PI) is computed for the cases of depolarisation in magneto-ionic media by
regular magnetic fields (differential Faraday rotation) or by turbulent
magnetic fields (internal or external Faraday dispersion), assuming that the
Faraday spectrum of the medium is dominated by one component or that the medium
is turbulent. Polarised emission from bright galaxy disks, spiral arms and
cores of galaxy clusters are best observed at wavelengths below a few
centimeters (at frequencies beyond about 10 GHz), halos of galaxies and
clusters around decimeter wavelengths (at frequencies below about 2 GHz).
Intergalactic filaments need observations at meter wavelengths (frequencies
below 300 MHz). Sources with extremely large intrinsic $|RM|$ or RM dispersion
can be searched with mm-wave telescopes. Measurement of the PI spectrum allows
us to derive the average Faraday rotation measure $|RM|$ or the Faraday
dispersion within the source, as demonstrated for the case of the spiral galaxy
NGC 6946. Periodic fluctuations in PI at low frequencies are a signature of
differential Faraday rotation. Internal and external Faraday dispersion can be
distinguished by the different slopes of the PI spectrum at low frequencies. A
wide band around the optimum frequency is important to distinguish between
varieties of depolarisation effects.
