Student Research Article - Charlotte Summers
PhD Student Charlotte Summers has written the following article as part of our series of Student Research Articles!
What are gamma ray bursts?
Gamma ray bursts (GRBs) are the most energetic explosions to ever be observed by astronomers; in a few seconds they release as much energy as the Sun will emit over the whole of its 10 billion year lifetime. The first detection of a GRB was made in 1967 by the Vela satellite system. The satellites, which were monitoring for gamma ray emission from nuclear weapons testing, detected a flash of gamma rays coming from space unlike anything that could be generated by a nuclear test. When the data was declassified in 1973, many theories were initially proposed for what caused these events. Because they were so bright, it was assumed that GRBs must be originating from sources within our galaxy, but there was no consensus on the exact nature of the progenitor systems (close to a hundred different theories were proposed in the following two decades!). This continued until the early 1990s, when data from the Compton gamma ray telescope revealed that the bursts were likely originating in other galaxies, meaning that the amount of energy being released in these events was far larger than previously thought. Following this, one prominent theory was that a subclass of GRBs known as short GRBs were generated by the merging of two neutron stars or a neutron star and a black hole. If this was the case, astronomers expected to observe other emissions coming from these events, including light at other wavelengths, neutrinos and gravitational waves. In 2017 this was precisely what happened, when the LIGO gravitational wave detectors observed a signal from a pair of merging neutron stars, and at the same time the Fermi gamma ray telescope detected a short GRB originating from the same place.
Although the LIGO/Fermi observation is strong evidence to support the theory that merging neutron stars produce short GRBs, the processes that actually produce them are not well understood. The large number of complex physical effects that need to be taken into account in these events makes modelling and understanding them difficult. The two neutron stars are incredibly massive and dense, and so they have strong gravitational fields, making general relativistic effects important. Their internal structure is also poorly understood, and the high densities and temperatures encountered during the merger make nuclear reactions and the emission of neutrinos important. Additionally, neutron stars can have very strong magnetic fields, and these are thought to play a key role in the formation of jets, which then produce the GRB emission we observe. In my research I am using computational models to simulate neutron star mergers, to understand how the different physical effects present affect how much energy is produced and available to power a GRB. For this, we use simulation codes that model the neutron star matter as a magnetised fluid, simulating its motion according to the equations of ideal magnetohydrodynamics (MHD). In addition to this we need to include the effects of other physical processes present, including gravity and the production and emission of neutrinos via particle interactions in the neutron star matter.
I completed my MPhys degree in Physics with Astrophysics at the University of Manchester, before beginning my PhD here in Edinburgh in 2018. Now in my third year of my PhD, I am working with the FLASH (magneto)hydrodynamics code to create a model of a merging neutron star and black hole. My day to day research involves a lot of programming, making changes to the code to incorporate new effects and studying the output. I work with the code on Eddie, the University of Edinburgh's supercomputer, as fluid dynamics models of this kind are too large and intensive to run on a home computer. I also spend time tutoring undergraduate courses during term, and this summer completed the Edinburgh Teaching Award.