Scorpius X-1

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**Scorpius X-1 / V818 Scorpii**
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Scorpius X-1 is an X-ray source some 9,000 light years (2,800 +/- 300 parsecs [1]) away. Apart from the Sun, it is the most powerful X-ray source in Earth's skies. Its X-ray output is 2.3×10³¹ W, about 60,000 times the total luminosity of the sun[2]. It was discovered in 1962 by a team under Riccardo Giacconi at American Science and Engineering, Cambridge, Massachusetts, who launched an X-ray detector on an Aerobee sounding rocket to look for X-ray emissions from Earth's moon. Instead, Scorpius X-1 became the first X-ray source discovered outside the Solar System. The object was later matched with a faint blue variable star (V818 Scorpii).

Scorpius X-1 shows regular variations of up to 1 magnitude in its intensity, with a period of around 18.9 hours. This is due to a companion star that regularly eclipses Scorpius X-1 from the point of view of Earth. Scorpius X-1 itself is a neutron star whose intense gravity draws material off this companion into an accretion disk, where it ultimately falls onto the surface, releasing a tremendous amount of energy. As this stellar material accelerates in Scorpius X-1's gravitational field, X-rays are emitted.

This system is classified as a low-mass X-ray binary; the neutron star is roughly 1.4 solar masses, while the donor star is only 0.42 solar masses.[3] The two stars were probably not born together; recent research suggests that the binary are may have been formed by a close encounter inside a globular cluster.[4]