March 20, 2008
Courtesy NASA
and World Science staff
Sadly, the show lasted only hours. But “if someone just happened to be looking at the right place at the right time, they saw the most distant object ever seen by human eyes without optical aid” on record, said Stephen Holland of NASA’s Goddard Space Flight Center in Greenbelt, Md.
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The afterglow of GRB 080319B was imaged by Swift's X-ray Telescope (left) and Optical |
As the jets plow into surrounding interstellar clouds, they heat the gas, often generating bright afterglows. Gamma ray bursts are believed to be the most luminous explosions in the universe, and this one “was a whopper,” said Swift principal investigator Neil Gehrels of the Goddard center. “It blows away every gamma ray burst we’ve seen so far.”
Swift’s Burst Alert Telescope picked up the burst at 2:12 a.m. Eastern U.S. time and pinpointed the coordinates in the constellation Boötes, researchers said. Telescopes in space and on the ground quickly moved to catch the afterglow. The burst is named GRB 080319B, because it was the second gamma ray burst found that day.
Two other Swift instruments also observed afterglows. Several ground-based telescopes saw the afterglow brighten to visual magnitudes between 5 and 6, in the scale used by astronomers. The brighter an object is, the lower its magnitude number. From a dark location in the countryside, people with normal vision can see stars slightly fainter than magnitude 6.
Thus the afterglow would have been dim, but visible to the naked eye, said Holland, a member of the Swift science team.
Later, the Very Large Telescope in Chile and the Hobby-Eberly Telescope in Texas measured the burst’s redshift at 0.94. A redshift is a measure of the distance to an object. A redshift of 0.94 translates into a distance of 7.5 billion light years, meaning the explosion took place 7.5 billion years ago, a time when the universe was less than half its current age and Earth had yet to form. The burst was seen occurring in the distant past because its light takes so long to reach us.
“No other known object or type of explosion could be seen by the naked eye at such an immense distance,” said Holland.
GRB 080319B’s optical afterglow was 2.5 million times more luminous than the most luminous supernova, or stellar explosion, ever recorded, scientists said. That would make it the most intrinsically bright object ever observed by humans. The most distant previous object that could have been seen by the naked eye is the nearby galaxy M33, a relatively short 2.9 million light-years from Earth.
Analysis of GRB 080319B is just getting underway, so astronomers don’t know why this burst and its afterglow were so bright. One possibility is the burst was more energetic than others, perhaps because of the mass, spin, or magnetic field of the progenitor star or its jet, scientists said. Or perhaps it concentrated its energy in a narrow jet aimed directly at Earth.
GRB 080319B was one of four bursts that Swift detected, a Swift record for one day—as it happened, the same day acclaimed science-fiction writer Arthur C. Clarke died. “Coincidentally, [his death] seems to have set the universe ablaze with gamma ray bursts,” said Swift science team member Judith Racusin of Penn State University.
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