Releasing more energy in 10 seconds than our Sun will emit in its expected 10-billion-year lifetime, some gamma-ray bursts (GRBs) are the most powerful explosions in the universe. These giant jets of plasma accelerate outwards from the core of a black hole, formed from the recent collapse of a massive star (and in some cases, from the merger of two neutron stars or a neutron star with a black hole). Whilst these seconds-long events occur on average once a day, the question of what powers these ultra-high-speed bursts has remained unresolved.
For the first time, an international team have gathered evidence to support one explanation for GRBs, called the magnetic model. In this hypothesis, the huge magnetic field of the dying star collapses within seconds of the GRB explosion, producing immense amounts of energy to propel the jets. An alternative explanation favored by some astronomers suggests that repeated violent collisions between material strewn out by the explosion and that surrounding the star are the driving force behind GRBs.
Researchers from the UK, Italy, Slovenia, Russia, South Africa, and Spain scrutinized the polarization data gathered from the collapse of a massive star in a galaxy 4.5 billion light-years away. They were surprised to find that in the seconds after the explosion, the polarization levels in the GRBs dropped sharply, suggesting that the stellar magnetic field had been destroyed.
“From previous studies, we expected to detect polarisation as high as 30 percent during the first hundred seconds after the explosion,” lead author Núria Jordana-Mitjans, a postgraduate student from Bath University, UK, said in a statement. “So we were surprised to measure just 7.7 percent less than a minute after the burst, followed by a sudden drop to 2 percent soon after.”
“This tells us that the magnetic fields collapsed catastrophically straight after the explosion,” Jordana-Mitjans continued, “releasing their energy and powering the bright light detected across the electromagnetic spectrum.”
These “cosmic flashbulbs” can be detected using dedicated satellites orbiting Earth, such as NASA’s Neil Gehrels Swift Observatory. In the case of this study published in The Astrophysical Journal, the gamma-ray flash (named GRB 190114C) was detected by Swift, which in a matter of seconds alerted robotic telescopes in the Canary Islands and South Africa to the event. Only 31 seconds after the onset of the GRB, the telescopes had repositioned and begun gathering data.
“Our innovative telescope systems are entirely autonomous, with no humans in the loop, so they slued very quickly and began taking observations of the GRB almost immediately after its discovery by the Swift satellite,” Professor Carole Mundell, head of Astrophysics at the University of Bath and co-author on the research, explained. “It is remarkable that from the comfort of our own homes, we were able to discover the importance of primordial magnetic fields in powering a cosmic explosion in a distant galaxy.”