Neutron Stars & PulsarsObservations of isolated neutron stars (or neutron stars in quiescent X-ray binaries) are extremely important in fundamental physics, as thermal emission from the surface of a neutron star carries signatures of its gravitational field. Detection of absorption lines corresponding to elements in the neutron star's atmosphere, and measurement of their gravitational redshift, would provide rather accurate data. From the gravitational redshift at the surface of the neutron star, the ratio between its mass and radius may be measured, providing a very strong constraint on neutron star models. Such models give Physicists an experimentally testable "handle" on properties of matter at (supra-)nuclear densities.Thanks to XMM-Newton, astronomers have been able for the first time to discover significant absorption lines in the spectra of the low-mass X-ray binary EXO0748-676. For an astrophysically plausible range of masses (M ~ 1.3 - 2.0 solar masses), the found redshift value of these lines (z = 0.35) is completely consistent with models of neutron stars composed of normal nuclear matter, while it excludes some models in which the neutron stars are made of more exotic matter. [Ref 18] The previous generation of imaging X-ray satellites (mainly ROSAT and ASCA) have detected a large population of young neutron stars which appear as rotation powered X-ray pulsars. X-ray emission in these systems may arise from a variety of physical processes. In many of them, non-thermal emission from relativistic particles accelerated in the pulsar magnetic field shows a power law spectrum over a broad energy range. The large collecting area of XMM-Newton has provided, for the first time, detailed spectral information for many of these weak X-ray sources. The EPIC camera can make very fast timing studies of X-ray pulsars, and with its excellent sensitivity, is able to search over much greater distances for such object. Its spatial resolution has also been necessary to separate X-ray pulsars from their surrounding supernova remnants (SNRs). [Ref 4] |
