A search for linear polarization in the active galactic nucleus 3C 84 at
  239 and 348 GHz
  We report a search for linear polarization in the active galactic nucleus
(AGN) 3C 84 (NGC 1275) at observed frequencies of 239 GHz and 348 GHz,
corresponding to rest-frame frequencies of 243 GHz and 354 GHz. We collected
polarization data with the IRAM Plateau de Bure Interferometer via Earth
rotation polarimetry. We do not detect linear polarization. Our analysis finds
3-sigma upper limits on the degree of polarization of 0.5% and 1.9% at 239 GHz
and 348 GHz, respectively. We regard the influence of Faraday conversion as
marginal, leading to expected circular polarizations <0.3%. Assuming
depolarization by a local Faraday screen, we constrain the rotation measure, as
well as the fluctuations therein, to be 10^6 rad/m^2. From this we estimate
line-of-sight magnetic field strengths of >100 microG. Given the physical
dimensions of 3C 84 and its observed structure, the Faraday screen appears to
show prominent small-scale structure, with \DeltaRM > 10^6 rad/m^2 on projected
spatial scales <1 pc.
