HST Image, New Results Highlight First Day Of 4th Huntsville Gamma Ray Burst Symposium

September 17, 1997

It was the equivalent of pulling the film out of the fixer and seeing that the culprit had not left the scene of the crime. More than six months after discovering the first visible component for a gamma ray burst, astronomers got a second look with the Hubble Space Telescope and found that the object had not moved.

"We got the data a week ago," explained Dr. Alan Fruchter of the Space Telescope Science Institute, "and we've been working non-stop to process it."

Fruchter spoke to about 200 astrophysicists meeting in Huntsville this week for the Fourth Biennial Huntsville Gamma Ray Burst Symposium hosted by NASA's Marshall Space Flight Center. Fruchter presented the most striking - and anticipated - results in a session on late-breaking observations.

No film is involved with the Hubble, but the comparison is apt: before the electronic picture could be released, it has to be cleansed of noise accumulated during its 78-minute exposure, multiple images had to be combined, and then the scene examined.

The quarry is the source of the now-famous February 28 event, known as GRB970228, spotted by the Beppo Sax astronomy satellite.

Weighty evidence

This new Hubble image of GRB970228 bolsters the theory that gamma ray bursts come from violent events billions of light years away, and thus are incredibly powerful.

It is rare that two pictures can carry so much weight. The first was taken just a week after Beppo SAX satellite saw a burst with a telescope and sparked a worldwide hunt. Within days, detectors on other spacecraft and optical telescopes on Earth had led astronomers to an inconsequential part of the sky. There they found a star in the middle of a nebula. A check of older film plates showed that the star was not there before. But the new star left a large, bright image, making the exact location uncertain.

Hubble was called up to provide the most detailed view possible in visible light. With just days left before GRB970228 slipped too close to the Sun, Hubble used the Wide Field Camera to take an image that showed a tiny bright spot in a nebula.

While it seemed to confirm that the burst source was connected with the nebula, scientists needed another picture, separated in time, to show whether the new star had moved or faded quickly.

On September 5, with GRB970228 now far enough from the sun that Hubble could safely look at it, the second picture was taken with the new Space Telescope Imaging Spectrometer.

Astronomers found what they were hoping to see.

Embarrassingly good data

"You can see a single bright pixel (picture element)," Fruchter explained, "lying on the side of an extended nebula about 0.8 arc-second across." That's about 1/2250th the apparent diameter of the Moon in the sky. The burst source takes just the one pixel, but that does not indicate its size, just that it was brighter than anything else in that point of the sky.

And it's at the same point as the March observation by Hubble.

"There is no proper motion," Fruchter said, so it is clearly associated with the nebula.

The position does not explain the whole story, he continued. Its gradual dimming shows that it is very big and bright.

"It's embarrassingly good," Fruchter said as he pulled out a graph showing the brightness against time. The graph uses logarithms that compress large numbers so they fit easily on a sheet of paper. "What I find so remarkable about these readings is that this is power law behavior."

Power law describes how something expands and loses energy. The March and September brightnesses of GRB 970228 fit exactly on the line. Most other telescope observations do, too, although three separate observations are far enough off that additional study may be required.

The crucial point, though, is that the object is taking so long to fade. The explosion of a star is like an explosion on Earth. The shock plows through material around it, compressing and pushing it ahead. In space, the blast accelerates the star's own material which then slams into interstellar gas.

It keeps going and going and ...

"If it was in our galaxy, it would be millions of times less powerful and it would run out of oomph a lot sooner," Fruchter said. Within days, certainly within weeks, a nearby gamma ray burst would fade away.

"This has been running for six months without turning over," Fruchter continued. He expects that it will start to fade in about a year.

"This supports the relativistic fireball model," he said, "but it doesn't tell us what caused the relativistic fireball." In a relativistic fireball, matter is accelerated almost to the speed of light, so fast that its light is blue-shifted and is emitted only along its line of flight, not in all directions as ordinary hot objects do.

Neither will hazard a guess on how far away the burst and nebula might be because the nebula's brightness is not known, making a good estimate difficult. Additional observations by ground based telescopes may tell the tale.

X-ray components and a repeater

While the search for counterparts for gamma ray bursters has taken years - six since the Gamma Ray Observatory was launched, and 30 years since bursts were discovered - now they seem to be coming out of the cosmic woodwork as bursts become the hottest topic in astrophysics.

Toshio Mirukami of Japan's Institute of Space and Astronautical Sciences said that his nation's Advanced Satellite for Cosmology and Astrophysics has seen several objects that appear to be X-ray components of gamma ray bursts. Pointing toward a burst which BATSE observed on Aug. 28, "We clearly detected the fading X-ray source. It's really fading," Mirukami said. No visible light counterparts have been found, though, and ASCA did not find other recent bursts.D.A. Smith described how the all-sky monitor on the Rossi X-ray Timing Explorer is searching for and finding gamma ray bursts. The monitor comprises three cameras that stare at a narrow stretch of sky, then move to a different view every 90 seconds, so detection depends on some luck - which the team had. The search started in April, and detected two bursts on Aug. 15 and 28. Good positional data were obtained and passed along to optical and radio telescope teams - in just two hours, the second time - so they could try to find counterparts to the burst.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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