GRB 160821A: Time varying polarised prompt gamma ray emission

For the first time, we report a high polarisation fraction of 66+ 26− 27% across a gamma ray burst 160821A at∼ 5.3 σ confidence level. The polarisation angle is found to swing by 80◦ twice during the burst concurrently with the change in high energy spectrum. The change in polarisation angle is not due to the change in width of the visible part of the jet and thereby rules out axi-symmetric emission models. The high energetics of the burst confirm that the jet is viewed on-axis and therefore, the observed radiation is synchrotron produced in ordered magnetic fields. The standard models of Poynting flux and baryon dominated jets find it difficult to explain these observed features.