The Swift X-ray Telescope (XRT) has seen first light, capturing a dazzling image of Cassiopeia A, a well-known supernova remnant in the Milky Way galaxy, and also has discovered its first gamma-ray-burst afterglow.

First-light image from the Swift X-ray Telescope, of the Cassiopeia A supernova remnant, shows the remnant of a gigantic stellar explosion that occurred in about 1680. Credit: NASA/PSU

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 with partners at the Brera Astronomical Observatory in Italy and the University of Leicester in England.

"We have a beautiful image of Cassiopeia A in all its fiery glory, and this is just a test," said David Burrows of Penn State, 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." 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 will help scientists unravel the mystery of gamma-ray bursts, the most powerful explosions known in the universe. These bursts are common, yet random and fleeting, lasting only a few millisecond to about a minute. Gamma-ray bursts likely signal the birth of a black hole. After the short, bright burst, the embers of the explosion linger 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, which is why they are so exciting.

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 sharp resolution 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 disappear quickly," said Alan Wells, leader of the XRT 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."

An artist's concept illustrates the Swift mission. Credit: Spectrum Astro

While the Cassiopeia A image is pretty, this was just a calibration test for the XRT. This 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 then sent to scientists around the world so that they can view the afterglow with other telescopes -- from small university observatories to the "big guns" like the Keck Telescope and the Hubble Space Telescope.

Next, the XRT collects spectra, or X-ray-light fingerprints, of the afterglow to reveal the nature of the explosion. Spectra 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.

The XRT obtained accurate positions and spectra of a gamma-ray-burst afterglow for the first time on 23 December 2004. "This first XRT afterglow demonstrates that the XRT will make quick, accurate measurements of gamma-ray burst positions, as well as spectral measurements that will provide important clues to the origins of these dramatic objects," said Guido Chincarini, the leader of Swift's Italian science team at the University of Milan and the Brera Astronomical Observatory.

Scientists 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. Some 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.

Swift is a medium-class explorer mission managed by NASA Goddard. Swift 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.