Research Fellows Directory
Dr Jonathan Granot
University of Hertfordshire
I work in high-energy astrophysics, studying some of the most energetic and extreme phenomena in the universe. For example, I study gamma-ray bursts (GRBs) – the most luminous cosmic explosions. GRBs lasting >2 seconds form when a rapidly-spinning star >25 times heavier than our Sun ends its life in a brilliant explosion, its core collapsing to form a black-hole that accretes the stellar envelope, producing relativistic jets - moving at almost the speed of light. I also study magnetars – neutron stars (extremely dense stars, slightly more massive than our Sun but the size of a big city) born with a huge magnetic field, a milliom-billion times that of the Earth. During the past year, I took part in a study where a powerful super-computer was used to simulate the merger of two such neutron stars, as they draw closer together by emitting gravitational waves – traveling ripples in the fabric of space-time. We found, for the first time, that this scenario creates just the right conditions to power a short duration GRB (lasting < 2 seconds), which is a big step forward for the viability of this 25 year old idea.
In the past year I have also discovered a new mechanism by which astrophysical outflows that show significant time variation and are extremely highly magnetized can accelerate up to very close to the speed of light. Initially almost all of their energy resides in their magnetic field, but as they accelerate it is gradually transferred into the energy of their motion (kinetic energy), until the latter eventually becomes dominant and the magnetic field becomes relatively weak. This allows for strong collisions between different parts of the outflow (“internal shocks”, without the “cushioning” effect of a strong magnetic field) that can result in efficient production of heat and radiation, which may help explain some of the observed properties of such astrophysical sources.