Public Release:  Mysterious pulsar with hidden powers discovered

University College London

Dramatic flares and bursts of energy - activity previously thought reserved for only the strongest magnetized pulsars - has been observed emanating from a weakly magnetised, slowly rotating pulsar. The international team of astrophysicists who made the discovery believe that the source of the pulsar's power may be hidden deep within its surface.

Pulsars, or neutron stars, are the collapsed remains of massive stars. Although they are on average only about 30km in diameter, they have hugely powerful surface magnetic fields, billions of times that of our Sun.

The most extreme kind of pulsars have a surface magnetic field 50-1000 times stronger than normal and emit powerful flares of gamma rays and X-rays. Named magnetars (which stands for "magnetic stars") by astronomers, their huge magnetic fields are thought to be the ultimate source of power for the bursts of gamma rays.

Theoretical studies indicate that in magnetars the internal field is actually stronger than the surface field, a property which can deform the crust and propagate outwards. The decay of the magnetic field leads to the production of steady and bursting X-ray emission through the heating of the neutron star crust or the acceleration of particles.

Now, research published today in Science, suggests that the same power source can also work for weaker, non-magnetar, pulsars. The observations, which were made by NASA's Chandra and Swift X-ray observatories of the neutron star SGR 0418, may indicate the presence of a huge internal magnetic field in these seemingly less powerful pulsars, which is not matched by their surface magnetic field.

"We have now discovered bursts and flares, i.e. magnetar-like activity, from a new pulsar whose magnetic field is very low," said Dr Silvia Zane, from UCL's (University College London) Mullard Space Science Laboratory, and an author of the research.

Pulsars are highly magnetized, and as they rotate winds of high-energy particles carry energy away from the star, causing the rotation rate of the star to gradually decrease. What sets SGR 0418 apart from similar neutron stars is that, unlike those stars that are observed to be gradually rotating more slowly, careful monitoring of SGR 0418 over a span of 490 days has revealed no evidence that its rotation is decreasing.

"It is the very first time this has been observed and the discovery poses the question of where the powering mechanism is in this case. At this point, we are also interested in how many of the other normal, low field neutron stars that populate the galaxy can at some point wake up and manifest themselves as a flaring source," said Dr Zane.

A crucial question is how large an imbalance can be maintained between the surface and interior magnetic fields. SGR 0418 represents an important test case.

"If further observations by Chandra and other satellites push the surface magnetic field limit lower, then theorists may have to dig deeper for an explanation of this enigmatic object," said Dr Nanda Rea, Institut de Ciencies de l'Espai (ICE-CSIC, IEEC) in Barcelona, who led the discovery.


Notes for Editors

1. For more information or to interview Dr Silvia Zane, please contact Clare Ryan in the UCL Media Relations Office on tel: +44 (0)20 7679 9726, mobile: +44 07747 565 056, out of hours +44 (0)7917 271 364, e-mail:

2. 'A low-magnetic-field Soft Gamma Repeater' is published in the 14 October issue of Science Express. Journalists can obtain copies of the paper by contacting UCL Media Relations.

3. Images of the SGR 0418 pulsar emissions are also available to journalists by contacting UCL Media Relations.

4. This discovery is the result of an international team work from CSIC-IEEC, INAF, University of Padua, MSSL-UCL, CEA-Saclay, Sabaci University and NASA-MSFC

5. This work was part funded by the Science and Technologies Research Council, UK. NASA's Marshall Space Flight Center manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations.

About UCL

Founded in 1826, UCL was the first English university established after Oxford and Cambridge, the first to admit students regardless of race, class, religion or gender, and the first to provide systematic teaching of law, architecture and medicine. UCL is the fourth-ranked university in the 2009 THES-QS World University Rankings. UCL alumni include Marie Stopes, Jonathan Dimbleby, Lord Woolf, Alexander Graham Bell, and members of the band Coldplay. UCL currently has over 12,000 undergraduate and 8,000 postgraduate students. Its annual income is over £600 million.

Science and Technology Facilities Council

The Science and Technology Facilities Council ensures the UK retains its leading place on the world stage by delivering world-class science; accessing and hosting international facilities; developing innovative technologies; and increasing the socio-economic impact of its research through effective knowledge exchange partnerships.

The Council has a broad science portfolio including Astronomy, Particle Physics, Particle Astrophysics, Nuclear Physics, Space Science, Synchrotron Radiation, Neutron Sources and High Power Lasers. In addition the Council manages and operates three internationally renowned laboratories:

  • The Rutherford Appleton Laboratory, Oxfordshire
  • The Daresbury Laboratory, Cheshire
  • The UK Astronomy Technology Centre, Edinburgh

The Council gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Laboratory for Particle Physics (CERN), the Institute Laue Langevin (ILL), European Synchrotron Radiation Facility (ESRF), the European Southern Observatory (ESO) and the European Space Agency (ESA). It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Chile, and the MERLIN/VLBI National Facility, which includes the Lovell Telescope at Jodrell Bank Observatory.

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