12 Accretion-powered X-ray pulsars

Summary

The spectrum of thermal radiation from a neutron star with surface temperature of order 10⁶ K peaks in the X-ray spectrum at a photon energy around 1 keV. The first observation of an X-ray source outside the Solar System, made in 1962 using a rocket-borne instrument (Giacconi et al. 1962), revealed an unexpected and powerful source, designated Sco X-1. The explanation of this source was given by Shklovsky in 1967; it is indeed a thermal source, but it is accreting matter in a hot circumstellar disc surrounding a neutron star in a binary system. Sco X-1 is now the prototype of a class of binary X-ray sources known as Low Mass X-ray Binaries (LMXBs).

Confirmation of the nature of Sco X-1 and other X-ray sources in the Galaxy revealed by the first X-ray astronomy satellite UHURU (launched in 1970) came when the source Cen X-3 was shown to be pulsating with a period of 4.8 seconds. Following the same arguments as in the interpretation of the binary radio pulsars, it soon became clear that the source must be a rapidly rotating neutron star in a binary system. The orbital periods are typically several days, indicating that the binary systems are close enough for mass transfer to occur.