Magnetic fields may be threaded through the ejected material and, as the spinning black hole forms, these magnetic fields twist into corkscrew shapes that are thought to focus and accelerate the ejected material. The magnetic fields can’t be seen directly, but their signature is encoded in the light produced by charged particles (electrons) that whiz around the magnetic field lines. Earth-bound telescopes capture this light, which has travelled for millions of years across the Universe.
Head of Astrophysics at Bath and gamma-ray expert Professor Carole Mundell, said: “We measured a special property of the light — polarisation — to directly probe the physical properties of the magnetic field powering the explosion. This is a great result and solves a long-standing puzzle of these extreme cosmic blasts — a puzzle I’ve been studying for a long time.” Mundell’s team was first to discover highly polarised light minutes after the burst that confirmed the presence of primordial fields with large-scale structure. But the picture for expanding forward shocks has proved more controversial.
This model predicts light with high levels of polarisation (>10%) soon after the burst when the large-scale primordial field is still intact and driving the outflow. Later, the light should be mostly unpolarised as the field is scrambled in the collision. Teams who observed GRBs in slower time — hours to a day after a burst — found low polarisation and concluded the fields had long-since been destroyed, but could not say when or how. In contrast, a team of Japanese astronomers announced an intriguing detection of 10% polarised light in a GRB, which they interpreted as a polarised forward shock with long-lasting ordered magnetic fields.
The challenge is to capture the light as soon as possible after a burst and decode the physics of the blast, the prediction being that any primordial magnetics fields will ultimately be destroyed as the expanding shock front collides with the surrounding stellar debris. CAPTURING THE LIGHT EARLY
The mystery remained unsolved for over a decade, until the Bath team’s analysis of GRB 141220A. Lead author of the new study, Bath PhD student Nuria Jordana-Mitjans, said: “These rare observations were difficult to compare, as they probed very different timescales and physics. There was no way to reconcile them in the standard model.”
Ms Jordana-Mitjans said: “This new study builds on our research that has shown the most powerful GRBs can be powered by large-scale ordered magnetic fields, but only the fastest telescopes will catch a glimpse of their characteristic polarisation signal before they are lost to the blast.” In the new paper, published today in the Monthly Notices of the Royal Astronomical Society, Professor Mundell’s team report the discovery of very low polarisation in forward-shock light detected just 90 seconds after the blast of GRB 141220A. The super-speedy observations were made possible by the team’s intelligent software on the fully autonomous robotic Liverpool Telescope and the novel RINGO3 polarimeter — the instrument that logged the GRB’s colour, brightness, polarisation and rate of fade. Putting together this data, the team was able to prove that: The light originated in the forward shock. The magnetic field length scales were much smaller than the Japanese team inferred. The blast was likely powered by the collapse of ordered magnetic fields in the first moments of the formation of a new black hole. The mysterious detection of polarisation by the Japanese team could be explained by a contribution of polarised light from the primordial magnetic field before it was destroyed in the shock.
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