ABSTRACT
X-ray The part of the electromagnetic spectrum whose radiation has energies in the approximate range from 1 to 150 keV. Typically used to observe the solar corona and, in particular, solar flares.
X-ray binary A binary star in which one component is a neutron star or black hole and the other is a main sequence star or red giant. Material is transferred from the latter to the former through Roche lobe overflow or a stellar wind, sometimes via an accretion disk. The accreting gas gets very hot, liberating gravitational potential energy of up to 1020 ergs per gram (about 10% of mc2). In the case of a neutron star accretor, there can be variability associated with the neutron star’s rotation period and with explosive nuclear reactions on the surface. Variability in the black hole systems is associated with changes in accretion rate and disk structure. Either kind of accretor can occur with either high mass (stellar wind) or low mass (Roche lobe overflow) donor. Most of the 10 or so known black hole systems have black hole masses of 5 to 10 solar masses. Most of the neutron star masses are 1.5 to 2.0 solar masses, permitting
a clean separation of the classes. Luminosities can reach 105 solar luminosities.
