High-Energy Astrophysics
(Granot, Hardcastle, Jakobsson, Croston, Genet, Priddey, Chapman, Goodger)
Traditionally, high energy astronomy refers to the study of astronomical objects that release electromagnetic radiation of highly energetic photons (quanta of light). As such, it includes observations in the extreme ultra-violet (UV), X-rays, and gamma-rays. However, with time its extent has broadened to include other high-energy particles that can reach us from astronomical objects, such as neutrinos (weakly interacting nearly massless particles) and cosmic rays (mainly protons or heavier nuclei travelling very close to the speed of light). The physical study of astrophysical phenomena that involve such high-energy particles is referred to as high-energy astrophysics. Astronomical objects commonly studied in this field include black holes, neutron stars, active galactic nuclei, supernova remnants, Gamma ray bursts, and quasars. These objects often release large amounts of energy within a small volume over a short period of time.
These phenomena are of great importance, since they probe extreme physical conditions which are very hard to reproduce in terrestrial laboratories, such as:
- strong gravitational fields
- very high and low densities
- unusually high temperatures
- vast spatial scales and masses
- extremely large magnetic fields
- relativistic bulk motions (coherent ordered motions of large amounts of matter at velocities close to the speed of light)
- individual particles of extremely high energies
They can therefore improve our understanding of basic physics and help address topics that are both fundamental and poorly understood, such as the launching of relativistic jets from accretion of matter onto a compact object (such as a black hole or neutron star), the physics of relativistic collisionless shocks, and how particle acceleration (individual particles acquiring large energies) occurs in nature. These physical processes, as well as others, are common to many of the objects studied in high energy astrophysics.
The high energy astrophysics research at the University of Hertfordshire includes both the study of the physics of these energetic objects, and their uses as probes for other fields of research in astronomy. The latter involves mainly observational work, while the former combines observations, phenomenology, and theory. The theoretical work involves both analytic studies and numerical simulations that require powerful computer facilities. Observationally, satellites that operate at X-ray and gamma-ray energies, such as the gamma-ray burst satellite Swift and the X-ray observatories Chandra and XMM-Newton, are a key element of our work. However, since the radiation emitted by high-energy particles often spans the entire observable electromagnetic spectrum, we also use facilities ranging from ground-based low-frequency radio telescopes (the NRAO VLA, the GMRT, and in the future LOFAR), through to the mid-infrared (Spitzer) and the optical and UV (the Hubble Space Telescope and numerous ground-based telescopes).
The main topics that we study are:
- Gamma-ray bursts (GRBs): highly relativistic outflows from the explosion of rapidly rotating
very massive stars or from the merger of two very compact stars. The following aspects of GRBs are important to us:
- GRBs as probes of galaxies, star formation, and the inter-galactic medium at high redshifts, including the epoch of reionization (observational)
- Statistical properties: energetics and luminosity function, and searches for angular correlation of GRB sub-samples/classes with nearby galaxies, constraining the fraction of nearby GRBs (observations and statistics)
- Modelling of the prompt and afterglow emission, in an attempt to probe the underlying physics (phenomenology)
- Structure and dynamics of GRB jets, relativistic fluid dynamics and magneto-hydrodynamics (MHD), as well as the structure and stability of relativistic shocks (theoretical).
- Soft gamma-ray repeaters (a small class of magnetars - neutron stars with a huge surface magnetic field exceeding 1014 Gauss).
- Radio-Loud active galaxies: systems in which accretion of matter on to the supermassive
black hole at the centre of the galaxy produces large amounts of radiation and often relativistic outflows that can
extend to large distances from the galactic centre. In particular, we are interested in the following aspects:
- The physics of the active nucleus, particularly the nature of the accreting material and 'unified models' that seek to explain the different appearance of active nuclei (observation and theory)
- Physical conditions, bulk speeds and particle acceleration in the large-scale jets and the lobes that they form (observation and modelling)
- The impact of the large-scale jets on their environment (observation and modelling)
- X-ray binaries: binary star systems in which a normal star loses mass to a neutron-star or black-hole companion. In many respects these are smaller-scale, local analogues of active galaxies and we are particularly interested in their relativistic jets.
- The hot intergalactic medium: plasma with temperatures between 107 and 108 K that fills the space between galaxies when they form groups or clusters.