The University of Michigan
412 Maynard Ann Arbor, MI 48109-1399
March 24, 1999 (35)
Contact: Sally Pobojewski, (734) 647-1844, E-mail: [email protected]
or John Gustafson, (505) 665-9197, E-mail: [email protected]
Thrifty telescope reveals new details of universe's most violent explosions.
ANN ARBOR---A low-cost, automated telescope built from recycled lenses and hardware is giving scientists important new information about gamma ray bursts---brief emissions of high-energy photons traveling to Earth from violent explosions in the deepest reaches of space.
Astronomers from the University of Michigan and the Department of Energy's Los Alamos and Livermore National Laboratories will describe these new details---including their measurements of the brightest optical celestial object ever recorded---in the April 1 issue of the science journal Nature.
The Nature paper presents analyses of measurements from a gamma ray burst observed Jan. 23 by a telescope called ROTSE-1, for Robotic Optical Transient Search Experiment. ROTSE-1, responding to a detection signaled by NASA's Compton Gamma Ray Observatory, captured optical emissions from the burst while the gamma rays were still arriving---the first time such an observation has ever been made---and recorded the burst while the optical emission was still peaking. Previous detections of the optical counterparts of gamma ray bursts have caught only the faint, fading afterglow of the event.
"It's like the difference between watching two cars collide and coming on the accident scene several hours later," said Carl Akerlof, U-M professor of physics. "In the first case, you have a much better chance of understanding what caused the crash."
ROTSE-1 uses four 35-mm telephoto lenses "of a variety favored by paparazzi for photographing elusive subjects under dim light conditions," Akerlof said. The 4-inch-diameter lenses are connected to charge-coupled devices---basically the same technology found in digital cameras now on the consumer market. To reduce costs, ROTSE's components were gathered from junk yards, used-camera shops and the amateur astronomy market.
The four cameras, strapped to a single mount, point to slightly different, but overlapping, sections of the sky. Together they capture 250 square degrees of the sky at once, about what would be covered by a dinner plate held at arm's length. Under computer control, ROTSE-1 automatically scans from horizon to horizon, taking a series of short exposures that together cover the entire visible night sky. In clear weather, ROTSE can compile a complete sky record twice each night. But the telescope also is designed to respond immediately to unexpected events flaring in the night sky.
On Jan. 23, ROTSE-1 interrupted its normal sky search after the orbiting Compton Gamma Ray Observatory spotted a powerful emission of gamma rays, triggering an automated alert network. Ten seconds after the CGRO detection, ROTSE-1 aimed itself at the estimated location of the outburst and recorded a series of seven images over 10 minutes, beginning 22 seconds after the initial detection of gamma rays.
Akerlof and his colleagues used a precise location provided by the European satellite Beppo-Sax to locate the optical transient in their images. After a short time the team was able to reject other possible sources for the signal, such as flare stars, meteors or Earth-orbiting satellites. After completing this analysis, the team broadcast a worldwide announcement via the Internet.
>From ROTSE-1's first exposure to the second, the optical brightness increased about 16-fold. At its peak brightness the object, estimated to be 9 billion light-years from Earth, was about 6 million times brighter than a typical supernova---an exploding star that can by itself briefly outshine an entire galaxy.
The ROTSE-1 observation represents the most luminous optical object ever detected. "If you had been gazing at that spot with binoculars, you would have seen a 'star' appear, brighten, and fade within minutes, an unbelievably violent event from the very edge of our universe," said Galen Gisler, an astrophysicist at Los Alamos National Laboratory.
By detecting optical emissions coincident with gamma rays, the ROTSE-1 observations suggest that gamma rays are not produced when the explosion slams into some surrounding material, as some models of gamma ray bursts hold, but rather are produced internally near the site of the original cataclysm, Akerlof explained.
"We are not likely to understand the gamma-ray burst phenomenon from a single event, however," Akerlof added. "Only with more coincident observations of gamma rays, optical and other emissions can we begin to sort out the common elements. That job will be carried on by a new generation of more powerful telescopes, including one we are developing called ROTSE-2."
ROTSE-1 currently is located at Los Alamos National Laboratory in New Mexico. The telescope cost about $200,000---a bargain among today's astronomical instruments---with principal funding from NASA, the Research Corporation (a philanthropic organization) and the Planetary Society.
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