5. Low-Mass X-Ray Binaries

The LMXB consist of a compact object accreting matter from a companion whose mass is less than $\sim$ 2M_$\odot$ (e.g. Hayakawa (1985)). These companions are cooler than the early-type companions of HMXB (Lang (1991)) and do not have strong winds so the orbit must be compact enough for Roche lobe overflow to take place. The optical emission of an LMXB is dominated by thermal emission from the accretion disk. Evaporating disk material may form an accretion disk corona (ADC; Kallman (1993)). Other characteristics of the source will depend on the magnetic dipole moment of the compact component.

LMXB in which the accreting star is a white dwarf are known as cataclysmic variables. The available gravitational binding energy for radiation production is a factor of 10³ less due to their larger radii ( 10⁹ cm compared with 10⁶ cm for neutron stars) and they are relatively dim in X-rays unless thermonuclear detonation of accreted material occurs, creating a nova. The type I bursts which occur in some neutron star LMXB are also due to runaway thermonuclear burning. This phenomenon is suppressed in pulsars due to the higher surface magnetic fields (e.g. Lewin et al. (1995)). This may happen because the magnetic field reduces the radiative and conductive opacities and inhibits convective mixing of the helium-burning layer (Joss and Li (1980)). Another effect may be the funneling of accreting matter onto the polar caps which increases the rate of mass accretion per unit area (Joss (1978)). Type II bursts are another phenomenon that occur in accreting neutron star systems (e.g. Lewin et al. (1995)). They are believed to be due to impulsive accretion of matter caused by instabilities in the inner accretion disk. In AM Her systems a magnetic white dwarf co-rotates with its companion and no accretion disk forms. This can occur with surface magnetic fields as low as 10⁹ G because the magnetic dipole moment increases with stellar radius.