# X-ray Binaries

### High Mass X-ray Binaries

X-Ray binaries (XRBs) are accreting systems hosting a star and a compact object. Giacconi (1962) serendipitously discovered the first of these sources, i.e. Sco X-1, during an X-ray observation of the Moon.

There are three types of compact objects that can take part in a binary system: a white dwarf (WD), a neutron star (NS), or a black hole (BH). The figure below shows the different types of XRBs depending on the nature of the compact object. If the compact object is either a NS or a WD, it can feature a strong surface magnetic field B~1012-15 Gauss, or B~106 Gauss, respectively. The magnetic field influences the accretion flow, by driving matter to the accreting column. Strong magnetic field in NS, also, imprints the X-ray spectrum emitted by the compact object, in the form of absorption/emission lines, referred to as cyclotron lines.

A further distinction is based on the mass (and therefore the spectral type) of the companion star. If the mass of the companion is more than 10 Msun the system is referred to as a High Mass X-ray Binary (HMXB). If the mass of the companion star is less than 1 Msunthen it is referred to as a Low Mass X-ray binary (LMXB). Finally, if the mass of the companion falls in between 1 Msun and 10 Msun thenit is referred to as an Intermediate Mass X-ray Binary (IMXB).

My research focuses on the class of HMXBs, hosting high mass, early type stellar companion (OB spectral classification). HMXBs can be further divided by the luminosity class of the companion star. If the companion is a dwarf of the main sequence (luminosity class V-III) the system is known as a Be/X-ray Binary (or BeX). If the companion is a supergiant (luminosity class I-II) the system is known as a sgHMXBs.