Thrifty telescope reveals new details of Universe's most violent explosions

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LOS ALAMOS, N.M., April 1, 1999 -- A low-cost, automated telescope built from recycled lenses and hardware recently fulfilled a quest astronomers have pursued for 25 years. In doing so it has provided critical new details of gamma ray bursts, brief emissions of high-energy photons traveling to Earth from unimaginably 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 described 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. It also recorded the outburst 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 of the University of Michigan. "In the first case, you have a much better chance of understanding what caused the crash.

"We are not likely to understand the gamma-ray burst phenomenon from a single event, however. 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," Akerlof said.

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 four-inch-diameter lenses are connected to charge-coupled devices -- basically the same technology found in digital cameras now on the consumer market. In fact, Akerlof said, to keep costs low, the components for ROTSE were gathered from sources such as junk yards, used-camera shops and the amateur astronomy market.

ROTSE-1 cost about $200,000 to develop -- a bargain among today's astronomical instruments -- with principal funding from NASA, the Research Corporation (a philanthropic organization) and the Planetary Society.

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 ROTSE 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 had aimed itself at the estimated location of the outburst. A software glitch caused ROTSE-1 to erase its first exposure, but it subsequently recorded a series of seven images over ten minutes, starting at 22 seconds after the initial detection of gamma rays. The images showed the brightening and fading of the optical component of the burst.

Although many astronomers have sought the same goal, ROTSE-1 was the first instrument to respond quickly enough to succeed in the task.

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. Thereafter the object faded. At its peak brightness the object was an estimated six million times brighter than a typical supernova, an exploding star that can by itself briefly outshine an entire galaxy.

The ROTSE-1 observation thus represents the most luminous optical object ever detected. In fact, said Los Alamos' Jeff Bloch, "if you had been gazing at that spot with binoculars, you would have seen a 'star' appear, brighten, and fade within minutes. That an explosion nine billion light years away would be visible for a few seconds to any person with a pair of binoculars is truly mind boggling. What could cause such a vast release of energy is an enduring mystery."

ROTSE-1 during its routine sky patrol also observed the location of the outburst about two hours prior to the burst; that observation showed no object visible down to ROTSE-1's limit of sensitivity, setting the most stringent limit on an optical precursor ever obtained.

Finally, by detecting optical emission coincident with the gamma rays, the ROTSE-1 observations suggest that the 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.

ROTSE-1's observations of the Jan. 23 outburst will help astronomers constrain theories of what kind of cataclysmic event produces beacons that can be seen from the far edge of the universe.

To understand how much energy is involved, Akerlof noted that detonating a one-megaton thermonuclear weapon converts into energy the equivalent mass of a bran muffin in a fraction of a second. The January gamma ray burst converted into energy an amount of mass equal to the sun in just a few seconds.

ROTSE-1 currently is located at Los Alamos National Laboratory in New Mexico. The team plans to move the instrument to a much darker site at Fenton Hill in the Jemez Mountains near Los Alamos. From this location, ROTSE-1 will be able to detect much fainter objects. Several other telescopes, including the Milagro cosmic ray observatory, are being developed or planned for Fenton Hill.

Los Alamos scientists three decades ago published the first reports of gamma ray bursts, which were originally spotted by gamma ray detectors aboard the first satellites launched to spot evidence of clandestine nuclear weapons development.

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