Sagittarius X-4
Sagittarius X-4 , also called Sgr x-4 , is a low-mass X - ray binary star in the globular cluster NGC 6624 of the Sagittarius constellation . With a rotation time of only 11 minutes and 52 seconds, this is the closest known binary star system.
discovery
Sgr X-4 was discovered by the Uhuru1 artificial satellite in 1972 . As its name indicates (the X refers to the English word x-ray) this binary star system is one of the first known X-ray sources. Its periodicity was verified 15 years later, in 1987, using the EXOSAT satellite. The periodicity is shown by a very low modulation (3%) of the X-ray flux received from the source.
Physical Properties
The properties of the X-ray emission indicate that the compact star of the system is a neutron star , the pulsed emission of which is not observed as in a pulsar . The extremely short orbital period implies that the system is in a very tight orbit (on the order of 100,000 km). This in turn suggests that the companion star of the compact object in this system is a white dwarf (the only star likely to be small enough to be captured the size of the orbit). Within the globular cluster, it seems likely that this system results from a tidal interception of the neutron star by the star that is in its main sequence phase. The neutron star would then have been in the extended atmosphere of the star during its phase as a red giant and slowly spiraling towards the center of the star, whereby the process would have ended with the star's red giant phase.