Understanding neutron stars

The origin of high-energy emission from the Crab Nebula has been identified.Provided by the University of South Hampton, England
By | Published: August 29, 2008
Located about 6,500 light-years from Earth, the Crab Nebula is the remnant of a star that began its life with about 10 times the mass of our Sun.
August 29, 2008
Another piece of the puzzle in understanding how neutron stars work has been put in place. Scientists have discovered the origin of high energy emissions from rotation-powered pulsars.

Pulsar systems containing neutron stars accelerate particles to immense energies, typically one hundred times more than the most powerful accelerators on Earth. Scientists are still uncertain exactly how these systems work and where the particles are accelerated.

Now a team of researchers from the United Kingdom and Italy, led by Professor Tony Dean of the University of South Hampton, has detected polarized gamma-ray emission from the vicinity of the Crab Nebula &#8212 one of the most dramatic sights in deep space. By using spectroscopic imaging and measuring the polarization &#8212 or the alignment &#8212 of the waves of high-energy radiation in the gamma-ray band, they have shown that these energetic photons originate close to the pulsar. Their paper “Polarized gamma-ray emission from the Crab” is published this week in Science.

The Crab Nebula is the result of a supernova explosion that was seen from Earth on July 4, 1054. The explosion left behind a pulsar or rotating neutron star with a nebula of radiating particles around it.

The neutron star contains the mass of the Sun squeezed into a volume of about 10 kilometers radius, rotating very fast &#8212 about 30 times a second &#8212 thereby generating magnetic fields and accelerating particles. A highly collimated jet, aligned with the spin axis of the pulsar and a bright radiating torus around the pulsar, are also seen. The Crab is known to accelerate electrons, and possibly other particles, to extremely high energies, both along the jet and around the torus, where they can be traced in the gamma-ray domain.

Looking into the heart of the neutron star with a gamma-ray telescope on the European Space Agency’s INTEGRAL spacecraft orbiting Earth, the researchers made a detailed study of the high-energy radiation to assess their polarization. They deduced that the majority of the gamma-rays are derived directly from the jet.

They analyzed data from over 600 individual observations of the Crab by the INTEGRAL spectrometer to assess the polarization of the gamma-rays and compared this data to the output from a sophisticated computer model.

The results show polarization with an electric vector aligned with the spin axis of the neutron star, demonstrating that a significant fraction of the high-energy electrons responsible for the polarized photons are produced in a highly ordered structure close to the pulsar.

Professor Tony Dean of the University’s School of Physics and Astronomy comments, “The remarkable alignment of the electric vector with the rotational axis of the pulsar, together with its similarity to the optical polarization angle, suggests that both fluxes originate at the same site close to the neutron star. The findings have clear implications on many aspects of high energy accelerators such as the Crab.”

The result also may have profound implications for fundamental physics. Some theories of quantum gravity have predicted that at very short distances a particular direction in space would be picked out, breaking Lorentz invariance. A consequence of this would be that the polarization vector of light would slowly rotate as it propagated through space. The Crab Nebula has a well-known rotational axis and now the first measurements of the polarization of gamma-rays have been shown to be aligned with it and not rotated away from it. This very strictly limits non-Lorentz invariance.