Megan's Blog

In the News this month: the role of magnetic fields in GRBs

Gamma ray bursts are highly energetic explosions which release enormous amounts of energy in just a few seconds. So-called because they were first discovered through their intense gamma-ray emission, these explosions can be seen across the visible universe. The exact nature of the jets which give rise to this emission is however, not certain. Different jet models predict very different properties of the magnetic fields present in the out-flowing material. Some models suggest that weak magnetic fields are present only locally within parts of the jet where they are created by shock waves, while other models suggest that large scale magnetic fields are generated by the central engine itself and help to both accelerate and collimate the jets.

While the Earth's magnetic field is not particularly strong, it is easy to detect with a simple compass. Detecting the strength and location of magnetic fields in an object many millions of light years away is much, much harder, so determining which model is correct is not straightforward. Luckily, magnetic fields affect electromagnetic radiation, so the strength of the magnetic field can be inferred from the polarisation of a signal. The presence of a large-scale magnetic field would result in the oscillation of the electric field of an electromagnetic wave having a preferred orientation. Objects which emit light randomly have no preferred orientation of this field and are said to be unpolarised, so measuring how polarised the light is can tell you something about the strength and the extent of the magnetic field in the object which generated the emission.

Using a new instrument designed to measure the polarisation of visible light, a team led by Ian Steele at Liverpool John Moores University, have measured the polarisation of the emission from the gamma ray burst known as GRB 090102, first detected by the Swift satellite on the 2nd of January 2009. Because the emission from these objects is so short-lived, ground-based observations are best done by robotic telescopes which can respond rapidly to alerts from gamma-ray satellites such as Swift. In this case, the optical observations were made with the Liverpool Telescope, a 2-metre robotic optical telescope located in the Canary Islands.

What the team found from their observations was that the light coming from the GRB in the first few minutes after the initial explosion was about 10 per cent polarised, which suggests that large-scale magnetic fields are present in the jets. A polarisation of 10 per cent is actually quite high for an astronomical object. If the magnetic fields in the jet were only present locally in shocks fronts, then the overall polarisation would cancel out or be very small. The presence of such a high level of polarisation provides support to the idea that large-scale fields are present and have a great effect on the nature and dynamics of the outflows from a gamma-ray burst explosion.

---

Steele, I., Mundell, C., Smith, R., Kobayashi, S., & Guidorzi, C. (2009). Ten per cent polarized optical emission from GRB 090102 Nature, 462 (7274), 767-769 DOI: 10.1038/nature08590