Polarimetry of the transient relativistic jet of GRB 110328 / Swift J164449.3+573451

TL;DR

This study investigates the polarization properties of the transient relativistic jet in Swift J164449.3+573451 across infrared and radio wavelengths to understand jet physics and source geometry, finding modest infrared polarization and no radio polarization.

Contribution

It provides the first deep infrared polarimetry of this transient and constrains radio polarization, offering insights into jet interaction and magnetic field structure in a tidal disruption event.

Findings

Infrared linear polarization detected at ~7%

No significant radio polarization detected at 1.4 and 4.8 GHz

Radio emission consistent with jet-medium interaction models

Abstract

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.