Brighter than a Million Galaxies
September 11, 1997
"Gamma ray burster" might not be as appealing a name as "black hole" in cocktail conversations, but it's a far more mysterious and elusive cosmic character that you can call FRED.
FRED means fast rise, exponential decay. Suddenly the burst is there, shining 10 million times brighter than our galaxy. Next it fades away - rapidly, then slowly - over the course of several dozen or hundred seconds.
Since the 1960s, scientists have tried to pin down the source of flashes of energy that appear - at random - in space about once a day. Next week, more than 200 astrophysicists will gather for the Fourth Huntsville Gamma Ray Burst Symposium. Unlike past meetings, the scientists this year are closer to solving the mystery.
"We know that they live a long way away," said Dr. Charles Meegan, one of the conference coordinators, "but we don't know their address yet."
Meegan, at NASA's Marshall Space Flight Center, is a co-investigator on the Burst and Transient Source Experiment (BATSE), one of four instruments aboard the Compton Gamma Ray Observatory (left). Since the Compton observatory was launched in 1991, it has recorded more than 1,900 gamma ray flashes and helped resolve the first mystery: are the flashes local or distant?
Where to look, and what might be the cause, will be the subjects of the week-long 4th Huntsville Gamma Ray Burst Symposium, September 15-19. The conference has been held every two years since GRO was launched.
"This is a watershed year for the symposium," Meegan said, "because now we have what we've been trying to get for 30 years, counterparts in other wavelengths." The February 28, 1997, burst was seen by enough instruments to let scientists check a small part of the sky and confirm that it appeared to come from a faint, distant galaxy.
The energetic stranger
"The sense of the community is that the doubt is over," Meegan said. "Gamma ray bursts are cosmological." That means that instead of coming from within our galaxy or even immediately around the galaxy, they are deep in space, probably more than 8 billion light years away (by comparison, our galaxy is about 150,000 light years across).
"But they're still very strange," Meegan continued. "All of the questions about them being cosmological are still there." The principal question is, what produces so much energy?
Radio waves, visible light, X-rays, and gamma rays all are photons released when matter gives up some energy - or when matter is turned into energy.
You don't get gamma rays from a light bulb burning out. You get it from thermonuclear reactions, like mass being converted into energy (a simple electron-anti-electron annihilation produces gamma rays at 511,000 eV, about 500,000 times more energetic than visible light).
"I wouldn't want to be on any planet in that star system when a gamma ray flash goes off," Meegan said. "If you were some distance away in the galaxy, it would be spectacular but not life threatening."
Spectacular means as much energy in 10 seconds as our sun will emit in 10 billion years - assuming they are about 8 billion light years away. That figure is based on the estimate that the bursters have a red shift value of about 1, or Z=1.
The farther away a star is, the faster it is moving, and thus the greater the Z number. Depending on which model of the universe you use, Z=1 can mean 6 to 10 billion light years. A burst recorded on March 8 appears to be connected with an object with a Z-number of at least 0.835 and possibly as high as 2.
"The truth is, we don't know it's Z=1," Meegan said. "That seems to be the best fit for the brightness. This is not ironclad at all. It's the simplest approximation."
Meegan said that it is most likely that scientists are tracking one phenomenon rather than several that end in similar bursts: "The net sum of all of these bursts have funny properties. There's all sorts of reasons to think that it is one phenomenon."
Fill in the blanks
Bursts appear to be coming from faint galaxies or the apparently empty space between galaxies. One would expect the burst to appear where matter is denser so it's easier to build whatever it is that goes off.
Models of what is happening vary. Most explanations involve matter falling into black holes or neutron stars, and questions about how the energy is released.
One model says it's a pulsar briefly beaming energy like a laser is reasonable. But then you need millions of bursters to account for the 1,600 aimed right at BATSE. A blast wave of material that moves almost as fast as light and "blue shifts" its radiation into gamma rays is reasonable, but then you have to design the stellar bomb that does it.
|Scientists at the gamma ray burst symposium are looking forward to new Hubble Space Telescope images of the February 28, 1997, burst source. The ones at left were taken in early March.|
The solution won't come out of this conference (although the scientists are eagerly awaiting a Hubble Space Telescope observation that may show the remnant of the February 28 event).
The sharing of ideas, and vigorously debating opposing viewpoints, will help shape the course of investigations that will lead, eventually, to the answer.
Invited presentations at the 4th Huntsville Gamma Ray Burst Symposium are:
The new BATSE catalog of burst sources - adding 514 since the 3B catalog two years ago, for a total of 1,636 through August 29, 1996 - will be formally released by Dr. Meegan at the opening session on global properties of bursts.
The search for counterparts, and how they flare then fade, will be discussed by Dr. Kevin Hurley of the University of California at Berkeley. Only two bursts have counterparts, GRB970228 in light and GRB970508 in light and radio. No counterparts have been found for others with similar counterparts.
The famous February 27 event and other optical counterparts will be covered by Dr. Jan van Paradijs of the University of Alabama in Huntsville. Van Paradijs discovered the optical counterpart to GRB970228 in images taken by telescopes in Chile.
Radio counterparts of gamma ray bursters will be reviewed by Dr. Dail Frail of the National Radio Astronomy Observatory. Frail used arrays of radio telescopes electronically connected to see the same detail as a single, larger telescope, and detected a radio counterpart to GRB970508. Again, most similar bursts do not have radio counterparts.
Soft gamma ray repeaters, a special case in gamma ray bursts, will be discussed by Dr. Shri Kulkarni of the California Institute of Technology. They are special because they are known to be caused by bodies within the galaxy. Kulkarni will interpret the observations in the context of the "magnetar" theory, that the repeaters are caused by neutron stars with magnetic fields more than a trillion times stronger than Earth's magnetic field.
Dr. Peter Meszaros of Pennsylvania State University will explain how the observations of counterparts and of afterglows restrict theoretical models of bursters, and in what directions they point us in solving the puzzle.
And Bohdan Paczynski of Princeton University will discuss theory that gamma ray bursts are caused by hypernovae, exceptionally violent stellar explosions. In this model, a massive star's core collapses and leaves a torus as dense as a neutron star. The intense magnetic field then accelerates the torus and creates a fireball that appears as a gamma ray burst.
Gamma Ray Bursts. New Observations Illuminate the most powerful explosions in the universe. Dr. Gerald Fishman, BATSE principal investigator at NASA's Marshall Space Flight Center, and Dr. Dieter H. Hartmann, a theoretical astrophysicist at Clemson University, explain bursts in as we understand them now in the July 1997 issue of Scientific American (pp 34-39)
December 17:BATSE Discovers Unique Gamma-Ray Bursts
December 13:BATSE Pins Down Space Oddity
December 10:BATSEÂ Scientists and Data Top 1996 Citations
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