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New Mexico State astronomers report on Sco X-1 optical and X-ray emissions

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?

covery 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 Astronomy.

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 X-rays.

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.

Karl Hill
June 3, 2002