Scorpius X-1 (α 16h 19m 55.1s,
δ -15° 38' 25'') was the first X-ray source detected
outside of our solar system, in June 1962, by astronomers at
MIT. It was detected by mounting a Geiger counter on a sounding rocket
launched from White Sands Missile Range in southern New Mexico. The
rocket flight was undertaken to study X-rays from the moon, but the detector
also detected Sco X-1. It was detected first because it is far brighter than
any other extrasolar X-ray source; Sco X-1 is
intrinsically bright in X-rays, but it is
relatively close by -- only a few thousand light years away.
Scorpius X-1 is a low-mass X-ray binary, which means that it
consists of a massive, compact object like a neutron star or a
black hole, and a low-mass stellar companion. The X-rays come
from accretion, where material from the companion
overflows its Roche lobe and spirals down onto the compact object. The
luminosity comes from the transformation of the falling material's
gravitational potential energy to heat by viscosity in the
accretion disk.
The system is also a
Z-source, a sub-class of X-ray binaries
which follow a Z-shaped pattern in a plot of the high-energy versus low-energy
X-ray luminosity. The position in the Z is related to the how fast the
star is accreting. They are at the top of the Z when they are near their
Eddington limit -- when the pressure of the light is nearly strong enough
to overcome gravity.
Besides being bright in X-rays Scorpius X-1 is also an optically
variable star. However, it is very faint with an apparent magnitude of
only 12.5. Therefore it is only visible with a telescope and a good star
chart, in the constellation Scorpius. It is best observed (if you can find
it!) between May and July.
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