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At least half of the stars visible on the night sky are actually so-called binaries. Here two stars are orbiting around their common center of mass. There were even systems detected with more than two stars. During the evolution of the binary one of the stars will exhaust the fuel in its center. If the star is heavier than a certain mass it will explode as a supernova. What will remain (the star's "corpse") depends on the former mass of the star and might be a white dward, a neutronstar, or a black hole. In general, they are a subclass of a so-called compact object, which is due to their high densities. In case of a white dwarf there will be, however, no preceding supernova. Now the binary consists not of two stars anymore. Rather it harbors a compact object and an ordinary star. Under certain conditions mass from the star can be transfered onto the compact object, called mass accretion. During that process the matter is strongly accelerated, reaching velocities near the speed of light. The kinetic energy will be released as X-rays once the matter hits the surface of the compact object (or the accretion disk in case of a black hole). Now the system is called an X-ray binary.
X-ray binaries with a neutron star as the compact object are my favorite research field.
X-ray binaries are classified by the type of the companion star: for lower masses of the companion the systems are called Low Mass X-ray Binaries (LMXB), and for higher masses High Mass X-ray Binaries (HMXB). These classes can be devided into subclasses as well. For instance in XMXBs, if the companion star is of type Be, the systems are usally denoted BeXRBs. Here, the optical spectrum of the Be type companion typically shows fluorescence lines of hydrogen. These originate from a circumstellare disk (see figure on the left). A particular feature of BeXRBs with compact objects are that they are invisible in X-rays for most of the time. This is related to an elliptical orbit of the compat object and the activity of the companion star. Once both stars are near to each other (at the so-called periastron) material from the Be disk can be transfered onto the compact object and X-rays are produced. However, if the Be star is inactive, which means its disk is rather small or even gone completely, no mass can be accreted. In the case of possible mass accretion the binary gets visible in the X-rays within a few days, an X-ray outburst results. In some cases an outburst may become the brightest object on the X-ray sky. As sudden as the binary got visible they get invisible again, which is why they are known as transient X-ray binaries. Thus, it is not trivial to observe such objects because they can be discovered during X-ray outbursts only. Unfortunately, the time between those X-ray outbursts strongly depends on the system and may range from a few weeks up to decades. An important aspect of BeXRBs for my kind of research is that during outbursts they cover a wide range of luminosities. They are an ideal target to check theoretical predictions or to dsicover properties, which depend on the binary X-ray luminosity. In particular, they play an important role in understanding the so-called cyclotron lines, which are visible in the spectra of several neutron stars.