Ancient Interstellar Collision Helps Explain Source of Radiation
The New York Times
October 5, 2005
Ancient Interstellar Collision Helps Explain Source of Radiation
By DENNIS OVERBYE
With a screech of high-energy radiation brighter than a million billion suns, a pair of stars in a faraway galaxy collided two billion years ago and disappeared into a black hole.
That cataclysm, recorded by a battery of telescopes and NASA satellites on July 9, has provided scientists with the answer to the last remaining piece of 35-year-old astronomical mystery: the origin of explosions that sporadically shower outer space with gamma rays, the most energetic and deadly form of electromagnetic radiation.
"This is the real deal," said Donald Lamb of the University of Chicago, a co-author of one of four papers by an international cast of astronomers being published on Thursday in the journal Nature.
The results from July 9 and a handful of other events, the astronomers said in interviews and at a news conference today at NASA headquarters in Washington, suggest that the universe is peppered with the titanic collisions of the remnants of dead stars, either the dense cinders known as neutron stars or black holes too dense for even light to escape. Such a collision would result in a black hole, theorists said.
According to Einstein's general theory of relativity, such collisions should cause the very fabric of space and time to rock and roll with so-called gravitational waves. These events now seem to be more common than theorists thought, and detecting them will open a window on some of the most exotic and violent processes in nature.
Nobody knows how many such objects exist or why some supernova explosions leave neutron stars behind and others leave black holes.
Neil Gehrels, an astronomer at NASA's Goddard Space Flight Center and the lead author of another paper in Nature, said astronomers knew of double star systems - the hypothesized arena of such apocalyptic mergers - in which both objects are neutron stars. "Until now," he said, "we have never seen them merge or vaporize. This is the first evidence of it happening."
Gamma ray bursts come in two types: long and short. Two years ago, astronomers were able to trace the long bursts, which last from two seconds up to a minute or more, to certain kinds of supernova explosions in very distant galaxies.
But about 10 percent of the bursts are shorter, often lasting less than a second, and have thus been harder to study. Theorists have speculated that they could originate in double-star systems where a pair of neutrons stars or a neutron star and a black hole spiral ever tighter in a death dance toward merger and oblivion.
But on May 9, by homing in on the X-ray afterglow of a short burst, a team using NASA's Swift satellite was able to trace a 70-millisecond burst to the vicinity of an elliptical galaxy about 2.9 billion light-years away in the constellation Coma Berenices. This was significant, because such galaxies lack the massive young stars that give rise to the more violent core-collapse explosions.
The breakthrough came on July 9, when a NASA satellite called the High-Energy Transient Explorer (HETE for short, pronounced hetty) detected a short gamma ray pulse from the constellation Grus.
"We caught it with everything," said Derek Fox of Pennsylvania State University, lead author of another paper. An X-ray camera on the satellite was able to detect and pinpoint a faint afterglow accurately enough so that the Chandra X-Ray Observatory was able to swing into action.
As a result, a team led by Jens Hjorth of the University of Copenhagen was able to use a Danish telescope at the European Southern Observatory to find the afterglow in visible light on the outskirts of a small blue galaxy about 2.1 billion light years away. It was the first time that had been achieved for a short burst. Dr. Fox and his colleagues were able to watch it fade for over a month using the Hubble Space Telescope.
From the distance, Dr. Fox and his colleagues were able to calculate the magnitude of the explosion and found that it was a hundredth to a thousandth as bright as a typical long burst, in line with the predictions for a marriage of two neutron stars. The location of the burst - on the outskirts of the galaxy far from the regions where young massive stars form - also favored a collision.
But perhaps most important of all, as Dr. Ricker pointed out, is what did not happen. There was no supernova in that blue galaxy, leaving a collision as the only likely possibility. "The July 9 burst was like the dog that didn't bark," said George Ricker of M.I.T., a co-author of one Nature paper and HETE's principal scientist. He called the outcome "an open-and-shut case."
"Our baby satellite came through," he said.
Astronomers are not sure, however, if they can tell what kind of objects they have observed colliding in these bursts - a pair of neutron stars, which would be exotic enough, or a black hole eating a neutron star, an even more exotic and presumably rarer occurrence?
Some theorists, among them Dr. Gehrels and his Swift teammate Peter Meszaros of Penn State, say there is reason to think that what was observed on July 9 - and, more emphatically, in another burst on July 24 in the Scorpio galaxy - was a black hole swallowing a neutron star.
Referring to that spectacular idea, Dr. Gehrels said, "I'd say it's not convincing, but there is a hint of that in the data."
On both July 9 and on July 24, the X-ray afterglow did not simply fade away but flared about 100 seconds after the initial blast, as if part of the neutron star had been thrown upward and then had fallen back into the black hole. In the July 24 burst, which will be the subject of a future Nature paper, the X-rays flared again half a day later.
The July 24 burst was also five times as powerful as the July 9 blast. Since they have more mass, black holes can produce a more powerful explosion.
In their Nature paper, Dr. Fox and his colleagues write, "The stage is now set for detailed studies of these exotic cosmic explosions, the most exciting of which would be the detection of their associated bursts of gravitational waves."
* Copyright 2005 The New York Times Company
http://www.nytimes.com/2005/10/05/science/space/05cnd-star.html?pagewanted=print
October 5, 2005
Ancient Interstellar Collision Helps Explain Source of Radiation
By DENNIS OVERBYE
With a screech of high-energy radiation brighter than a million billion suns, a pair of stars in a faraway galaxy collided two billion years ago and disappeared into a black hole.
That cataclysm, recorded by a battery of telescopes and NASA satellites on July 9, has provided scientists with the answer to the last remaining piece of 35-year-old astronomical mystery: the origin of explosions that sporadically shower outer space with gamma rays, the most energetic and deadly form of electromagnetic radiation.
"This is the real deal," said Donald Lamb of the University of Chicago, a co-author of one of four papers by an international cast of astronomers being published on Thursday in the journal Nature.
The results from July 9 and a handful of other events, the astronomers said in interviews and at a news conference today at NASA headquarters in Washington, suggest that the universe is peppered with the titanic collisions of the remnants of dead stars, either the dense cinders known as neutron stars or black holes too dense for even light to escape. Such a collision would result in a black hole, theorists said.
According to Einstein's general theory of relativity, such collisions should cause the very fabric of space and time to rock and roll with so-called gravitational waves. These events now seem to be more common than theorists thought, and detecting them will open a window on some of the most exotic and violent processes in nature.
Nobody knows how many such objects exist or why some supernova explosions leave neutron stars behind and others leave black holes.
Neil Gehrels, an astronomer at NASA's Goddard Space Flight Center and the lead author of another paper in Nature, said astronomers knew of double star systems - the hypothesized arena of such apocalyptic mergers - in which both objects are neutron stars. "Until now," he said, "we have never seen them merge or vaporize. This is the first evidence of it happening."
Gamma ray bursts come in two types: long and short. Two years ago, astronomers were able to trace the long bursts, which last from two seconds up to a minute or more, to certain kinds of supernova explosions in very distant galaxies.
But about 10 percent of the bursts are shorter, often lasting less than a second, and have thus been harder to study. Theorists have speculated that they could originate in double-star systems where a pair of neutrons stars or a neutron star and a black hole spiral ever tighter in a death dance toward merger and oblivion.
But on May 9, by homing in on the X-ray afterglow of a short burst, a team using NASA's Swift satellite was able to trace a 70-millisecond burst to the vicinity of an elliptical galaxy about 2.9 billion light-years away in the constellation Coma Berenices. This was significant, because such galaxies lack the massive young stars that give rise to the more violent core-collapse explosions.
The breakthrough came on July 9, when a NASA satellite called the High-Energy Transient Explorer (HETE for short, pronounced hetty) detected a short gamma ray pulse from the constellation Grus.
"We caught it with everything," said Derek Fox of Pennsylvania State University, lead author of another paper. An X-ray camera on the satellite was able to detect and pinpoint a faint afterglow accurately enough so that the Chandra X-Ray Observatory was able to swing into action.
As a result, a team led by Jens Hjorth of the University of Copenhagen was able to use a Danish telescope at the European Southern Observatory to find the afterglow in visible light on the outskirts of a small blue galaxy about 2.1 billion light years away. It was the first time that had been achieved for a short burst. Dr. Fox and his colleagues were able to watch it fade for over a month using the Hubble Space Telescope.
From the distance, Dr. Fox and his colleagues were able to calculate the magnitude of the explosion and found that it was a hundredth to a thousandth as bright as a typical long burst, in line with the predictions for a marriage of two neutron stars. The location of the burst - on the outskirts of the galaxy far from the regions where young massive stars form - also favored a collision.
But perhaps most important of all, as Dr. Ricker pointed out, is what did not happen. There was no supernova in that blue galaxy, leaving a collision as the only likely possibility. "The July 9 burst was like the dog that didn't bark," said George Ricker of M.I.T., a co-author of one Nature paper and HETE's principal scientist. He called the outcome "an open-and-shut case."
"Our baby satellite came through," he said.
Astronomers are not sure, however, if they can tell what kind of objects they have observed colliding in these bursts - a pair of neutron stars, which would be exotic enough, or a black hole eating a neutron star, an even more exotic and presumably rarer occurrence?
Some theorists, among them Dr. Gehrels and his Swift teammate Peter Meszaros of Penn State, say there is reason to think that what was observed on July 9 - and, more emphatically, in another burst on July 24 in the Scorpio galaxy - was a black hole swallowing a neutron star.
Referring to that spectacular idea, Dr. Gehrels said, "I'd say it's not convincing, but there is a hint of that in the data."
On both July 9 and on July 24, the X-ray afterglow did not simply fade away but flared about 100 seconds after the initial blast, as if part of the neutron star had been thrown upward and then had fallen back into the black hole. In the July 24 burst, which will be the subject of a future Nature paper, the X-rays flared again half a day later.
The July 24 burst was also five times as powerful as the July 9 blast. Since they have more mass, black holes can produce a more powerful explosion.
In their Nature paper, Dr. Fox and his colleagues write, "The stage is now set for detailed studies of these exotic cosmic explosions, the most exciting of which would be the detection of their associated bursts of gravitational waves."
* Copyright 2005 The New York Times Company
http://www.nytimes.com/2005/10/05/science/space/05cnd-star.html?pagewanted=print
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