Polarimetry of the transient relativistic jet of GRB 110328 / Swift
  J164449.3+573451
  We present deep infrared (Ks band) imaging polarimetry and radio (1.4 and 4.8
GHz) polarimetry of the enigmatic transient Swift J164449.3+573451. This source
appears to be a short lived jet phenomenon in a galaxy at redshift z = 0.354,
activated by a sudden mass accretion onto the central massive black hole,
possibly caused by the tidal disruption of a star. We aim to find evidence for
this scenario through linear polarimetry, as linear polarisation is a sensitive
probe of jet physics, source geometry and the various mechanisms giving rise to
the observed radiation. We find a formal Ks band polarisation measurement of
P_lin = 7.4 +/- 3.5 % (including systematic errors). Our radio observations
show continuing brightening of the source, which allows sensitive searches for
linear polarisation as a function of time. We find no evidence of linear
polarisation at radio wavelengths of 1.4 GHz and 4.8 GHz at any epoch, with the
most sensitive 3 sigma limits as deep as 2.1%. These upper limits are in
agreement with expectations from scenarios in which the radio emission is
produced by the interaction of a relativistic jet with a dense circumsource
medium. We further demonstrate how the polarisation properties can be used to
derive properties of the jet in Swift J164449.3+573451, exploiting the
similarities between this source and the afterglows of gamma-ray bursts.
