An international team of scientists, led by astrophysicists from the University of Bath in the UK, measured the magnetic field in a distant burst of gamma rays, confirming for the first time a decades-old theoretical prediction – that the magnetic field in these shock waves become scrambled after the ejected material crashes and strikes the surrounding environment.
Black holes are formed when massive stars (at least 40 times the size of our Sun) die in a catastrophic explosion that fuels a shock wave. These extremely energetic events chase matter at speeds close to the speed of light and fuel bright, short-lived gamma flashes that can be detected by satellites orbiting the Earth – hence their name, Gamma-Ray. Bursts (GRB).
Magnetic fields can be threaded through the ejected material, and as the rotating black hole forms, these magnetic fields twist into a corkscrew shape that is believed to focus and accelerate the ejected material.
Magnetic fields cannot be seen directly, but their signature is encoded in the light produced by charged particles (electrons) which hiss around magnetic field lines. Terrestrial telescopes capture this light, which has traveled for millions of years across the Universe.
Professor Carole Mundell, head of astrophysics at Bath and expert on gamma rays, said: “We measured a special property of light – polarization – to directly probe the physical properties of the magnetic field fueling the explosion. It’s a great result that solves a long-standing puzzle of these extreme cosmic explosions – a puzzle I’ve been studying for a long time.
CAPTURE THE LIGHT TLYT
The challenge is to capture the light as soon as possible after a burst and decode the physics of the explosion, with the prediction being that all of the primordial magnetic fields will eventually be destroyed when the expanding shock front collides with the surrounding stellar debris. .
This model predicts light with high levels of polarization (> 10%) soon after the burst, when the large-scale primordial field is still intact and drives the flow. Later, the light should be largely unpolarized because the field is scrambled during the collision.
Mundell’s team were the first to discover highly polarized light within minutes of the burst which confirmed the presence of primordial fields with large-scale structure. But the image of the expansion of front shocks proved to be more controversial.
Teams that watched GRBs more slowly – hours to a day after a gust – found weak polarization and concluded that the fields had long been destroyed, but couldn’t say when or how. In contrast, a team of Japanese astronomers reported an intriguing detection of 10% polarized light in a GRB, which they interpreted as a polarized forward shock with long-lasting ordered magnetic fields.
Lead author of the new study, Nuria Jordana-Mitjans, a doctoral student in Bath, said: “These rare observations were difficult to compare because they probed very different timescales and physics. There was no way to reconcile them in the standard model.
The mystery remained unsolved for over a decade, until the Bath team’s analysis of GRB 141220A.
In the new document, published in the Monthly notices from the Royal Astronomical Society, Professor Mundell’s team reports the discovery of a very low polarization in the forward shock light detected only 90 seconds after the explosion of GRB 141220A. Ultra-fast observations were made possible by the team’s smart software on Liverpool’s fully autonomous robotic telescope and the new RINGO3 polarimeter – the instrument that recorded color, brightness, polarization and fade rate of the GRB. By gathering this data, the team was able to prove that:
- The light comes from the front impact.
- The magnetic field length scales were much smaller than what the Japanese team deduced.
- The explosion was likely fueled by the collapse of ordered magnetic fields in the early moments of the formation of a new black hole.
- The mysterious polarization detection by the Japanese team could be explained by a supply of polarized light from the primordial magnetic field before it was destroyed during the shock.
Ms Jordana-Mitjans said: “This new study builds on our research which has shown that the strongest GRBs can be powered by large-scale ordered magnetic fields, but only the fastest telescopes will see their polarization signal. characteristic before they are lost. to the explosion.
Professor Mundell added: “We now need to push the boundaries of technology to probe the early moments of these explosions, capture statistically significant numbers of explosions for polarization studies, and place our research in the larger context of multi-messenger tracking. in real time from the Extreme Universe.