Low Mass Xray Binaries LMXB

The optical companions of LMXB are generally of spectral type A to M with less than two solar masses. The mass transfer through Roche lobe overflow feeds an optically thick accretion disk, the X-ray emission of which often dominates the optical and UV appearance of the binary. The Lx/Lopt is typically 100-1000, except in those systems where an accretion disk corona (ADC) hides the central source leading to a much reduced Lx/Lopt of <20. Heating of the optical star by the compact X-ray source is generally significant, leading to an optical modulation from which the binary nature can be inferred. LMXB are less likely to show eclipses, but often show so called dips, which occur when the line of sight to the X-ray source is intercepted by the accretion stream or by material splashing off the accretion disk when the gas stream hits its outer edge. The periods of the binary orbits in LMXB (known only of one third of all systems) range from 0.19 h to 17 days. The catalogue of LMXB [29] shows 150 entries. About 20% of those (<30) are BH candidates. NS in LMXB are weakly magnetized (B <1010 G) and, therefore, they are usually not pulsars but rather bursters and QPO sources. There is, however, the class of ms pulsars that clearly contain weakly magnetized NS [7] (Sect. 15.6.4). A number of objects, historically named Z- or Atoll-sources, show interesting spectral/timing behavior. Roughly half of the known LMXB are transient X-ray sources (Sect. 15.6.1).

The spectra of LMXB are generally the sum of two components (a very soft and a hard component), with some showing an additional very hard tail.

One might expect a population of intermediate mass X-ray binaries (IMXB) with optical companions in the mass range (1-10 M0). However, these systems are rare because of selection effects: the masses are not high enough to produce the required high mass loss rates in stellar winds and when they evolve through Roche lobe overflow the mass transfer phase is very short. Two known NS systems may be placed into this group: Her X-1 and Cyg X-2.

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