On September 9, 1999, a NASA satellite was struck by a massive blast of X-ray radiation. The pulse had originated in a distant star cluster some 25,000 years earlier, but was only now reaching Earth because of the enormous distance in between.
The satellite had been monitoring the star system that produced the blast, watching for the smaller X-ray blasts that it regularly produces, lasting just a few seconds each. By chance, they captured this much rarer event, which lasted for hours and released thousands of times more energy-enough to power the sun for more than 100 years.
Using new tools, a NASA scientist Tod Strohmayer and a U of T astronomer David Ballantyne have analyzed the satellite data to produce a minute-by-minute view of the star system and its surroundings during and after the blast.
“This explosion that occurred on the neutron star basically acted like a spotlight,” disturbing the surrounding environment and illuminating details of the system that have never been seen before, said Ballantyne.
At the centre of the system is a neutron star, a super-dense sphere weighing as much as the sun, but just 15 kilometres across-all that remains behind from the death of a star in an ancient supernova explosion. Packing a large mass into such a small space endows the neutron star with an incredibly powerful gravitational field. A smaller companion star orbits at about half the distance between Earth and the moon, but while the moon takes a month to orbit the Earth, the companion star whips around the neutron star every eleven minutes.
The neutron star also sucks a stream of gas off the surface of the companion, which forms a disk as it spirals towards the neutron star, like water going down a drain. It settles onto the surface of the neutron star, but it is highly unstable. When enough of it accumulates, a nuclear explosion occurs, creating a burst of x-rays a few seconds long detectable on earth.
The burst detected by NASA’s Rossi X-Ray Timing Explorer (RXTE) in 1999 came from a much bigger nuclear storm which raged for hours. It was only the second such “superburst” to be recorded, and one of a total of eight observed to date.
Now Strohmayer and David Ballantyne of the Canadian Institute for Theoretical Astronomy (CITA) at U of T have revisited the data, focussing on the subtle changes to the X-ray radiation as it passed through the disk. These changes provide a sort of probe of the disk, showing it evolving in detail, something that’s impossible to see any other way.
“I think this is ultra, ultra cool,” said Omer Blaes, an astronomer at the University of California in Santa Barbara. Until now, astronomers hadn’t been able to test the various theories about what the disks are made of and how they should behave. “There’s been a lot of argument about that.” This new analysis could help resolve the dispute, he says.
There are still a number of mysteries surrounding these superbursts. At least one star system has been seen producing repeated superbursts, but “at this time we can’t predict when they’re going to go off,” says Ballantyne. He is also puzzling over a period during the eruption where the data doesn’t fit with any existing model. “My guess is that the system is undergoing some sort of rapid evolution,” he says, with either the disk or the surface of the neutron star-or both-quickly cooling and contracting.