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Born to Spin

The question is: How fast?
Published: 02-14-2008

Composite image of the Crab Nebula, showing the remnants of a supernova explosion that occurred in AD 1054 about 6300 light years from Earth. The dot in the center is a rapidly spinning young neutron star. The red color represents radio emission; the green, visible emission; and the blue, X-ray emission. Credit: J. Hester (ASU), CXC, HST, NRAO, NSF, NASA

Neutron stars are born in supernovae, spinning very fast. How fast they spin at birth depends on a variety of factors including the initial rotation of the star that goes supernova and what takes place during the supernova explosion. So, if you want to understand these phenomena, one place to start is by investigating how fast a new neutron star can initially spin.

Fellow Rosalba Perna and colleagues Roberto Soria (University College London), Dave Pooley (University of Wisconsin at Madison), and Luigi Stella (Osservatorio Astronomico di Roma) recently did exactly that. The researchers studied X-ray emissions from supernovae in nearby galaxies observed by the Chandra, XMM-Newton, and Swift X-ray observatories and compared them with theoretical simulations of X-ray emissions from young neutron stars. Because neutron stars emit X-rays from the time they are born until their spins slow down (when they become faint and hard to detect), the researchers were able to use observational correlations between X-ray emissions and rotational energies to determine limits on how fast new neutron stars could spin.

Perna and her colleagues showed that most newly formed neutron stars cannot spin any faster than 25–30 rotations per second. This limit is roughly 10 times slower than spin rates estimated from many radio-wave observations of older rotating neutron stars (pulsars). Theorists must now include this new limit in their models of how the cores of stars collapse and neutron stars evolve.

As they revisit the evolution of neutron stars, they may now be able to answer such important questions about young neutron stars as: How common are these objects in the Universe? How bright do they shine on average? Is the young and very bright neutron star in the Crab Nebula (shown above) just an everyday neutron star, or is it one of the brightest?   - Julie Phillips


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