Low-mass X-ray binaries

Low-mass X-ray binaries (LMXB) are binary stars where one of the components is a black hole or a neutron star. The other component (donor) is filling its Roche lobe and therefore transfers mass to the compact star. The donor is less massive than the compact object, and usually on the main sequence. In some cases the donor can be either degenerate or evolved, (subgiant or red giant). About one hundred LMXB:s have been detected from the Milky Way. Several LMXB have been discovered from globular clusters.

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 20 magnitudes. The brightest part of the system is the accretion disk around the compact object. The orbital periods of LMXB range from a few hours to a few days.