Hypernovae: the most efficient power sources in the Universe
Cosmic rays were first detected by the Austrian physicist Victor Hess, a discovery for which he won the Nobel Prize for Physics in 1936. But only relatively recently have the astronomical sources that generate gamma radiation been localised and characterised. “Gamma-ray bursts are the product of some of the most powerful cosmic particle generators we know,” says Mannheim. GRB 190114C was most probably the product of what’s called a hypernova -- the cataclysmic explosion of a rare type of extremely massive star at the end of its evolution. Within the interior of such a star, the dense core undergoes a sudden and catastrophic gravitational collapse, and is transformed into a black hole within a matter of seconds. The gravitational energy released during this process is emitted in the form of two oppositely directed plasma jets that are oriented parallel to the rotational axis of the star.
“In hypernovae, a large fraction of the entire rest mass of the collapsed star is converted into radiation. They are the most efficient power sources in the Universe,” says Mannheim. As the analysis of the observational data for GRB 190114C has revealed, the elementary particles emitted in the plasma jets were accelerated to unimaginably high energies. The ultrahigh-energy gamma-ray photons detected during the event were on the order of a billion times more powerful than X-ray photons.
Follow-up observations carried out by telescopes that capture light in other regions of the electromagnetic spectrum complement those of the twin MAGIC instruments. In particular, they suggest that the most energetic photons can be attributed to the interaction of the plasma jets with the interstellar gas surrounding the hypernova. This is expected to produce an extremely powerful shock wave that propagates at speeds very close to the speed of light, and further accelerates the particles in the plasma jets. “Such cosmic catastrophes also occur in our own galaxy, the Milky Way. They are extremely rare, but if one were to occur in our stellar neighbourhood it would have deadly consequences for us,” says Karl Mannheim. The high-energy radiation would ionise the atmosphere, and destroy the ozone layer that shields us from ultraviolet light. In fact, it has been suggested that some of the mass extinction events recorded in Earth’s rocks might have been triggered by GRBs. However, Professor Mannheim is convinced that “evolution is capable of coping with cosmic and other sorts of catastrophes. After all, extreme phenomena such as GRB 190114C demonstrate that “nature’s powers of invention will always surpass the capacity of human imagination.”