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The Violent Universe Transcript The Violent Universe Transcript Date: Wednesday, 19 January 2011 - 1:00PM Location: Museum of London Gresham Lecture, Wednesday 19 January 2011 The Violent Universe Professor Ian Morison In this lecture we are going to examine what are the most powerful explosions in the universe that can be observed today and also study the Big Bang origin of the universe itself - an explosion of a very different and unique type. The story begins with the serendipitous discovery of what are termed “Gamma Ray Bursts or GRBs - a discovery that came out of the cold war. It’s an interesting point as to what “today” means. We see these events now but, as we will see, they arise in galaxies in the distant reaches of the universe and so we are seeing events that actually happened many billion of years ago. The discovery of Gamma Ray Bursts As nuclear test ban treaties were negotiated in the late 1950s, President Eisenhower's science advisors cautioned him that the USSR might try to secretly carry out nuclear tests in space. It was decided to design and launch a series of satellites that could detect the characteristic double burst of gamma rays (very highly energetic photons) that result from a nuclear blast. The project was code-named Vela (meaning “watchers”) and the first spacecraft was launched in October 1963 orbiting at an altitude of 120,000 km (74,400 miles). It carried six gamma ray detectors along with other instruments. The gamma ray detectors were made cesium iodide which scintillates – giving flashes of visible light - when gamma rays pass through it. The data had to be analysed by hand and in 1969 scientists, working with data recorded on July 2nd 1967, found a spike in the data, a dip, a second spike, and a long, gradual tail off. As the team leader, Ray Klebasabel said: "One thing that was immediately apparent was that this was not a response to a clandestine nuclear test". His team checked for possible solar flares and supernovae and found none that might have caused the mysterious event. The number of recorded events rose rapidly as more sensitive detectors were carried by later generations of Vela satellites. Later, as pairs of satellites were launched with improved timing capabilities, it became possible to approximately determine the directions from which the gamma ray pulses originated. The arrival times of the pulses at the satellite pairs Vela 5a and 5b and 6a and 6b could be measured to an accuracy of 1/64 of a second whilst the light travel time between the satellite pairs across their orbital diameters was around 1 second. This enabled the direction of the event relative to the line between each pair of satellites to be determined to about 1/5th of a radian or 10 degrees. Given the two pairs of satellites one could then derive one or two possible directions for the source of the event. As they suspected, they found was that the bursts came from outside the solar system and also by their random scatter across the sky, the data hinted that the sources lay, not in our galaxy (in which case one would expect the sources to lie along the plane of the milky way) but in the universe beyond. Klebasabel published the first results in 1973, detailing 16 confirmed bursts in a paper in the journal Nature entitled “Observations of Gamma-Ray Bursts of Cosmic Origin". As a result, a far more sensitive gamma ray satellite observatory was designed and built. Called the Compton Gamma Ray Observatory, it was launched in 1991 and joined a wide array of Earth satellites and deep space probes that carried much smaller detectors. Over a period of 6 years it observed nearly 2000 bursts which showed that they had an isotropic distribution across the sky and so confirmed that they were not associated with our own galaxy. [Note: On September 22, 1979, the Vela satellites did detect one possible nuclear test that appeared to have taken place over the Atlantic and is sometimes referred to as the South Atlantic Flash. In addition, the Arecibo ionospheric observatory in Puerto Rico detected an anomalous ionospheric wave during that morning - an event which had not been observed previously by the scientists. Unconfirmed reports indicate that it was a nuclear test initiated by South Africa with possible assistance from Israel.] Gamma ray burst profiles: those on the left are typical of the short bursts (less than 2 seconds) whilst those on the right are typical of long bursts (greater than 2 seconds). What causes the Gamma Ray bursts? For many years after the discovery of GRBs, astronomers searched for a counterpart: an astronomical object whose position agreed with that of a recently observed burst. All such searches were unsuccessful, and where, in a few cases, the position of the GRB was particularly well defined, no bright objects of any nature could be seen. This suggested that the origin of these bursts were either very faint stars or extremely distant galaxies. What was really required were exceedingly fast follow up observations at other wavebands so that, should a gamma ray burst be observed, its source could be immediately identified. The breakthrough came in February 1997 when the satellite BeppoSAX detected a gamma-ray burst (GRB 970228). Its X-ray camera was immediately pointed towards the direction from which the burst had originated and detected rapidly fading X-ray emission. More significantly still, 20 hours after the burst, the UK’s William Herschel Telescope on La Palma was able to identify a fading optical counterpart. Once the GRB had faded, deep imaging was able to identify a faint, distant host galaxy at the location of the GRB. Because of the very faint luminosity of this galaxy, its exact distance was not measured for several years but well before, a further breakthrough occurred with the BeppoSAX discovery of GRB 970508 later that year. The position of this event was found within four hours of its discovery so allowing research teams to begin making observations much sooner than for any previous burst. The spectrum of the object revealed a redshift of z = 0.835, placing the burst at a distance of roughly 6 billion light years from Earth so providing the first accurate determination of the distance to a GRB. This proved that GRBs occur in extremely distant galaxies. As time is of the essence in making follow up observations after the detection of a GRB, the locations determined by the current gamma-ray telescopes such as Swift, are instantly transmitted over the Gamma-ray Burst Coordinates Network (GRBCN). These positions can then be used to rapidly slew earth based telescopes onto the source position in time to observe the afterglow emission at longer wavelengths. The Swift spacecraft, which was launched in 2004 and still operational, is equipped with on-board X-ray and optical telescopes which can be rapidly and automatically slewed to observe the afterglow emission following a burst detected by its very sensitive gamma ray detector. The swift satellite observing a GRB with an artist’s impression of how one might look. On the ground, numerous optical telescopes have now been built or modified to incorporate robotic control software that responds immediately to signals sent through the GRBCN. This allows the telescopes to rapidly slew towards a GRB within seconds of receiving the positional data and make follow-up observations whilst the gamma-ray emission is still present. There was an interesting, though not realised, possibility in 2008. The GRB, 080319B, had an extremely luminous optical counterpart that peaked at a visible magnitude of 5.8. Given a very dark and transparent sky this could have been seen with the unaided eye. Should anyone have been looking in the right direction at this time, the photons that fell on their retina would have been travelling for 7.5 billion light years as so he or she would have looked back in time more than halfway towards the origin of the universe! In 2009, the Swift Gamma-Ray Burst Mission detected GRB 090423 in the constellation Leo. Its afterglow was detected in the infrared and this allowed astronomers to determine its redshift. Having a z of 8.2, this makes GRB 090423 the second most distant object currently known in the universe. At the time of its discovery it was earliest object ever detected and its light was emitted when the universe was only 630 million years old! [In October 2010, the European Southern Observatory’s Very Large Telescope in Chile observed a galaxy in the infrared that has a redshift of 8.55 giving a distance of 13.12 billion light years. Its light was emitted just 600 million years after the origin of the universe. As the universe has been expanding since its light was emitted, it is now though to be at a distance of 30 billion light years!] So let us summarise what was known: gamma-ray bursts are flashes of gamma rays associated with extremely energetic explosions in distant galaxies and are the most luminous electromagnetic events known to occur in the universe. Bursts can last from milliseconds to several minutes, although a typical burst lasts a few seconds. The bursts are classified into two types, short – less than 2 seconds in length – and long – greater than 2 seconds. The initial burst is usually followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared and radio). How much energy is released? The measurement of the approximate distance to the gamma-ray burst 970508 in 1997 made it possible to calculate the energy emitted during the event.
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