High-Energy astrophysics (HEA) deals with the behaviour of matter where the typical energy of individual particles is higher than those on the surfaces of stars. It features atomic and subatomic processes under extreme conditions of high (>100000K) temperature, relativistic gravity and velocities, and magnetic fields with intensities up to a quadrillion times that of the earth. A significant fraction of research in HEA is focused on the physical processes associated with compact objects such as neutron stars, white dwarfs and black holes. This includes studies of supernovae, gamma-ray bursts, accretion disks and associated relativistic jets, production of neutrinos and gravitational waves, radio and x-ray emission of pulsars and magnetars, and physics at supranuclear densities inside neutron stars.
A significant area of research in high-energy at Leiden Observatory is synchrotron emission from active galaxies. This emission is powered by magnetized, relativistic jets emanating from rapidly spinning supermassive black holes at the centers of these galaxies. From the emission we can deduce the energetics of the jets, the characteristics of the diffuse media surrounding the nuclei, and the evolution of these characteristics with cosmic time. Leiden astronomers are also working on questions related to the energetics of the hot intergalactic medium in clusters of galaxies: what is its composition, how is is heated (possibly, by the same type of black hole powered jets), how does it cool, and how does it interact with the intermixed cooler gaseous components? Theoretical research in HEA in Leiden concerns mainly questions related to rapidly spinning (up to 700Hz!), accreting neutron stars. What is the physics of the nuclear explosions on their surfaces which are observed on Earth as a seconds-long bursts in X-ray flux? What sets the observed upper limit on the neutron star spin? Will gravitational wave observatories like LIGO and VIRGO be able to detect neutron stars and if so, what can we learn from them?