Jan 27, 1999

GOTCHA! The Big One That Didn't Get Away

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GOTCHA! The Big One That Didn't Get Away

Robotic telescope captures visible light from a powerful gamma-ray burst

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Jan 27, 1999: For the first time, scientists have witnessed the visible light emitted at the same time as a gamma-ray burst, a mysterious explosion in the far reaches of the universe.

"This discovery signals yet another new era in the study of these fantastic objects. It is now shown that they can be observed from the ground, in different wavelength regions, while the main part of the explosion is in progress," said Dr. Jerry Fishman, principal investigator for the Burst and Transient Source Experiment (BATSE), a NASA instrument on board the Compton Gamma Ray Observatory which captured the gamma-ray burst, and alerted other observers.

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Right, above: The negative image sequence shows the visible light from the blast at its brightest apparent magnitude of 9 as captured by the Robotic Optical Transient Source Experiment (ROTSE). Interestingly, the peak of the optical brightness did not correspond in time to the peak in brightness of the gamma-ray burst. Credit: Carl Akerlof/University of Michigan/Los Alamos National Laboratory/Lawrence Livermore National Laboratory. Click on the picture to see the (128 KB). The top three images were taken during the gamma-ray burst, the bottom three were taken of the optical afterglow, after the prompt gamma-ray burst event had ended.

This particular burst had the power of nearly ten million billion suns, and the light grew so bright that anyone gazing at the night sky could have seen it using only a pair of binoculars. The chances were slim, however, that someone would be looking at that exact point in the sky at 4:47 a.m. EST, on Jan. 23. But thanks to the use of two satellites, a unique ground-based telescope, and the Internet, scientists around the world were able to pinpoint the location of the burst, and to monitor it from start to finish.

Gamma-ray bursts, brief flashes of gamma-ray energy lasting from a few milliseconds to a few hundred seconds, disappear almost as fast as they appear. These bursts occur randomly, and their source is unknown. Gamma-rays are far outside the visible part of the spectrum and cannot be detected by the human eye. Scientists, using gamma-ray telescopes, detect only a few hundred gamma-ray bursts a year. The combination of all of these factors makes it extremely difficult to study these puzzling objects. Instruments used to capture this most recent burst were BATSE, on board NASA's Compton Gamma Ray Observatory, in co-operation with the Los Alamos National Laboratory's ROTSE (Robotic Optical Transient Search Experiment) and the Italian-Dutch satellite BeppoSAX.

Making Lemonade out of a Lemon

The detection of optical emission at the same time as a gamma-ray burst was made possible by the Gamma-Ray Burst Coordinates Network. Ironically, this outstanding capability was possible only because of an unfortunate hardware failure on the Compton Gamma-Ray Observatory.

GRO was designed to store its data on on-board tape recorders which would be dumped to ground stations several times each day. However, in 1992, early in the mission, both tape recorders failed. Now, with no on-board storage, the only way to get the data is to transmit it in real-time through the Tracking and Data Relay Satellite System, which allows more than 80 percent of the GRO data to be recovered.

Scientists at NASA's Goddard Space Flight Center wondered how they might "make lemonade out of this lemon" by using the real-time data. Led by Dr. Scott Barthelmy, and with the cooperation of the BATSE team, a system was set up to intercept the real-time data at NASA/Goddard, strip out the BATSE data, capture the burst data, compute the direction to the burst, and transmit this information immediately to anyone who could use it to make observations.

The gamma-ray telescope, BATSE, developed at NASA's Marshall Space Flight Center in Huntsville, Ala., continually monitors the sky for gamma-ray sources. Normally, scientists must wait about a day for a sky position to be calculated from BATSE. In order to determine the gamma-ray burst location more quickly, a team of scientists, led by Dr. Scott Barthelmy at NASA's Goddard Space Flight Center, implemented the Gamma-ray burst Coordinates Network (GCN). This system automatically intercepts BATSE data at Goddard, calculates a rough gamma-ray burst position, and within seconds distributes the location over the Internet to eager observers around the world.

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Due to the challenges of determining the burst position in the sky, previous observations of visible light from gamma-ray bursts have all been made hours after the burst had ended. The light by that time was so dim it could only be detected by the world's largest telescopes. But thanks to GCN's rapid response time, the ROTSE-I telephoto camera array in Los Alamos, N.M. was able to watch a gamma-ray burst as it occurred. Dr. Carl Akerlof of the University of Michigan in Ann Arbor, along with several colleagues, designed ROTSE to point quickly to a gamma-ray burst location after receiving GCN data. ROTSE uses four telephoto lenses and charge-coupled devices (CCDs) on an automated pointing system, to image a piece of sky 16.5 degrees across as fast as every 8 seconds.

Above: The ROTSE-1 telescope, which automatically captured the first-ever visible images during a gamma-ray burst. Click for a


ROTSE received a GCN location 4 seconds after the burst was identified by BATSE, and was able to observe the gamma-ray burst a mere 22 seconds after it had begun. At its brightest, the burst reached an apparent magnitude of 8.9, approximately 15 times dimmer than the faintest stars visible to the naked eye.

"If the burst had occurred somewhere in our galactic neighborhood, it would have been so bright that night would've turned into day," according to Dr. Chryssa Kouveliotou, of Universities Space Research Association at the Marshall Space Flight Center.

The entire duration of the gamma-ray burst was only 110 seconds, but in that brief span of time Akerlof's team was able to do what had never been done before: follow the variations of visible light throughout the burst.

Above: This figure shows the how the brightness in gamma-rays, as observed with BATSE, varied during GRB 990123. The three intervals marked by vertical lines indicate the times during which ROTSE obtained its first three visible images. Credit: Dr. Michael Briggs, NASA/Marshall.

sky map
Right: The gamma ray and optical counterpart burst is near the constellation Corona Borealis. In late January, Corona Borealis transits around 7 a.m. local time in the northern hemisphere. The best time to view this part of the sky is in the pre-dawn hours between 4 and 5 a.m.when it will be high in the southeast. A heavenly object "transits" when it passes through a meridian, an imaginary line in the sky drawn from north to south and directly observer's head.

GCN, however, only directs observers to a general area of the sky. Because the images are so detailed - each has almost 17 million pixels - ROTSE scientists used data from the BeppoSAX x-ray satellite to pinpoint the burst location within their image. Dr. Luigi Piro of the Instituto di Astrofisica Spaziale in Rome, Italy, with his BeppoSAX collaborators, provided a precise location of the burst 5 hours after it had begun, within a diameter of 10 arc-minutes. After an additional 3 hours, they were able to refine the position to an accuracy of 4 arc-minutes. (The apparent diameter of the sun in the sky is approximately 30 arc-minutes.)

Other scientists have been quick to use the precise locations provided by BeppoSAX to gather even more detailed observations. A team led by Dr. Stephen Odewahn at the California Institute of Technology used a CCD camera, mounted on a 60-inch telescope at the Mt. Palomar Observatory, to make sensitive optical observations of the burst's afterglow. They found that the light from the burst had dimmed to about 18th apparent magnitude, roughly 5,000 times dimmer than at its peak. They also noticed a faint galaxy located close to the line of sight between the Earth and the burst source, but this galaxy does not appear to be related to the burst.

Left: These are the discovery images of the optical afterglow emission from GRB 990123. The left panel shows the star field as it appeared prior to the explosive gamma-ray burst. A faint galaxy, indicated with an arrow, was originally proposed to contain the gamma-ray burst source. Now, however, the galaxy is thought to be in the foreground. The right panel shows the optical emission, observed about 4 hours after the gamma-ray burst on January 23, dominating the light from the faint galaxy. Credit: Dr. Stephen Odewahn/Caltech-NRAO-CARA GRB Collaboration.

A little more than a day after the burst observation, Dr. Dan Kelson of the Carnegie Institute of Washington and his colleagues used a spectrograph, mounted on the 10-meter (33-ft.) Keck II telescope on Mauna Kea in Hawaii, to analyze the ultraviolet and visible light from the gamma-ray burst. Although the burst had faded considerably by this time, they were able to measure a redshift of 1.6 for the visible light. This redshift measurement implies that the gamma-ray burst occurred at a distance so far away that its light, traveling at 299,792 km/s (186,000 miles per second), took about 10 billion years to reach us.

The intrinsic power of the gamma-ray burst has been estimated to be about 1016(or ten million billion) times that of our Sun. This estimate would be reduced if the faint foreground galaxy observed by Kelson's team acted as a gravitational lens. Such a lens would pull light we normally wouldn't see toward our line of sight, intensifying the amount of light directed toward us. Gravitational lensing would make the burst appear brighter than it actually is. The estimate of the burst intensity would also be reduced if the source were not emitting equally in all directions. Like the headlight of a car, the light from the burst may be concentrated toward us.

Astronomers are planning further observations to learn more about these mysterious gamma-ray bursts. To check on the possibility of lensing, BATSE scientists at NASA/Marshall are monitoring their data for other gamma-ray bursts coming from the same location in the sky. In addition, once the optical light has faded sufficiently, astronomers will use the world's most sensitive telescopes, including NASA's Hubble Space Telescope, to try to identify any galaxies lying near the gamma-ray burst.

Defining magnitude

Astrophysicists express the brightness of stars in visible light in two related forms.

Apparent visible magnitude, mv, measures the light that actually reaches us on Earth. But that is not the true measure of a star or galaxy's brightness because distance makes things appear dimmer. So, absolute magnitude - Mv - is used to compare how bright stars would be if they all were 32.6 light years (10 parsecs) away.

Bright stars are ranked first and get the low numbers, followed by dim stars with higher numbers (6th magnitude is the faintest that the naked eye can detect; reaching mv = 25 requires extremely sensitive telescopes and instruments). A few have negative magnitudes given when modern instruments showed them to be brighter than the initial magnitudes given by astronomers just using their eyes. A single magnitude is about 2.5 times brighter than the next dimmer magnitude. A 100-fold difference in brightness makes a difference of 5 in magnitude.

At mv = -1.45, Sirius A has the greatest apparent magnitude of any star in our sky. At Mv = +1.41, it's still in the Top 20 for absolute magnitude. But that parade is led by Deneb, a blue supergiant in Cygnus (the Swan), with a whopping Mv = -7.3. The closest competitors are Antares and Mimosa (beta Cruces) at Mv = -4.7.

Web Links
Cosmic Gamma Ray Bursts NEWS & RESEARCH

Gamma-ray Bursters cross the 'Line of Death' - Oct. 13, 1998. A study of gamma ray burst spectra shows one more thing that these mysterious, cosmological gamma ray bursts are not.

Blast from the past: the latest clue in solving the gamma-ray burst mystery (May 6, 1998).

Gamma-ray burst identification earns top prize (Jan. 12, 1998)

Twinkle, twinkle, massive fireball - reports from the 4th Huntsville Gamma-ray Burst Symposium (Sept. 17, 1997)
Discovery may be "smoking gun" in gamma-ray mystery (March 31, 1997).

Never before have scientists been able to watch the visible light during a gamma-ray burst. Ever since the bursts were first discovered in 1967, scientists have been eager to gather more data about these elusive, cataclysmic explosions. Now, thanks to this collaboration between teams of international astronomers made possible by the communication capabilities of the Internet, scientists can learn more than they ever have before.

"This discovery demonstrates the power of multiple spacecraft and ground-based instruments working together via the Internet to solve some of the greatest mysteries in astrophysics," Fishman said.

Thanks to this type of co-operation, scientists have now made an immense leap toward solving the enigma of gamma-ray bursts.

More web links

ROTSE - home page, University of Michigan

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Authors: Dr. Tom Koshut, Leslie Mullen
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