X-ray binary
The most common type of luminous galactic X-ray source, involving a close binary system in which gas flows (via the inner Lagrangian point) or blows (by a strong stellar wind) from a normal nondegenerate star on to a compact companion (see mass transfer; binary star). For the most luminous sources such as Centaurus X-3 and Cygnus X-1, radiating X-rays at 1029–1031 watts, this companion is probably a neutron star or black hole; in less luminous cases, such as SS Cygni and AM Herculis, it is more likely to be a white dwarf. Gravitational energy powers these sources and both the luminosity (L) and temperature (T) are proportional to the mass-to-radius ratio (M /R) of the accreting star: L ∝ (M /R )Gm ′
where G is the gravitational constant and m ′ is the rate of mass accretion, and T ∝ (M/R )G ∊
where ∊ depends on the efficiency of the gas heating, being high for shocks and low for viscous heating. Two main types of X-ray binary are distinguished: in high-mass binaries (HMXBs), such as Centaurus X-3 and Cygnus X-1, the nondegenerate star is a giant or supergiant of early spectral type (O, B, or A); in low-mass binaries (LMXBs) such as Hercules X-1, the visible star is a middle or late main-sequence star of near solar mass. Several binaries contain a pulsating X-ray source, probably involving a rotating magnetized neutron star; these binaries, which include Centaurus X-3 and Hercules X-1, are among the best-determined of all binary systems.
The most luminous X-ray binaries, including Cygnus X-3, Scorpius X-1, and Circinus X-1, are also strong variable radio sources, sometimes also emitting radio flares.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006