Scorpius X-1 (3U 1617-15) is a compact X-ray source that lies 800 to 1,600 light-years away in the constellation Scorpius. Scorpius X-1 (Sco X-1) was the first X-ray source to be discovered (in 1962) and is the brightest X-ray source in the sky apart from the Sun.
Sco X-1 is believed to be a neutron star with a mass of about 1.4 solar masses which is one component of a low-mass X-ray binary (LMXB) system. Its companion is a star of 0.42 solar masses. Material from this companion "donor" star enters an accretion disk around the neutron star and becomes heated to a very high temperature at which it emits X-rays. Material from the accretion disk falls on to the surface of the neutron star with the release of further large amounts of energy.
Sco X-1 shows regular variations of up to 1 magnitude in intenisty, with a period of about 18.9 hours. These variations are due to the neutron star being regularly eclipsed by the companion star as seen from Earth. The optical counterpart of Sco X-1 is a 13th magnitude blue variable named V818 Scorpii.
The path and motion of Sco X-1 around the center of the Galaxy are similar to those of the most ancient stars and globular clusters of the inner galactic halo. This suggests that Sco X-1 was formed by a close encounter in a globular cluster. Less likely, but possible, is that a supernova explosion launched Sco X-1 into its presnt orbit from a previous location in the galactic bulge. In any case, the Sco X-1's galactocentric orbit indicates an age for the object of less than 30 million years.