Astronomers Puzzled by a Pulse
Astronomers have discovered a strange sputtering and pulsing object in the center of the Milky Way galaxy that appears to be a completely new species of stellar object.
“This repeated bursting behavior . . . is unlike anything we’ve ever seen before,” said Don Lamb, a University of Chicago astronomer.
Astronomers are particularly excited by the new object because nothing like it was predicted by now-accepted theories of stellar evolution. “Every time you put up a new satellite that looks in a new wavelength range,” said UC Berkeley astrophysicist Kevin Hurley, “you start turning up these bizarre objects that nobody would have guessed existed.”
Since its X-ray signal was first picked up by Chryssa Kouveliotou of NASA’s Marshall Space Flight Center in December, scientists all over the world have been trying to make the first visual sighting of the object, said Massachusetts Institute of Technology astrophysicist Walter Lewin.
Depending on what they find, the object may cause the revision of some current stellar theories. “What makes it so unusual is that no single source has both bursted and pulsed before,” said astrophysicist Lynn Cominksi of Sonoma State University.
The object of excitement appears to be a strange hybrid of several different types of previously detected--but little understood--exotic stars. Adding to the mystery is that the rhythm of the object’s signals has changed substantially since December.
When National Aeronautics and Space Administration’s orbiting Compton Gamma Ray Observatory detected the object’s signal, it was spitting out erratic streams of high-energy X-rays every few minutes. But two days later, the bursts had settled down to one per hour. At the same time, the object was pulsing with the steady beat of rapidly spinning stellar core--a rhythm steadier than even atomic clocks, Cominski said.
This new “bursting pulsar” is but the latest addition to an increasingly bizarre and mysterious zoo of unfathomable objects discovered over the last 20 years.
One such mysterious species, bursting stars, was discovered in the 1970s by U.S. and Soviet satellites launched to detect high-energy bursts from secret nuclear explosions.
“They found them,” said Caltech astrophysicist Shri Kulkarni, “but instead of coming from the ground, [the explosions] were coming from the sky.”
During the past two decades, at least one such burst per day has lit up the sky with gamma rays (or very high-energy X-rays). But “we have no clues what they are,” Kulkarni said. “They could be Klingon warships.”
Other known astronomical objects seem to have a familial resemblance to the new object, but not all of them are well understood either. Perhaps the most bizarre example is a one-of-a-kind object called a “rapid burster,” which was discovered by Lewin’s group 20 years ago.
The new object appears to be in a class by itself. “This new one excels all others in many respects,” Hurley said. The fact that its pulse period is changing, he said, is a clear sign that it is sucking up matter from a companion star nearby. “As matter falls into it, its pulse period either speeds up or slows down.”
What happens then, however, is a matter of much dispute. At least two different scenarios have been proposed--both previously observed in burned-out stars under very different circumstances.
Lamb’s group is arguing that the companion star is dying, puffing up as our own sun will, billions of years from now, when it runs out of nuclear fuel. The compact companion pulls the hydrogen atmosphere off the sun-like star; when the hydrogen crashes to the surface, it sets off nuclear explosions--just like hydrogen bombs.
Lewin, however, is certain that the new object is more like his “rapid burster,” which apparently does not produce thermonuclear explosions. Instead, Lewin said, both the new object and the rapid burster have some kind of “mysterious screen”--possibly a strong magnetic field--that holds back the in-falling matter from the companion star. When the screen can hold back the hydrogen no longer, it falls, crashing to the core’s surface at one-third the speed of light--releasing far more energy than even a nuclear explosion.
Astronomers will know much more once they get a look at other types of signals coming from the object. It is extremely hard to see because the center of the galaxy is tightly packed with stars.
Lamb’s group announced last week that they might have found an infrared and visible signal, but they can’t be sure yet that it is coming from the same source.
Meanwhile, astronomers at the Very Large Array radio telescope in Socorro, N.M., found a promising radio source that might shed light on the nature of the object. It does seem to be emanating from the area, Lewin said.
The discovery is the subject of an article in the issue of the journal Science that appears today. A paper on the same object, by a different group of scientists, will appear in the journal Nature next week.
