Only some 30 days since its launch NASA's Swift X-ray Telescope (XRT), with key involvement from UK scientists at the University of Leicester, has discovered its first gamma-ray-burst afterglow during its initial activation phase. During its first view of the cosmos it captured a dazzling image of Cassiopeia A, a well-known supernova remnant in the Milky Way galaxy.
"We have a beautiful image of Cassiopeia A in all its fiery glory, and this is just a test," said Prof. David Burrows of Penn State University in the US, the lead scientist for the XRT. "Even more exciting is our discovery of our first X-ray afterglow of a gamma-ray burst, which is exactly what the XRT was designed for. No sooner had we turned this on than 'presto,' we bagged our first gamma-ray burst afterglow."
Prof. Burrows said that the first light and this first afterglow demonstrate that the XRT is working well and that spectacular observations are sure to follow soon.
The XRT is one of three instruments aboard the NASA-led Swift satellite, which was launched on 20 November 2004. The XRT was built at Penn State University with partners at the University of Leicester and the Brera Astronomical Observatory in Italy.
The XRT will help scientists unravel the mystery of gamma-ray bursts, the most powerful explosions known in the universe - emitting more than one hundred billion times the energy that our Sun emits in an entire year. These bursts are common, yet random and fleeting, lasting only a few milliseconds to about a minute. Gamma-ray bursts could signal the birth of a black hole. After the short, bright burst, the remnant of the explosion lingers on for hours or days as an 'afterglow' of X-rays and optical light. Much is still unknown about the causes of gamma-ray bursts, Swift has been eagerly awaited since it was planned in 1998.
A gamma-ray burst will trigger Swift's Burst Alert Telescope (BAT) to autonomously point Swift's XRT and UV/Optical telescopes toward the burst within about a minute. These two telescopes have fine resolution, enabling them to capture the afterglow before it fades away and to reveal details never previously seen.
"Speed is crucial, because clues to what caused the burst may disappear quickly," said Prof. Alan Wells, leader of the XRT development effort at the University of Leicester. "In the past it has taken hours to view the afterglow with a high-quality telescope. Now we'll be on the scene within minutes."
The XRT obtained an accurate position, spectrum and decay light curve of a gamma-ray-burst 'afterglow' for the first time on 23 December.
Dr Julian Osborne, Swift operational phase team leader at Leicester said, "This first GRB data is very exciting for us. The XRT performed beautifully. The results from this observation show that the XRT will provide unprecedented data from the very earliest phases of the GRB afterglow. We are looking forward to a wealth of new information about black hole creation, stellar death and the early Universe."
While the Cassiopeia A image is pretty, this was just a first test for the XRT. The telescope's full-time job will be capturing images and spectra of explosions far beyond the Milky Way galaxy, some farther than 12 billion light years.
The XRT will perform two important functions. First, it will pinpoint the location of the gamma-ray burst. The BAT gets close, but the XRT nails the location down because of its fine resolution. This information is sent immediately to scientists around the world - including UK astronomers using facilities such as the Faulkes Telescopes in Hawaii and Australia, the William Herschel and Liverpool Telescopes in La Palma and the European Southern Observatory's suite of telescopes in Chile - so that they can study the afterglow while it is still bright.
Next, the XRT collects spectra of the afterglow to reveal the nature of the explosion. They show the amount of X-ray light at different wavelengths, and can contain information about the type, temperature, velocity, and range of energies of the atoms in the regions surrounding the burst and also can provide a measure of how long ago in the early universe the gamma-ray burst actually occurred.
Scientists also hope to use the XRT to observe the afterglow of short bursts, less than two seconds long. Such afterglows have not yet been seen, and it is not clear whether they exist. Scientists think there are at least two kinds of gamma-ray bursts: longer ones (more than ten seconds) that generate afterglows and that seem to be caused by massive star explosions; and shorter ones that may be caused by mergers of black holes or neutron stars. The XRT will help rule out various theories and scenarios.
The XRT is a prime example of the value of international collaboration in reducing costs and bringing additional expertise and instrument technology to a NASA-lead mission. The Brera Astronomical Observatory supplied the X-ray mirror that images the X-ray sky. The University of Leicester provided system-design expertise and hardware for the telescope, particularly the camera system that detects the X-rays. Penn State provided the electronics and telescope tube, and is responsible for controlling the XRT in flight. The University of Leicester is also providing a UK Swift Science Data Centre.
Swift is a medium-class explorer mission managed by NASA Goddard. This is a NASA mission with participation of the Italian Space Agency and the Particle Physics and Astronomy Research Council in the United Kingdom. It was built in collaboration with national laboratories, universities and international partners, including Penn State University in Pennsylvania U.S.A.; Los Alamos National Laboratory in New Mexico U.S.A.; Sonoma State University in California U.S.A.; the University of Leicester in Leicester, England; the Mullard Space Science Laboratory in Dorking, Surrey, England; the Brera Observatory of the University of Milan in Italy; and the ASI Science Data Center in Rome, Italy.
A high-resolution image to illustrate this story is available
The image and further information can also be found at:
* NASA - Swift * UK Swift Data Science Centre
Figure Caption: First-light image from the Swift X-ray Telescope, of the Cassiopeia A supernova remnant. This object is the remnant of a gigantic stellar explosion that occurred in about 1680. The explosion heated the surrounding gas and the remnants of the star to temperatures of several million degrees Celsius. The hot gas has been expanding and cooling for the past 325 years. This image is a true X-ray color image: the lowest X-ray energies are shown in red, the medium energies are in green, and the highest energies are in blue. The bright green filaments are rich in silicon, while red portions are dominated by emission from iron. Supernova remnants like this are responsible for producing much of the material that makes up most of Earth-like planets and for mixing these "heavy" elements into the interstellar gas, where they can form new generations of stars and planets. The image demonstrates that the XRT is working as designed and can perform its job of imaging spectroscopy of astrophysical objects, including the gamma-ray bursts that it was designed to study.
UK Role in Swift
The UK role in Swift has been to provide core elements of the narrow field instruments (the X-ray telescope and the UV/Optical telescope), utilising mature technology already developed for the ESA XMM-Newton mission, and the JeT-X instrument.
University of Leicester
Lead role in the X-ray telescope design, focal plane camera assembly and X-ray design (using past experience from JET-X and XMM-Newton). The UK SWIFT Science DATA Centre, at Leicester, will provide an archive of all SWIFT data, with open access for the wider UK astronomical community.
Mullard Space Science Laboratory, UCL
The major part of the UV/Optical telescope was constructed at MSSL using designs and expertise from the XMM-Newton Optical Monitor.
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