Chaos and order: linear and circular polarimetry of GRB afterglows (invited)

Author: Klaas Wiersema

Abstract: It has long been known that polarimetry of afterglows may provide valuable, light-curve independent, insight into the magnetic field properties of the GRB jet, the jet opening angle and the internal structure of the jet itself. But unfortunately it is difficult to acquire data with sufficient quality and time span, as afterglows are faint and fade rapidly. To remedy this, we observed three Swift GRB afterglows with the Very Large Telescope, covering a broad time span (several nights) with high precision measurements. Using this unique, rich, dataset we can now confirm some of the earliest model predictions for afterglow polarisation, for example the expected 90 degree polarisation angle swing expected for jet breaks.

In addition, we find several unexpected new phenomena, that defy easy explanation, e.g. a completely new type of very rapid variability in the late time linear polarisation lightcurves, clear evidence for highly ordered magnetic fields in the forward shock, the possibility of stronger than expected plasma propagation effects, and the effects of large afterglow rebrightening episodes.

I will review future observing strategies to test the causes for these new phenomena.