Astronomers have stumbled upon a bizarre kind of star that stutters out fleeting flashes of radio waves.
"It's
quite unusual to find such a different type of object," says Andrew
Lyne, an astronomer from the University of Manchester's Jodrell Bank
Observatory, UK, and part of the international team that made the
discovery. "This is the opening of a new field in astronomy," he adds.
The
strange type of star was spotted using the Parkes radio telescope in
New South Wales, Australia. Astronomers have so far seen 11 examples of
the objects, which they call Rotating Radio Transients (RRATs). The
stars send out bursts of radio waves that last between 2 and 30
milliseconds, with a time interval between bursts varying unpredictably
from 4 minutes to 3 hours.
This
makes them a bit like jittery cousins of pulsars, explains Lyne.
Pulsars are rapidly rotating neutron stars that send out a concentrated
beam of radio waves, just like a lighthouse. Every time that beam
sweeps over the Earth our radio telescopes hear a regular click.
But no one has ever seen a pulsar-type object emit an irregular beam quite like the RRATs.
The truth is out there
Enduring mystery: why would some neutron stars only sometimes emit a pulse?
The
team was hunting for pulsars when they spotted the objects. "There were
11 areas of sky where all we saw was occasional flashes," recalls Lyne.
"It was difficult to believe that the flashes we saw came from outer
space, because they looked very much like man-made interference," adds
Lyne's Jodrell Bank colleague, Maura McLaughlin.
The
team then realized that although the time difference between flashes
varied wildly, that time span was always a whole-number multiple of
some smaller unit of time, between 0.4 and 7 seconds, depending on the
source.
The
team concludes that RRATs could be a previously unknown form of neutron
star, which spins regularly (like a pulsar), but only occasionally
emits a radio beam. As to why it only sometimes emits a beam, Lyne
admits, "we haven't a clue". They present the findings in this week's Nature1.
Vicky
Kaspi, an astrophysicist from McGill University in Montreal, Canada,
points out that some pulsars are already known to occasionally deliver
extra strong pulses, although astronomers are at a loss to explain why.
It's possible that RRATs are doing just this, she suggests, but that
their normal radio emissions are simply too faint for us to see.
Death and rebirth
The
RRAT may be one of many possible fates in store for a star in its death
throes. When an ageing star runs out of fuel, the outward pressure
generated by its fusion reactions is overcome by the star's own
gravity. Smaller stars collapse into glowing cinders known as white
dwarfs, whereas giant stars die with such force that the resulting
supernova explosion can create a black hole.
ADVERTISEMENT
Intermediate stars tend to become neutron stars in their
afterlife. Although not quite dense enough to form a black hole, they
are still so squashed by gravity that the protons and electrons in
their atoms merge to become neutrons.
Some neutron stars, known as magnetars, have enormous magnetic fields and radiate high-energy X-rays and -rays. Pulsars usually stick to lower-energy radio waves.
Curiously, one of the RRATs found in the survey seemed to be emitting both X-rays and radio waves2.
This makes Lyne think that pulars may perhaps turn into magnetars as
they age, passing through the RRAT stage in between. But this idea is
just a theory, he says.
"It's
a wonderful idea," says Kaspi. "The jury's still out, but it could
really fill a gap between pulsars and magnetars if it's right." Only
further study of RRATs will confirm what they really are.
Post a comment to this story by visiting our newsblog.
-rays. Pulsars usually stick to lower-energy radio waves. 
