A swift look at the biggest explosions in the universe

October 28, 1999

NASA selects a mission to rapidly locate gamma-ray burst sources
Oct. 29, 1999: The story of gamma-ray bursts is becoming like the biography of a film star who hits the jackpot after years of bit parts. Bursts were discovered in the late 1960s by nuclear test detection satellites. Until the 1980s they were monitored by instruments that were piggybacked on satellites designed for other missions.

By then the mystery had led NASA, in 1978, to select the the Burst and Transient Source Experiment (BATSE) as one of four instruments aboard the Compton Gamma Ray Observatory (launched in 1991). BATSE was envisioned as a fire alarm that would notify the other instruments on the observatory to help scientists settle this nagging little mystery.

In this role, BATSE wins as "best supporting actor" by showing that bursts are perhaps the most violent explosions we can observe in the universe. This has swept up the astrophysics community. More than 3,200 professional papers have been written about bursts, says Dr. Kevin Hurley of the University of California at Berkeley, and papers are being published at the rate of 1.3 per day, faster than bursts are recorded.

As with any star that has made good, the next step is his or her own starring role. That comes in 2003 with the planned launch of Swift. Breaking with NASA tradition, the name isn't an acronym. It describes how quickly the spacecraft is designed to swing around and put an array of telescopes on target and capture bursts before they fade.

Swift will carry three instruments, the Burst Alert Telescope (BAT), an X-Ray Telescope (XRT), and an Ultraviolet/Optical Telescope (UVOT). The primary mission is quite simple, said Dr. Neil Gehrels, the principal investigator at NASA's Goddard Space Flight Center. He described Swift in the last session (Instrumentation) of the 5th Huntsville Gamma Ray Burst Symposium held in Huntsville, Ala., last week.

"We know that long bursts are associated with faint galaxies at least halfway to the edge of the known universe," Gehrels said. "But what we don't know is, what are the physical origins of bursts? What are their progenitors [the stars that become bursts] and what is the physics that goes on inside?"

Swift's three instruments will help answer those questions.

First, BAT will detect the onset of a gamma-ray burst. Unlike BATSE, which has eight modules that view the entire sky (other than what the Earth blocks), BAT will view a smaller fraction of the sky. It will comprise a special kind of pinhole camera called a coded aperture mask placed in front of a large solid-state detector. This will let BAT calculate a burst's location to within a few arc-minutes (a fraction of the Moon's apparent diameter and much finer than BATSE can do).

Next, the spacecraft swings to aim the XRT and UVOT in the neighborhood of the burst. The XRT is based on a proven design for Spectrum X, a Russian/European/U.S. mission, set for launch in 2003. It is sensitive to X-rays in the 0.2 to 10 kilo-electron volt (keV) range and has a 24 arc-minute field of view, slightly smaller than the Moon's apparent diameter. The UVOT, derived from the optical telescope that Europe's X-ray Multi-Mirror Mission satellite (XMM) will carry, has a 30 cm (12 in.) primary mirror, equivalent to a 4-meter (13.2 ft) telescope on the ground, Gehrels said. It will have a 17 arc-minute field of view (slightly more than half the apparent diameter of the Moon) and sensitivity from 170 nm (ultraviolet) down to 650 nm (deep red).

Using the two telescopes, scientists should be able to locate bursts to within 0.3 arc-seconds, and to tell whether the burst has an optical transient that should be the target of follow-up observations by larger observatories in orbit or on Earth.

While waiting for bursts to go off, Swift will map the sky at high x-ray energies. This hasn't been done since the first High Energy Astronomy Observatory (HEAO-1), which orbited during 1977-79. BAT will be 50 times more sensitive that HEAO-1's Hard X-Ray/Low Energy Gamma Ray Experiment, so it will provide more detailed maps that will help observers find new targets for the Chandra X-ray Observatory and XMM.

This mapping mission will require some patience, though, since Swift will automatically chase a burst as soon as one is detected. Another break from NASA tradition, Gehrels explained, is that Swift's burst data will be made available as soon as they come through since time is of the essence in burst observations. Within 10 seconds, Swift should have a 10 arc-minute determination of a burst's location. In less than three minutes, it will have an X-ray or optical determination to less than an arc-second. And that will allow follow-up observations, in the weeks and months that follow, with large observatories like the Hubble Space Telescope that require much more planning to repoint. And beyond that? After a lead role, most stars look for a blockbuster role. In this case, it will be the Next Generation Gamma-Ray Burst Observatory. "We want NASA to begin to form a group within this next year to study the mission requirements" for what would come after Swift, said Dr. Gerald Fishman, principal investigator for BATSE at NASA's Marshall Space Flight Center. "Swift serves as a pathfinder" for the next-generation instrument, Fishman said. "We won't firm up plans until it makes its observations" since those could change the requirements. The design is so distant for now that the next-generation telescope might comprise several spacecraft operating together, and almost certainly will operate interactively with advanced missions like the Gamma-Ray Large Space telescope (GLAST) planned for launch in 2005. The next-generation instrument would also be used in concert with major observatories on Earth and in orbit.

"This mission is seen primarily as a NASA facility," Fishman continued, "designed by the entire NASA community and used by the science community. Although NASA would play a lead role, it is expected to have international support."

Engineers at MIT prepare HETE-II for launch. Credit: MIT

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Fishman said scientists are also looking at a new operational model that would involve the National Science Foundation as a full partner rather than having the observatory operated and funded primarily by and for NASA. NSF operates many of the United States' ground-based observatories.

"Since the science is something of interest to both agencies" - ground-based observatories often seek optical counterparts for bursts - "it should be funded by both agencies," Fishman said.

A Clemson University student examines the primary mirror for Super-LOTIS. Credit: Clemson University.

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Since Swift won't start chasing bursts until 2003, the Next Generation Gamma-Ray Burst Observatory won't even be designed until around 2005 for launch in 2010 or later.

Meanwhile, other instruments are helping keep gamma-ray bursts in the limelight. The High-Energy Transient Explorer (HETE-II; the first failed to reach orbit in 1998) is set for launch on Jan. 23. It has a smaller detector than Swift and will only stare at a section of sky away from the sun, so it will detect only 30 or so bursts a year. But HETE-II also has ultraviolet and X-ray instruments that will provide a refined location to help larger telescopes target bursts for follow-up observations.

Chasing those locations will be Super LOTIS, built from an old 60 cm (24-in) reflector telescope provided by the Lick Observatory. Super LOTIS - the Livermore Optical Transient Imaging System built by Lawrence Livermore National Laboratory - has been in tests since "first light" on Feb. 25, 1999. In addition to looking at night for optical afterglows of gamma-ray bursts, it is programmed to record burst triggers that happen during the day and then try to locate their afterglows at night. Super LOTIS is scheduled to be relocated from Lawrence Livermore to the Kitt Peak National Observatory near Tucson, Arizona where observing conditions are better.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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