Magnetic fields put the heat on neutron stars
Feb 21, 2007
Magnetic heating could be playing a much more prominent role in the evolution of neutron stars than previously expected, claim astrophysicists in Spain and the US. The researchers looked at data describing the surface temperature and magnetic field of about 30 neutron stars and found a mathematical relationship between the two properties that suggests that the stars are being heated by their own magnetic fields. While magnetic heating had been expected in "magnetars" -- neutron stars with very high magnetic fields -- the study provides the first evidence that heating also occurs in stars with much lower fields. This could cause astrophysicists to rethink current theories of how neutron stars cool. (Phys. Rev. Lett. 98 071101).
Formed by the collapse of massive stars, neutron stars are extremely dense objects containing mostly neutrons. They are typically only about 10 kilometres in diameter, but are at least 40% heavier than the Sun, which means that their core density is several times that of the density of an atomic nucleus. As they age, neutron stars are thought to cool first by emitting neutrinos, and then by emitting photons. By measuring the rate at which neutron stars cool, physicists can gain important insights into the subatomic physics that govern the innards of these objects.
José Pons and colleagues at Alacant University in Spain and a collaborator at Montana State University in the US used data from satellite X-ray telescopes and ground-based radio telescopes to show that magnetic heating appears to be happening in neutron stars with magnetic field strengths between about 1012-1015 Gauss. Astrophsyicists had previously thought magnetic heating would only be signficant in magnetars with magnetic fields above 1014 Gauss.
The next step for the researchers is to further test the relationship between temperature and magnetic field by analysing data from more neutron stars. However, this may have wait until the next-generation of X-ray telescopes such as NASA’s Constellation X satellite array or the ESA's XEUS satellite become available sometime in the next decade. Pons also believes that improved computer simulations of the interaction between neutron stars and their magnetic fields could shed further light on how neutron stars cool.
About the author
Hamish Johnston is editor of Physics Web