Public Release: 

Scientists Catch Another Gamma-Ray Burster In Visible Light

NASA/Marshall Space Flight Center--Space Sciences Laboratory

Eyes in the Southern Hemisphere are turning to capture the fading glory of a gamma-ray burst that appeared on May 10. The Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory observed the powerful burst.

"It was a strong burst with a lot of structure, about 100 seconds long," said Dr. Chip Meegan, a lead BATSE co-investigator here at NASA's Marshall Space Flight Center. "It's in the top 4 percent for peak flux," that is, maximum intensity.

At the same time, Beppo-SAX, a Dutch-Italian satellite, saw the burst in gamma rays and X-rays, and used its wide-field camera to provide a more precise location than BATSE can provide. The Beppo-SAX team reported that the X-ray brightness peaked at 4.3 times that of the Crab Nebula, and had an average brightness of 0.4 Crab.

The team also pinned down GRB 990510, as it is designated, in the southern hemisphere in a constellation known as the Chamaeleon, at right ascension of 13h38m, and a declination of -80 deg. 29'.

With a refined location, several southern observatories were aimed at GRB990510 in search of an visible-light counterpart (because GRB 990510 is in the Southern Hemisphere, it cannot be seen by the ROTSE array that captured the Jan. 28, 1999, burst). Astrophysicists are eager to determine if burst sources are associated with galaxies or other objects, and to measure how they fade through optical and radio wavelengths.

The Australian National University's 1.3-meter Mount Stromlo telescope was first to spot the counterpart, but was delayed in reporting it.

The first on record was P.M. Vreeswijk of the University of Amsterdam using the 1-meter Swope telescope at the European Southern Observatory, in the Chilean Andes just 9 hours after the burst was noted by BATSE. Vreeswijk is a member of the science team, led by Jan van Paradijs, that found the first GRB optical counterpart in February 1997. Van Paradijs works at the University of Alabama in Huntsville.

In quick succession, sightings were reported by ANTU, one of the 8.2-meter Very Large Telescopes at ESO's Paranal Observatory - which just recently had "first light" - and the 1.3-meter Warsaw telescope in Las Campanas, Chile, operated by Poland, and by the 1-meter South African Astronomical Observatory telescope at Sutherland, South Africa.

With those observations, astronomers now report a redshift of z=1.619, putting the burst source about 10 billion light years away. Astronomers are now trying to determine if it is associated with a galaxy that can't be seen until the burst fades away.

The redshift is a measure of how far known spectral lines are shifted due to the expansion of the Universe. Astronomer Edwin Hubble noted early on in this century that objects in deep space appear to be moving away from our own Milky Way, and the farther away, the faster they appear to be moving.

It is this apparent motion that causes the shift in the spectral lines. Scientists now know that this is apparent velocity is not actually caused by distant galaxies moving through space, but instead we are observing the actual expansion of the Universe and everything in it. A redshift of 1.6 means the expansion of the universe has caused lines in the object's spectrum to be shifted by a factor of 2.6 in wavelength (1+z).

The total energy from the burst is estimated at 1.6x1053 ergs (that's 16 followed by 52 zeroes), equivalent to our sun's output for 1.3 trillion years - about 88 times the current age of the universe.

The ANTU VLT team also reports detecting slight polarization of the light, indicating that at least part of the light is emitted by electrons spiraling along strong magnetic field lines.

As observed by BATSE, GRB990510 looks like two bursts with an initial burst lasting about 10 seconds, followed by a 30-second gap, and then another burst that trails off gradually until it makes a final hiccup about 90 second after the first flash of gamma rays.

"Bursts always look longer when they're stronger," Meegan explained. "This one may be a little longer than average, but it's not unusual." Within each of the spikes in the burst profile are four of five narrower spikes.

"It looks like overlapping FREDs," Meegan said, referring to Fast-Rise, Exponential-Decay. These are bursts that appear quickly, and then weaken and fade over a long period of time.

"That's common and it's one of the things that make bursts so baffling," Meegan said. "Somehow the energy release is done in episodes and the shock waves from each of these are hitting each other. The question is, why is the central engine doing things sporadically?"

Meanwhile, astronomers in the Southern Hemisphere are checking back on GRB 990510 several times a day to measure the fading brightness of the optical counterpart.

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