Scorpius X-1

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**V818 Sco or Scorpius X-1**
Observation data Epoch J2000.0
Constellation (pronunciation)	Scorpius
Right ascension	16^h 19^m 55.07^s
Declination	-15° 38' 24.8"
Apparent magnitude (V)	12.2
Distance	9,000 ly (2,800 pc)
Other designations
V818 Sco, H 1620-15, 2RE J161955-153824, 1XRS 16170-155, 2A 1616-155, INTEGRAL1 21, RE J1619-153, XSS J16204-1536, 3A 1617-155, INTREF 685, RE J161956-153814, X Sco X-1, 2EUVE J1619-15.6, KOHX 20, SBC7 569, EUVE J1619-15.6, 1M 1617-155, 2U 1617-15, AAVSO 1614-15, 1H 1617-155, 2MASS J16195506-1538250, 3U 1617-15, H 1617-155, 2RE J1619-153, 4U 1617-15.

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 may have been formed by a close encounter inside a globular cluster.[4]