Gamma-ray BurstsThought to be from Edge of Universe
Gamma-Ray Bursts Thought to be from Edge of Universe
January 15, 1997
The latest measurement and analysis of the sky positions of over 1,700 gamma-ray bursts (sky map) provide strong indications that these bursts originate from the remotest parts of the universe, at distances of billions of light years from Earth. These conclusions contradict theories that the bursts must somehow be associated within or just outside of our own Milky Way Galaxy. At cosmological distances, each gamma-ray burst must release about as much energy in ten seconds as our Sun emits in its entire lifetime.
This is the conclusion of a scientific presentation by Dr. Charles Meegan of NASA/Marshall's Space Sciences Laboratory, on behalf of the science team operating the Burst and Transient Source Experiment (BATSE) aboard NASA's Compton Gamma Ray Observatory. These results will be presented on January 16, at the 189th meeting of the American Astronomical Society in Toronto, Ontario, Canada.
By taking a different approach to looking at how bursts are distributed on the sky, Meegan estimates that compared to theories that place bursts just ouside of our own Milky Way Galaxy, bursts are about 300 times more likely to come from the edge of the universe at distances billions of light years away.
Either way, it has led astronomers at NASA's Marshall Space Flight Center to conclude that gamma-ray bursts most likely come from far outside our own Milky Way Galaxy. And that would mean that the universe is peppered with incredibly powerful explosions, each releasing as much energy in about ten seconds as our Sun emits in its entire ten-billion-year lifetime.
These results will be presented January 16 by Dr. Charles Meegan, on behalf of the science team operating the Burst and Transient Source Experiment (BATSE), at the 189th annual meeting of the American Astronomical Society, held in Toronto, Ontario, Canada
If your eyes could see them, each gamma-ray burst might look like a giant flashbulb going off. The bursts are detected by BATSE at random times and in random positions in the sky. Since its launch in 1991, BATSE has detected about one burst per day, for a total exceeding 1,700 so far. You cannot predict when or where the next burst might occur. But once BATSE has detected a burst, the experiment is able to determine (to within about 1 degree, or about twice the diameter of the full moon) the position of the burst on the sky.
By looking at the positions of all the bursts together, astronomers learn about how the bursts are distributed in space. "Take the band of the Milky Way, for example," said Meegan, a co-investigator on BATSE. "If you look up at a dark sky, you see most of the stars concentrated in a broad band across the sky. It's clearly not random. You're seeing the frisbee-like structure of our own Galaxy from a position inside the disk. When BATSE was built, we thought that we'd see a similar distribution from the gamma-ray bursts, especially the weak ones, because we thought the bursts were in our Galaxy."
However, instead of mirroring the structure of the Milky Way on the sky, gamma-ray bursts provided one of the greatest scientific surprises: an unexpected answer that leads to another puzzle.
The positions of the bursts are almost perfectly random. This random distribution, which astronomers call isotropic, combined with the observation of very few dim bursts, makes the distribution of gamma-ray bursts unlike that of any known Galactic objects.
Just how random are the burst positions? Two of the sky-maps below are generated by a computer using a program to randomly put 1719 dots on the sky. One is the actual BATSE data for 1719 gamma-ray bursts. Can you tell which one is the real burst position map? The answer is at the bottom of the page.
"What could cause something like this?" mused BATSE Principal Investigator Dr. Gerald Fishman of NASA/Marshall. "We think the data are telling us that the bursts are literally billions of light years away, coming from the most remote parts of the universe, at what we call cosmological distances."
But if bursts are cosmological, the distribution must be perfectly random, not just appear random, or be nearly random. Astronomers require more than just a visual appearance of randomness, or an indication from just a few bursts. "At first we only had a few hundred bursts," Fishman said, "and they looked random too. But it was kind of like trying to judge the fairness of a coin after only a few tosses." If you flipped a coin only three times, you'd either have three heads, three tails, two heads and a tail or two tails and a head. That's just not enough trials to figure out if the coin is really fair or not. The same was true initially with the bursts. But now with over 1,700, astronomers think they have enough bursts to really pin-down the randomness of the distribution.
Astronomers assess the randomness of the distribution of bursts on the sky by measuring a dipole moment and quadrupole moment of the distribution, and comparing these measured values to those one would get if the distribution were perfectly random.
The dipole moment tells astronomers what tendency exists for bursts to appear in the direction of the Galactic Center, located in the constallation Sagitarius. The BATSE team computes a measure of the dipole moment by taking the cosine of the angle between each burst and the galactic center, and computing the average. If the distrubition is isotropic, the average will be zero, as bursts will be just as likely to appear in the half of the sky nearer the Galactic center as the one farther from the Galactic center.
The quadrupole moment is a measure of the tendency for bursts to appear in the plane of the Galaxy, like the stars in the band of the Milky Way. To measure of this property, the BATSE team takes the galactic latitude of each burst, computes the sine of the angle, squares it, and then takes the ensemble average for all bursts. From this number, they subtract 1/3, so that a truly random distribution also has a value of 0 for this parameter.
The BATSE data provide a measure of the dipole moment of -0.014 +/- 0.014, and a value of -0.004 +/- 0.007 for the quadrupole moment, both consistent with zero, and therefore with a cosmological origin of bursts.
"Cosmology predicts 0, and nothing else," says Meegan, "while a galactic model can give you just about anything you want. So the fact that when we make the measurement, we come up with exactly what cosmology would predict tells me that they (the bursts) are cosmological." Meegan added, "When you look at the distribution this way, it's the most compelling piece of evidence yet that indicates bursts come from the edge of the universe."
To illustrate the point, consider that you have two boxes containing 300 slips of paper, each with numbers on them. In one box, which represents the cosmological model, each piece of paper has a zero written on it. In the other, which represents galactic models, one piece of paper has a zero written on it, all the others have 'not zero' written on them. You don't know beforehand which box is which.
"Stick your hand in one of the boxes and pull out a piece of paper," Meegan says, "If you get a zero, which box do you think you've pulled from? Did you pull from the box with all zeros, or did you just get lucky and pull the one zero out of the Galactic box? I'd put my money on the cosmology box."
For more information on the Burst and Transient Source Experiment, please contact: Dr. Gerald Fishman Mail Code ES-81 NASA/Marshall Space Flight Center Space Sciences Laboratory Huntsville, Alabama 35812 firstname.lastname@example.org
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