X-ray Binaries

X-ray binaries (XRBs) are systems in which a compact object—either a neutron star (NS) or a black hole (BH)—accretes matter from a companion star, typically a low or high-mass stellar object which transfers material onto the compact object through either Roche-lobe overflow or stellar winds. As this matter spirals inward, it forms an accretion disk, heating up to millions of degrees and emitting copious amounts of X-rays. This radiation is modulated by a variety of physical processes due to variations in the accretion flow.

Illustration of an X-ray binary system. Source: ESA

The behaviour of these sources is largly dependent on mass accretion rate. At a very high accretion rate, large influx of material into the accreation disk can cause instability in the disk, causing outbursts. These events can last from weeks to several months. Most of the Low Mass X-ray Binaries (LMXBs) are discovered during their outbursts, as they get very bright. If the compact object is a Neutron Star, the accreated matter gets deposited onto the Neutron Star and upon reaching critical conditions, can cause a runaway reaction resulting in a bright flash of X-rays. These are called Thermonuclear Bursts. The LMXBs can reach luminosities near eddington limit during these bursts. Thermonuclear bursts are excellent laboratories through which measurements such as distance, radius and spin period of th Neutron Star can be performed.