Forty years after the discovery of Scorpius X-1, the first X-ray
source to be detected beyond the solar system, New Mexico State
University astronomers are sorting out one of its nagging mysteries:
Why does it seem to have split personalities when scientists compare
its X-ray emissions with its optical brightness?

The discovery of Sco X-1 in 1962 — by an Aerobee rocket launched
from White Sands Missile Range in New Mexico to look for soft X-rays
emitted from celestial objects — marked the birth of X-ray astronomy
and gave scientists a new tool for studying the universe, said
Professor Bernard McNamara of New Mexico State’s Department of

Astronomers soon learned that Sco X-1, so named because it is the
first-discovered X-ray source in the constellation Scorpius, is a
binary system — an extremely dense neutron star that pulls matter
away from a companion low-mass star. Matter flowing from the
low-mass star forms what is known as an accretion disk around the
neutron star, and as it is transported through this disk to the
neutron star, an enormous amount of energy is released, mostly as

Scientists had assumed that Sco X-1’s optical emission — its visible
light, which is less intense than its X-ray luminosity — results
from reprocessed X-rays, McNamara said. “The model was that the
optical light arose from the X-rays, that some of the X-ray energy
was absorbed in the process and re-radiated in visible wavelengths.”

But if that were the case, fluctuations in the optical brightness
should correlate closely with fluctuations in X-ray emissions, and
studies comparing optical and X-ray observations have not found
that correlation, McNamara said.

In a paper presented today (June 3, 2002) at a national meeting of
the American Astronomical Society in Albuquerque, N.M., McNamara
and his New Mexico State University colleague Thomas Harrison suggest
that Sco X-1 has three major optical states that are associated with
the rate at which mass falls onto the neutron star.

McNamara and Harrison, with NMSU astronomy students, obtained X-ray
data from the orbiting Compton Gamma Ray Observatory before it was
decommissioned in 2000. Optical observations for the same time
periods were obtained using the 1-meter Yale telescope and the
People’s Photometer at the Cerro Tololo Inter-American Observatory
(CTIO) in Chile. They interpreted the data using a new theoretical
model of Sco X-1 developed by Dimitrios Psaltis, Frederick Lamb and
Guy Miller.

“One of the surprising things we found was that the two signals were
correlated only when the system was at its brightest,” McNamara said.
“Why weren’t they always correlated? What we came up with is a
realization that the system is a lot more complex than a simple
reprocessing of X-ray energy.”

One important factor in the complexity, he said, seems to be how
quickly the system moves from one state to another. “We found
there’s probably only one stable state” when the optical and X-ray
flux are correlated, he said.

McNamara and Harrison’s study also helped validate the new
theoretical model developed by Psaltis and colleagues.

“We were able to take our observations and interpret them on the
basis of this model to find that it does a very good job of
describing what’s going on in the system,” McNamara said.

Even though it is about 40,000 trillion miles from Earth, it is the
brightest non-solar X-ray source in the sky, McNamara said. “This is
a very powerful X-ray source and we want to understand it,” he said.

Sco X-1 also serves as the prototype for a class of objects known as
low-mass X-ray binaries (LMXBs), he noted. It serves as a model for
understanding similar systems elsewhere in the universe in which
one star is affecting the evolution of another.