X-ray binaries are stellar systems consisting of a star and a stellar remmant such as black-hole, neutron star or a white dwarf. When material from the star (or donor) is falling onto the stellar remnant (or compact object) it is heated to temperatures of several million degrees and produces copious X-ray emission. In this process we may also observe jet-like colimated outflows or wide-angle winds of highly ionized plasma. The properties of this emission depend on the conditions close to the compact object and therefore can be used to study the behaviour of matter under the influence of strong gravitational fields. In addition the properties of the binary stellar system (e.g. parameters of the two objects and their orbit, long-term evolution) depend on the past of the two objects. Therefore, X-ray binaries are very usefull laboratories for studies of the properties of compact objects and stellar evolution.

Our group has a deep interest in observational as well as theoretical studies of X-ray binaries. We use data from a wide range of X-ray telescopes: Rossi X-ray Timing Explorer (X-rays), Chandra X-ray Observatory (X-rays), XMM-Newton (X-rays), as well as ground based telescopes in order to understand their spectral properties and variability patterns.

In particular our research is focused on the variability of the X-ray emission as a means to understand the nature of the compact objects in X-ray binaries and the properties of the accretion flow. The vast majority of X-ray binaries with massive companions harbour X-ray pulsars. The detection of pulsations from an accreting X-ray source provides one of the strongest evidence that the compact object is a neutron star. X-ray pulsations result from the misalignment of the neutron star spin and magnetic axis. Gas is accreted from the stellar companion and is channeled by the magnetic field on to the magnetic poles producing two or more localized X-ray hot spots. As the neutron star rotates, pulses of X-rays are observed as the hotspots move in and out of view. The change in the neutron star rotation velocity (spin-up or spin-down) allows measurements of accretion torques, which can provide a measure of accretion rate and magnetic field.

In addition we use multi-band observations of X-ray binaries to study the nature of their donor stars, their orbital parameters and address their long-term variability. We combine these observational data with theoretical models of the emission from the accretion flow and jet outflows in order to obtain a better understanding of the physical processes which take place in those extreme environments.

The current generation of X-ray telescopes offer us a very clear picture of the X-ray source populations in nearby galaxies. This allows us to study for the first time in detail the populations of X-ray binaries in a large number of galaxies in our local Universe and obtain a more clear picture of their properties and their dependence on star-formation and galactic parameters.

• "Be/X-ray binaries", (Reig, P., 2011, Ap&SS, 332, 1)
• "Patterns of variability in Be/X-ray pulsars during giant outbursts", (Reig, P., Nespoli, E., 2013, A&A, 551, 1)
• "Disc-loss episode in the Be shell optical counterpart to the high-mass X-ray binary IGR J21343+4738" (Reig, P., Zezas, A., 2014, A&A, 561, A137)
• "An extensive spectroscopic study of the Be X-ray binary populations in the SMC" (Antoniou V., Hadzidimitriou D., Zezas A., Reig P., 2009, ApJ, 707, 1080)

N. Kylafis, I. Papadakis, P. Reig, A. Zezas, G. Maravelias