Public Lectures for the Session 2006-2007 held in Room E7of the Renold Building, University of Manchester.
Delivered to the Manchester Astronomical Society

21 September 2006

''The Stop-Start Pulsar and RRATS (Rotating Radio Transients)"

Prof. Michael Kramer
(Head of Pulsar Group - Jodrell Bank, University of Manchester)

The Stop-Start Pulsar and RRATS…the discovery of the known and the unknown

Professor Kramer began his talk describing the discovery of transient neutron stars by explaining that Pulsars or pulsating radio sources, a name coined by the Daily Telegraph back in 1967, had been one of the most studied phenomena at Jodrell Bank for nearly 40 years. They were thought to be rapidly rotating neutron stars formed by the implosion and then rebounding explosion of a massive star during its destruction in a supernova event They were also the fastest objects in interstellar space, some moving at over 1000km/sec, after being expelled at high velocity from asymmetrical supernova blasts.

Pulsars are neutron stars, perhaps no more than 20km in diameter, that spin about once a second but some with spin rates measured in milliseconds. They are often described as cosmic lighthouses. Their magnetic poles emit radio waves from bipolar rotating beams detected as a flash of radio energy if they intercept Earth-based instruments. Although repeated observations show that their spin rate slows as rotational energy is lost by the braking action of the radio beams interacting with the surrounding interstellar medium, they have incredibly accurate periods, so accurate that they can be used as cosmic clocks.

RRATS (Repeating Radio Transients) are a new class of pulsar. Only 11 such objects have been found. They are like a flickering lighthouse. Instead of a radio flash every time it spins, there is one every few minutes or every few hours. The RRATs' short, intermittent bursts last only a few seconds, 10 of the objects show a mean periodicity of only 3.1 seconds, and their outbursts are infrequent, spaced from four minutes to three hours apart. They were discovered in a search for isolated radio signals from pulsars in data recorded during a four-year survey in the plane of our Milky Way galaxy by the Multi-Beam Survey with the Parkes 200-foot radio telescope in Australia from where the centre of the Milky Way passes overhead. This survey discovered objects that are now being more closely monitored from Jodrell Bank with its more sophisticated instrumentation and data analysis.

Given the fact that the single, 35-minute observation that first revealed these new stars had only a small chance of catching one of these outbursts, there may be as many as four times more of these RRATs than the 100,000 or so constant pulsars in the Milky Way. This raises its own problems, not least that it suggests a far higher number of supernovae in our galaxy than hitherto thought to have occurred.

Yet another type of pulsar has been discovered, 'moody pulsars'… Prof. Kramer would like a more inspired term for them. These are pulsars that are active for about a week and then apparently switch off for 3-4 weeks. If these stars belonged to binary systems, as some pulsars do, their reappearance from behind their partners might explain the periodic bursts. Perhaps the term 'eclipsing or occulting pulsars' might be apt. The problem is that these pulsars shut off in less than 10 seconds, implying a very small object in a close binary system.

For over 50 years, Manchester University's world leading Jodrell Bank observatory has been at the cutting edge of radio astronomy.

Synopsis by Kevin J. Kilburn (Secretary)

Maintained by Michael Oates
Page modified 28 October, 2006