Part-time pulsar yields new insight into inner workings of cosmicclocks (Forwarded)

Jodrell Bank Observatory
University of Manchester
Macclesfield, U.K.

Contacts:

Dr Michael Kramer, University of Manchester
+44 1477 571321

Professor Andrew Lyne, University of Manchester
+44 1477 571321

Julia Maddock, PPARC Press Office
Tel +44 1793 442094

3rd March 2006

Press Release 2006/06

Part-time pulsar yields new insight into inner workings of cosmic clocks

Astronomers using the 76-m Lovell radio telescope at the University of
Manchester's Jodrell Bank Observatory have discovered a very strange
pulsar that helps explain how pulsars act as 'cosmic clocks' and confirms
theories put forward 37 years ago to explain the way in which pulsars emit
their regular beams of radio waves -- considered to be one of the hardest
problems in astrophysics. Their research, now published in Science
Express, reveals a pulsar that is only 'on' for part of the time. The
strange pulsar is spinning about its own axis and slows down 50% faster
when it is 'on' compared to when it is 'off'.

Pulsars are dense, highly magnetized neutron stars that are born in a
violent explosion marking the death of massive stars. They act like cosmic
lighthouses as they project a rotating beam of radio waves across the
galaxy. Dr. Michael Kramer explains, "Pulsars are a physicist's dream come
true. They are made of the most extreme matter that we know of in the
Universe, and their highly stable rotation makes them super-precise cosmic
clocks. But, embarrassingly, we do not know how these clocks work. This
discovery goes a long way towards solving this problem."

The research team, led by Dr. Michael Kramer, found a pulsar that is only
periodically active. It appears as a normal pulsar for about a week and
then "switches off" for about one month before emitting pulses again. The
pulsar, called PSR B1931+24, is unique in this behaviour and affords
astronomers an opportunity to compare its quiet and active phases. As it
is quiet the majority of the time, it is difficult to detect, suggesting
that there may be many other similar objects that have, so far, escaped
detection.

Prof. Andrew Lyne points out that, "After the discovery of pulsars,
theoreticians proposed that strong electric fields rip particles out of
the neutron star surface into a surrounding magnetised cloud of plasma
called the magnetosphere. But, for nearly 40 years, there had been no way
to test whether our basic understanding was correct."

The University of Manchester astronomers were delighted when they found
that this pulsar slows down more rapidly when the pulsar is on than when
it is off. Dr. Christine Jordan points out the importance of this
discovery, "We can clearly see that something hits the brakes when the
pulsar is on."

This breaking mechanism must be related to the radio emission and the
processes creating it and the additional slow-down can be explained by a
wind of particles leaving the pulsar's magnetosphere and carrying away
rotational energy. "Such a braking effect of the pulsar wind was expected
but now, finally, we have observational evidence for it," adds Dr Duncan
Lorimer.

The amount of braking can be related to the number of charges leaving the
pulsar magnetosphere. Dr. Michael Kramer explains their surprise when it
was found that the resulting number was within 2% of the theoretical
predictions. "We were really shocked when we saw these numbers on our
screens. Given the pulsar's complexity, we never really expected the
magnetospheric theory to work so well."

Prof. Andrew Lyne summarized the result: "It is amazing that, after almost
40 years, we have not only found a new, unusual, pulsar phenomenon but
also a very unexpected way to confirm some fundamental theories about the
nature of pulsars."

Publication

"A Periodically Active Pulsar Giving Insite into Magnetospheric Physics"
M.Kramer, A.G.Lyne, J.T.O'Brien, C.A.Jordan and D.R. Lorimer

To be published in the journal Science.

Background Information

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 unique activity of the pulsar PSR B1931+24 was discovered during
routine timing measurments carried out by the University of Manchester's
76m Lovell Telescope. It became clear a few years ago that the pulsar was
not detected in many of the regular observations and the pulsar appeared
to be bimodal, the pulsar being either ON or OFF in a quasi-periodic
fashion. Studying the data amassed from 1998 to 2005 showed that the
periodicity varied between 30 to 40 days. No other known pulsar behaves in
this way.

The Jodrell Bank work was supported by funding from the UK Particle
Physics and Astronomy Research Council (PPARC). Jodrell Bank Observatory
is part of the School of Physics and Astronomy at The University of
Manchester. The Observatory is home to the Lovell Radio Telescope and the
MERLIN/VLBI National Facility which is operated by the University on
behalf of PPARC.

Further information on pulsars can be found on the Jodrell Bank
Observatory Pulsar Group pages,
http://www.jb.man.ac.uk/research/pulsar/

The Particle Physics and Astronomy Research Council (PPARC) is the UK's
strategic science investment agency. It funds research, education and
public understanding in four areas of science -- particle physics,
astronomy, cosmology and space science.

PPARC is government funded and provides research grants and studentships
to scientists in British universities, gives researchers access to
world-class facilities and funds the UK membership of international bodies
such as the European Laboratory for Particle Physics (CERN), and the
European Space Agency. It also contributes money for the UK telescopes
overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy
Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI
National Facility, which includes the Lovell Telescope at Jodrell Bank
observatory.

PPARC's Public Understanding of Science and Technology Awards Scheme funds
both small local projects and national initiatives aimed at improving
public understanding of its areas of science.

Images

[Image 1:
(Labelled version) http://www.jb.man.ac.uk/news/brakingpulsar/onatten.gif
(62KB)
(Unlabelled version) http://www.jb.man.ac.uk/news/brakingpulsar/onBIG.gif
(223KB)]
The current understanding of a pulsar. The central neutron star is highly
magnetised and emits a radio beam along its magnetic axis, which is
inclined to the rotation axis. The strong magnetic field eventually leads
to the extraction of particles from the surface, filling the surrounding,
so-called magnetosphere with plasma. The size of the magnetosphere is
given by the distance where plasma co-rotation reaches the speed of light,
the so-called light-cylinder. The plasma creating the radio emission
eventually leaves the light cylinder as a pulsar wind, which provides a
torque onto the pulsar, contributing about 50% to its observed slow-down
in rotation.

Credit: Jodrell Bank Observatory

[Image 2:
http://www.jb.man.ac.uk/news/brakingpulsar/off.gif (61KB)]
When the radio emission stops, the pulsar wind also vanishes, and the
slow-down of the pulsar is dominated by its magnetic field.

Credit: Jodrell Bank Observatory

[Animation:
(.avi format) http://www.jb.man.ac.uk/news/brakingpulsar/1931.avi (3.2MB)
(.mpg format) http://www.jb.man.ac.uk/news/brakingpulsar/1931.mpg (1.1MB)]
The pulsar switches on and off in a periodic fashion. Shown here in a very
time-compressed version not to scale, the pulsar is on for about a week
before shutting off for about a month. When the pulsar is on, the radio
beam sweeps the sky, and a particle winds slows down the pulsar 50% more
than during times when the pulsar is off.

Credit: Jodrell Bank Observatory