Gamma-ray space observatory fully operational

The third telescope aboard NASA’s Swift gamma-ray observatory, the Ultraviolet/Optical Telescope (UVOT) with key involvement from UK scientists at University College London’s Mullard Space Science Laboratory, has seen first light and is now poised to observe its first gamma-ray burst. The UVOT captured an image of the Pinwheel Galaxy, known by amateur astronomers as the ‘perfect’ face-on spiral galaxy. With the UVOT turned on the Swift observatory is fully operational. Swift’s two other instruments – the Burst Alert Telescope (BAT) and the X-ray Telescope (XRT) with University of Leicester involvement – were turned on over the last few weeks and have been snapping up gamma-ray bursts ever since.

Swift is a NASA-led mission dedicated to unravelling the mysteries of gamma-ray bursts – random and fleeting explosions that signal the likely birth of black holes. “This was a real treat to point the UVOT toward the famous Pinwheel Galaxy, M101,” said Dr. Peter Roming, UVOT Lead Scientist at the US Penn State University. “The ultraviolet wavelengths in particular reveal regions of star formation in the galaxy’s spiral arms. But more than a pretty image, this first-light observation is a test of the UVOT’s capabilities.”

Swift’s three telescopes work in unison. The BAT instrument detects gamma-ray bursts and autonomously turns the satellite in seconds to bring the burst within view of the XRT and the UVOT, which provide detailed follow-up observations of the burst afterglow. Although the burst itself is gone within seconds, scientists can study the afterglow for clues about the origin and nature of the burst, much like detectives at a crime scene.

The UVOT serves several important functions. First, it will pinpoint the gamma-ray burst location a few minutes after the BAT detection. The XRT provides a burst position within a 1- to 2-arcsecond range. The UVOT will provide sub-arcsecond precision, a spot on the sky about as wide as the eye of a needle at arm’s length. This information is then relayed to scientists at observatories around the world so that they can view the afterglow with other telescopes.

As the name implies, the UVOT captures the optical and ultraviolet component of the fading burst afterglow. Prof. Keith Mason, the UK UVOT lead at University College London’s Mullard Space Science Laboratory explains, “The ’big gun’ optical observatories such as Hubble and Keck have provided useful data over the years, but only for the later portion of the afterglow. The UVOT isn’t as powerful as these observatories, but has the advantage of observing from the very dark skies of space. Moreover it will start observing the burst afterglow within minutes, as opposed to the day or weeklong delay inherent with heavily used observatories. This is extremely important because the bulk of the afterglow fades within hours.”

The ultraviolet portion will be particularly revealing, says Roming. “We know nearly nothing about the ultraviolet part of a gamma-ray burst afterglow,” he said. “This is because the atmosphere blocks most ultraviolet rays from reaching telescopes on Earth, and there have been few ultraviolet telescopes in orbit. We simply haven’t yet reached a burst fast enough with an ultraviolet telescope.”

The UVOT’s imaging capability will enable scientists to understand the shape of the afterglow as it evolves and fades. The telescope’s spectral capability will enable detailed analysis of the dynamics of the afterglow, such as the temperature, velocity and direction of material ejected in the explosion.

The UVOT will also help scientists determine the distance to the closer gamma-ray bursts, within a redshift of 4, which corresponds to a distance of about 12 billion light years. The XRT will determine distances to more distant bursts. Scientists hope to use the UVOT and XRT to observe the afterglow of short bursts, less than two seconds long. Such afterglows have not yet been seen; it is not clear if they fade fast or simply don’t exist. Some scientists think there are at least two kinds of gamma-ray bursts: longer ones (more than two 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 UVOT and XRT will help rule out various theories and scenarios.

The UVOT is a 30-centimeter telescope with intensified CCD detectors and is nearly identical to an instrument on the European Space Agency’s XMM-Newton mission. The UVOT is as sensitive as a four-meter optical ground-based telescope. The UVOT’s day-to-day observations, however, will look nothing like the Pinwheel Galaxy. Distant and faint gamma-ray burst afterglows will appear as tiny smudges of light even to the powerful UVOT. The UVOT is a joint product of Penn State and the Mullard Space Science Laboratory.