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Scientists catch another gamma-ray burster in visible light

Several telescopes observe optical counterpart

May 18, 1999: 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.

Right: A view of GRB 990510 - labeled OT - seen through the University of Copenhagen's 1.54-meter telescope on La Silla at the European Southern Observatory in Chile. Links to 736x461-pixel, 131K GIF. Credit: University of Copenhagen, ESO

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"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, an Italian-Dutch 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.

 

Thirteen years ago, a survey by the European Southern Observatory's 1-meter Schmidt telescope showed nothing of note in the tiny patch of sky where scientists now believe they see a fading optical counterpart to the GRB 990510. To the left is a reproduction of a 30 second centering exposure in green-yellow light, obtained with the ESO ANTU 8.2-meter Very Large Telescope and the multi-mode FORS1 instrument on May 11, 1999, at 03:48 UT under mediocre observing conditions (image quality 1.0 arcsec).The optical image of the afterglow of GRB 990510 is easily seen in the box, by comparison with a 120 minute exposure in 1986 on IIIa-F emulsion behind a R(ed) filter. The field shown measures about 6.2 arcmin wide. North is up and East is left. Full-size images, up to 2700 pixels wide, are available at the ESO web site. Credit: ESO.

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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'.

Left: The twin peaks of the GRB 990510 light curve, as seen by BATSE, conceals even more detailed structure. Links to 451x349-pixel, 9K GIF. Credit: Marc Kippen (UAH & NASA/MSFC) and the BATSE Rapid Burst ResponseTeam.

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.

Web Links

Cosmic Gamma Ray Bursts NEWS & RESEARCH
GOTCHA! The Big One That Didn't Get Away - Jan. 27, 1999. For the first time, images of visible light from a cosmic explosion are captured by a robotic telescope while spectacular gamma-ray data are captured by orbiting satellites.
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).

External links

European Southern Observatory at La Silla and Paranal, Chile. Check their news page for updates on optical observations of GRB 990510.

Observation details and announcements from the University of Amsterdam, Danish 1.54-meter La Silla telescope, the Astronomical Observatory of Brera, and the University of Warsaw 1.3-meter telescope, and the South African Astronomical Observatory.

Beppo-Sax WWW page on the GRB 990510 burst, the 18th strong event observed in three years by the Italian-Dutch satellite.



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 Sutherland telescope at the South African Astronomical Observatory 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 other ESO telescopes.

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.

Right: The location of GRB 990510 as refined by BATSE, Beppo SAX, and Ulysses. Links to 715x439-pixel, 9KB GIF. Credit: Marc Kippen (UAH & NASA/MSFC) and the BATSE Rapid Burst ResponseTeam.

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.

Left: GRB 990510 - labeled O.T. - as seen by the University of Warsaw 1.3-meter telescope at Las Campanas, with an enlarged view from ESO's Very Large Telescope in the inset. Links to 494x494-pixel, 199K GIF. Credit: UW, ESO

"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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