Gamma-Ray Bursts with MOPTOP

Prof Iain Steele
Dr Helen Jermak
Prof Shiho Kobayashi

Gamma-ray bursts are some of the most energetic transient events in the universe, they are powered by relativistic jets produced by neutron star mergers and the collapse of massive stars. Although the acceleration and collimation processes of GRB jets (and other relativistic jets from compact objects)  are still open questions, recent observations imply the importance of magnetic acceleration processes (e.g. the magnetic field around the compact object becomes twisted and amplified by the rotation and the magnetic pressure accelerates the jet). According to the magnetic models, the magnetic field in the jet is expected to be well ordered, while alternative models predict random magnetic fields or a patchwork of unaligned magnetic regions. Since photons are produced by the synchrotron process, they are polarised perpendicular to the magnetic field lines in each fluid element of the emission region. The net polarisation signals depend on the geometry and relativistic effects. Polarimetry offers a unique probe of the physical conditions and geometry of the jets, and gives constraints on the jet acceleration process and plasma physics in a relativistic regime.

The Liverpool Telescope has made most of the (still relatively few) successful measurements of early-time optical GRB afterglow polarisation through the RINGO series of polarimeters. These instruments' unique capability was to measure polarisation on very short timescales for poorly localised sources. We have now built a new polarimeter (MOPTOP) which has ~4 times the sensitivity of RINGO. This will allow us, in only 3 years, to double the sample size of GRB polarization measurements. This will make it possible, for the first time, to start to disentangle correlations between the observed polarization and derived jet properties over a representative sample of bursts. In addition the low systematic errors of MOPTOP will mean that for the first time we will be able to constrain behaviour at low polarization. In this regime the turbulent patch model is expected to operate and we will test the prediction that this model will give polarisation angle variability as opposed to the constant orientation we observe in the highly polarized and therefore magnetically collimated bursts.

The PhD student will lead the collection and analysis of the GRB data, working within a wider collaboration incorporating staff from LJMU as well as the Universities of Bath, Ferrara (Italy) and Nona Goricia (Slovenia) who will provide expert advice and support on the theoretical interpretation of the data.  

The following two papers provide some more information about GRB polarimetry and MOPTOP: