Black holes, neutron stars and Gamma Ray Bursts

William Crarer, a masters student studying physics at the University of Bath shares some of his exciting research with the Mayes Creative team.

“Abstract: Numerical simulations of an unstructured Top Hat GRB jet and a jet with a Gaussian distribution of energy across its opening angle were run from 110 to 700 days post-GRB. These simulations showed qualitatively that the Gaussian jet spread and decelerated sooner and faster than the Top Hat jet. This supports the suggestion of previous simulations that the strange afterglow behaviour associated with GRB170817a resulted from a jet with Gaussian angular structure viewed off axis. Quantitative comparison with existing Top Hat. jet numerical simulations and spreading models was not possible as the simulations lacked robustness due to an insufficient starting Lorentz factor and questions over jet energy content.”

When did you first appreciate space? “I was raised in Cornwall from when I was 1 and I’ve lived my whole life, pre-university, near Helston. I knew nothing about the history of astronomy in Cornwall (Goonhilly, John Couch Adams etc.) but the dark skies were free from light pollution so I had a deep appreciation for the beauty of the night sky and the Milky Way arcing across it.”

How did you end up studying space? “At university, I did not specialise in astrophysics or astronomy but instead focused on learning skills that I thought would be useful in getting a graduate job. This lead me to studying computational modelling, which sounds less fun, but it is really useful for understanding things that are hard to see or experiment on in the real world. Space is a prime example of this; a lot of what we want to study is incomprehensibly far away, unimaginably old and either impossibly bright or impossibly faint. This makes space phenomena very hard to study and so simulating them can really help scientists try to understand them and make predictions.”

My master’s project aimed to use computer simulations to understand Gamma Ray Bursts. These are the brightest events we see in the universe, outshining whole galaxies for a few seconds before fading away to a faint afterglow. The initial flash is so bright we can see them from billions of light years across the universe and use the light from them to study the far away galaxies that host them. They can also teach us about how the laws of nature work in the most extreme conditions; in a way they are the hottest, brightest and fastest laboratories in the universe.”

What can you tell us about Gamma Ray Bursts? “Gamma Ray Bursts produce jets of matter that shoot out at close to the speed of light, so fast that the laws of physics that we know on Earth no longer apply and we have to use Albert Einstein’s theory of relativity to understand them. It was these jets that I simulated in my project, comparing 2 different models of the structure of these jets and how they spread out over time. The flash of a Gamma Ray Burst is made up of gamma rays, which are part of the electromagnetic spectrum just like the visible light we see with our own eyes or the x-rays we use to see inside our bodies. The difference is gamma rays are the most energetic part of the spectrum, more energetic even than x-rays. Since we first saw these Gamma Ray Bursts in the 1960s, astrophysicists have been painstakingly working out what causes these cataclysmic events. One cause is the collision of 2 very massive, dense objects such as a black hole with a very small type of star called a neutron star. Both black holes and neutron stars are made when massive stars reach the end of their lives and blow up in massive explosions called supernova. While most of the star is blown apart, some of it collapses inwards and forms a dense ball of matter. If this ball is dense enough it collapses in on itself and forms a black hole, an infinitely small, infinitely dense point near which the laws of physics break down. If its not quite dense enough to become a black hole it is called a neutron star. These stars are still unimaginably heavy, a teaspoon of neutron star material has the same mass as Mount Everest. It is mot surprising that when these 2 impossible things collide they produce the biggest fireworks in the universe.”

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“As the dense objects collide they create a jet of material that shoots out into space at speeds close to the speed of light. At these speeds, the rules of space and time change; time goes at different rates for matter going at different speeds and space itself is squished and stretched differently. To understand this behaviour, we have to use a theory called Relativity that was thought up by Albert Einstein over 100 years ago. The material in the jet is battered around as it flies outwards and it produces the flash of gamma rays that light up the universe and that we see on Earth. Later on the jet smashes into the dust and gas that floats in space and more light is produced in all parts of the spectrum like visible light, infra-red and ultra-violet. If one of these Gamma Ray Bursts happened close enough to Earth, you would be able to see this afterglow with your own eyes.”

What did your simulations find? “The attached clips show how the simulations predicted each of these models of jet would evolve over time. The more yellow areas represent dense parts of the jets and the green and blue areas represent empty parts of space outside and behind the front of the jet. The differences between the two are reasonably subtle and the reasoning behind them is quite involved. In both clips, you can see a dense bubble that expands outwards and eddies form inside the bubble.”

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Top Hat Density

“Recently, scientists have detected the ripples in space and time caused by these cosmic collisions at the same time as they saw the Gamma Ray Burst and then the afterglow that followed. This was a massive breakthrough for astronomers as never before have we seen the same event in so many different ways. Our observations of this event showed some weird and unexpected behaviour in the flash of gamma rays and in the afterglow that followed. This weirdness suggested that our models for the jet of material may be missing out on some key things. My project aimed to include something that may be one of the missing elements and to see what difference it would make to what we saw. We used a special software to simulate how the jet of fluid would move outwards and how it would spread out sideways as it flows away from where the collision happened. Our simulations of both the original model and the updated models are shown in the clips discussed before. The original model is known as the top hat model because in this model the jet looks like a top hat. Scientists are an inventive bunch.”

“Our results showed that the updated model could explain some of the strange observations but a lot more work is required to confirm this.”

Interested in science and art? Why not visit the Mayes Creative website and look at some of our other projects, like the meteor film pod project. This project combines both artistic film and cosmic rays, the films speed and quality was changed depending on the cosmic radiation.

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