Astronomers of the University of Manchester's Jodrell Bank Observatory (UK) have led an international team which used the Parkes radio telescope in Australia to find a new kind of cosmic object which sends out radio flashes. These flashes are very short and very rare: one hundredth of a second long, the total time the objects are visible amounts to only about one tenth of a second per day.
The discovery will be published in this week's issue of the journal Nature.
Eleven sources of flashes have been found in different parts of the plane of the Milky Way in a survey for radio pulsars, which are small, compressed, highly-magnetised, neutron stars that produce regular pulses as they rotate, like cosmic light-houses. While that survey found over 800 pulsars and is the most successful in history, it also uncovered this new type of star. Rather than searching only for the periodic trains of pulses, the astronomers developed new techniques for detecting single short bursts of radiation.
Dr Maura McLaughlin explained: "It was difficult to believe that the flashes we saw came from outer space, because they looked very much like man-made interference". The isolated flashes last for between 2 and 30 milliseconds. In between, for times ranging from 4 minutes to 3 hours, the new stars are silent.
After confirmation of their celestial nature, studies over the next 3 years revealed that 10 of the 11 sources have underlying periods of between 0.4 seconds and seven seconds.
"The periodicities found suggest that these new sources are also rotating neutron stars, but different from radio pulsars", says Professor Andrew Lyne. "It is for this reason that we call them Rotating Radio Transients or RRATs. It's as if, following a flash, a RRAT has to gather its strength during perhaps a thousand rotations before it can do it again!"
RRATs are a new flavour of neutron stars in addition to the conventional radio pulsars and to the magnetars, which are also believed to be rotating neutron stars and are known to give off powerful X-ray and gamma-ray bursts. It is possible that RRATs represent a different evolutionary phase of neutron stars to or from magnetars.
The new objects probably far outnumber both their cousins. "Because of their ephemeral nature, RRATs are extremely difficult to find and so we believe that there are about 4 RRATs for every pulsar," says Dr Richard Manchester of the Australia Telescope National Facility. He is part of the team which also includes astronomers from the US, Canada and Italy.
M.A. McLaughlin, A.G Lyne, D.R Lorimer, M. Kramer, A.J Faulkner, R.N Manchester, J.M. Cordes, F. Camilo, A. Possenti, I.H. Stairs, G. Hobbs, N. D'Amico, M. Burgay and J.T, O'Brien. "Transient radio bursts from rotating neutron stars" 2006. Nature, 439, pp 817-820. Online at http://xxx.lanl.gov/abs/astro-ph/0511587
A pulsar is a neutron star, which is the collapsed core of a massive star that has ended its life in a supernova explosion. Weighing more than our Sun, yet only 20 kilometres across, these incredibly dense objects produce a beam of radio waves which sweeps around the sky like a lighthouse, often hundreds of times a second. Radio telescopes receive a regular train of pulses as the beam repeatedly crosses the Earth so the object is observed as a pulsating radio signal.
The Parkes survey which discovered 800 pulsars and the RRATs reported here, uses a multi-beam system installed on the 64-metre Parkes Radio Telescope in New South Wales, Australia. The powerful "multibeam" receiver was built as a joint venture between engineers at the Australia Telescope National Facility and the University of Manchester's Jodrell Bank Observatory.
The receiver gives the telescope 13 beams capable of scanning the sky simultaneously and, as Professor Andrew Lyne of the University of Manchester explained, "It's like having over a dozen giant radio telescopes operating at once". As a result, the system requires 13 sets of sophisticated data acquisition systems, one for each beam, which were largely developed and built by the UK and Italian groups.
The Jodrell Bank work was supported by funding from the UK Particle Physics and Astronomy Research Council (PPARC).
Further information on pulsars can be found on the Jodrell Bank Observatory Pulsar Group pages, http://www.jb.man.ac.uk/research/pulsar/
[Image 1: http://www.jb.man.ac.uk/news/rrats/virtual.jpg (550KB)] Visualisation of a neutron star, showing the magnetic field lines (cut away) and the radio beam (Credit: Russell Kightly Media).
[Image 2: http://www.jb.man.ac.uk/news/rrats/RRATS.jpg (1.46MB)] Visualisation of a neutron star, showing the magnetic field lines and the radio beam emanating from a magnetic pole (Credit: Michael Kramer/Ian Morison).
[Image 3: http://www.jb.man.ac.uk/news/rrats/RRATSpulse.jpg (2.27MB)] A flash from a RRAT, showing the sweep in radio frequency which is the signature of a particular RRAT.
[Image 4: http://www.jb.man.ac.uk/news/rrats/pksmultibeam.jpg (238KB)] The multibeam receiver used in the discovery being installed on the 64 meter radiotelescope at Parkes, New South Wales, Australia.
[Image 5: http://www.jb.man.ac.uk/news/rrats/Parkes.jpg (4.58MB)] The 64 meter Parkes radiotelescope, New South Wales, Australia. (Credit: Shaun Amy).