14 DAY TRIAL // What happens when a neutron star gradually loses its fuel? The neutron star is itself the result of a supernova explosion, so where can it go any further? An international team of astronomers seem to have answered this question.
As usual stars consume their hydrogen neutrons get produced. If the star is large enough a supernova explosion will eventually happen. If the star was not too large the "end" result will not be a black hole, but a neutron star - black hole's smaller cousins. Such a neutron star is an incredibly dense star (it might be twice as heavy as our Sun and only 20 km in diameter!) which is made up almost entirely by neutrons. It is so dense that it isn't even a gas (like usual stars), but a liquid.
More over, as they are so compacted, they end up spinning extremely fast. It's the same phenomenon you observe in ice skating when an ice skater spins: when she closes her arms around her body she starts spinning faster. Physicists call this phenomenon the "conservation of angular momentum". This is a universal phenomenon similar to the conservation of energy. It also holds in case of stars.
Because of the magnetic fields at the surface, when a neutron star spins it emits radio waves in a highly periodic fashion. This period corresponds to its rotation period. They are also called pulsars because they emit periodic "pulses".
The new discovery
Right now, the team of astronomers lead by Maura McLaughlin of the University of Manchester used the Parkes radio telescope in Australia (image below) to map the pulsars. They discovered more than 800 pulsars, but they have also discovered 11 weird objects. These objects seem inactive for most of the time but for intermittent, extremely short and violent bursts of radio waves. They were dubbed rotating radio transients (RRATs).
Their isolated outbursts last for as few as two milliseconds and are separated by gaps as long as three hours. They flash from once every four minutes to once every three hours. Some flash even less frequently. "These things were very difficult to pin down," says Dick Manchester of the Commonwealth Scientific and Industrial Research Organization's (CSIRO) Australia Telescope National Facility. "For each object we've been detecting radio emissions for less than one second a day. And because these are single bursts, we've had to take great care to distinguish them from terrestrial radio interference."
Since August 2003, each RRAT has been observed at least nine more times to detect multiple bursts. Despite their intermittent nature, the bursts make these new stars among the strongest radio sources in the universe.
"The flashes of radio waves are comparable with, or stronger than, the pulses from the brightest pulsars," said Andrew Lyne of the University of Manchester. "I guess that there are only a few dozen brighter radio sources in the sky."
The intermittent nature of these bursts raised various questions. For example: Do they spin in a highly irregular fashion? Or: Are they part of a binary system and their companion blocks most of the radiation?
The astronomer's analysis suggests they are not part of binary systems and that they rotate like more regular pulsars, but only pulse occasionally. The scientists don't know yet what exactly makes them flash the way they do.
"It may be that these objects are older radio pulsars which are dying and unable to produce normal radio emission," McLaughlin said. This isn't 100% certain. They argue that maybe RRATs have high magnetic fields, similar to a magnetar (another type of neutron star), that prohibit regular emissions. However, magnetars emit huge amounts of high-energy light (X-Rays and gamma rays) while nothing similar is seen in case of RRATs. So, the hypothesis that these stars are the leftovers of regular neutron stars seems most probable.
How many such stars are there?
Considering the small likelihood of detecting such short pulses and the number of detections actually made, the astronomers estimate there may be as many as five times more of these RRATs than the 100,000 constant pulsars in the Milky Way.
"This discovery increases the current galactic population estimates [for neutron stars] by at least several times," the team writes in their report on the findings, published today in Nature. "We therefore expect the emerging generation of wide-field radio telescopes to discover many more RRATs."
"We expect the galaxy to harbor roughly 400,000 of these objects," McLaughlin said. "This is four times as many as conventional radio pulsars."
