Madrid, 16 years old (Europe Press)
Measurement of the magnetic field of the distant gamma-ray burst confirmed the theoretical prediction that it would become turbulent after the ejected material hits the surroundings.
Black holes form when massive stars (at least 40 times larger than our Sun) die in a catastrophic explosion that generates a shock wave. These highly energetic events eject material at speeds close to the speed of light and generate bright short-range gamma-ray flashes that can be detected by Earth-orbiting satellites, hence their name, gamma-ray bursts (GRBs).
Magnetic fields can be strung into the ejected material, and as a spinning black hole forms, these magnetic fields twist into a spiral shape that is thought to focus and accelerate the ejected material.
Magnetic fields cannot be directly seen, but their signature is encoded in the light produced by charged particles (electrons) orbiting around magnetic field lines. Ground-based telescopes capture this light, which has traveled through the universe for millions of years.
Study author and director of astrophysics at the University of Bath, Carol Mundell, explains: “We measured a special property of light – polarization – to directly probe the physical properties of the magnetic field that led to the explosion. This is a fascinating result and solves an ancient mystery from these extreme cosmic explosions, And it’s a mystery I’ve been studying for a long time.”
Capture the light before time
The challenge is to capture the light as soon as possible after the explosion and decipher the physics of the explosion, with the expectation that any primordial magnetic fields will eventually be destroyed when the expanding shock front hits the surrounding stellar debris.
This model predicts light with high levels of polarization (>10%) shortly after bursting, when the broad-band primordial field is still intact and is driving the outflow. Later, the light must be mostly unpolarized, as the field becomes turbulent in the collision.
Mondel’s team was the first to detect highly polarized light minutes after the explosion, confirming the existence of primitive fields with a large-scale structure. But the spectacle of sprawling incidents head-on has proven more controversial.
Teams observing the GRBs at slower times — hours to a day after the explosion — found low polarization and concluded that the fields were destroyed long ago, but they could not say when or how. Instead, a team of Japanese astronomers announced the exciting discovery of 10% polarized light on the GRB, which they interpreted as a direct collision with ordered long-lasting magnetic fields.
The new study’s lead author, University of Bath doctoral student Nuria Jordana-Metgans, explains: “It was difficult to compare these rare observations because they investigate completely different physical and temporal scales. There was no way to reconcile them in the Standard Model.”
The mystery remained unsolved for more than a decade, until Bath’s team analyzed GRB 141220A.
In the new article, published Wednesday in the Monthly Notices of the Royal Astronomical Society, Professor Mundell’s team reports the detection of very low polarization in forward collision light that was detected just 90 seconds after the explosion of GRB 141220A.
The ultra-fast observations were made possible by the team’s intelligent software on the fully autonomous robotic Liverpool Telescope and the new RINGO3 polarimeter, the instrument that recorded the GRB’s colour, brightness, polarization and fade rate. By putting this data together, the team was able to show that the measurements of magnetic field lengths were much smaller than what the Japanese team inferred. Perhaps the impetus for the explosion was the collapse of the magnetic fields arranged in the first moments of the formation of a new black hole.
The mysterious discovery of polarization by the Japanese team can be explained by the contribution of polarized light from the primordial magnetic field before it was destroyed in the collision.
“This new study builds on our research showing that the most powerful GRBs can operate through widely arranged magnetic fields, but only the fastest telescopes will be able to look at their characteristic polarization signal before it gets lost in the explosion,” says Jordana Mitgans.
Professor Mondel adds that the frontiers of technology now need to be pushed “to explore the first moments of these outbursts, capture a statistically large number of bursts for polarization studies, and place our research in the broader context of multi-message time tracking of the extreme universe.”