Low-mass X-ray binaries
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Low-mass X-ray binaries (LMXBs) are binary stars where one of the components is either a black hole or neutron star. The other component (donor) is usually filling its Roche lobe and therefore transfers mass to the compact star. The donor is less massive than the compact object, and can be on the main sequence, a degenerate dwarf (white dwarf), or an evolved star (red giant). Approximately one hundred LMXB:s have been detected in the Milky Way, and of these, thirteen LMXBs have been discovered in globular clusters. New data from the Chandra X-ray Observatory has revealed LMXBs in many distant galaxies.
A typical LMXB emits almost all of its radiation in X-rays, and typically less than one percent in visible light, so they are among the brightest objects in the X-ray sky, but relatively faint in visible light. The apparent magnitude is typically around 15 to 20. The brightest part of the system is the accretion disk around the compact object. The orbital periods of LMXBs range from ten minutes to hundreds of days.