The Biggest Explosions in the Universe

Tuesday 20th August, 2019

Our speaker for the evening was Dr Phil Evans from Leicester University who came to talk to us about "The Biggest Explosions in the Universe". He said that he is a postdoctoral researcher and spends 90% of his time working with the data collected by NASA's SWIFT spacecraft that is designed to analyse the gamma rays given off by gamma ray bursts. His main work involves writing software that automatically distributes the spacecraft's data to other scientists.

He commented that gamma ray bursts are the most powerful explosions since the Big Bang and were first spotted in the 1960s by Vela military satellites. These satellites were launched to monitor the 1963 Partial Test Ban Treaty between the USA and Russia. The Vela scientists were able to roughly triangulate the source of the gamma rays using the Vela system of satellites in orbit and concluded that their source was extraterrestrial. However, they were not able to tell whether the gamma rays were coming from inside our own galaxy, the Milky Way, or whether they emanated from the distant Universe. This led to scientists proposing hundreds of scientific models to explain the observations.

It was not until the Compton Gamma Ray Observatory was launched into Low Earth Orbit using the Space Shuttle in 1991 that astronomers were able to use its results to show that the gamma ray bursts came from all over the sky. This spacecraft was then followed by BeppoSAX in 1996, which could provide accurate positions of the gamma ray bursts. This allowed other instruments on BeppoSAX or ground-based telescopes to examine the source of the explosions further using longer wavelengths such as X-rays or even visible light.

Dr Evans then continued by saying that the SWIFT spacecraft that he works with was launched in 2004 and is still operational unlike the previous spacecraft and satellites he had mentioned. At their end of their missions they were de-orbited and had burnt up in the Earth's atmosphere.

He then explained that gamma ray bursts are divided into two types: long and short duration. The long bursts are the most numerous, accounting for roughly 70% of bursts, and have a duration of more than two seconds. Each has been associated with a galaxy where rapid star formation is still occurring. They are thought to be the result of a massive star coming to the end of its life and exploding. The short gamma ray bursts last less than two seconds and make up about 30% of observations. They come from areas where there are few new stars being born and it is proposed that they occur when a pair of neutron stars or a neutron star and a black hole collide.

Dr Evans ended his talk by saying that there is a proposed mission by the European Space Agency called Theseus that would study the early Universe using gamma and X-rays. If selected it would launch in 2032, although it has to compete for funding against other proposed missions: SPICA an infrared space telescope, and EnVision, a Venus orbiter.