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The Story of SCO X1: QPO's and BeyondInterestingly enough, although SCO X-1 was the first discovered source, and the one with the greatest X-ray brightness in the sky, it has been reluctant to reveal its secrets. It was only many years later that scientists found convincing evidence for its binary nature, even though they were quite certain that there must be a companion star. Now, we have satellites that allow us to observe the sky for far longer than five minutes, thereby opening up new time domains for studying potentially exotic behavior. Orbiting X-ray telescopes provide much more complete coverage and sensitivity, and SCO X-1 has obliged us... A scant ten years ago, a strange signal was detected from SCO X-1, (as well as from certain other X-ray sources). Named "quasi-periodic oscillations" (or QPO's), these elusive pulses of X-rays come and go like the wind, lasting a few hundred seconds, only to return and do the same thing again, but slightly differently each time. During these hundred second outbursts, the light comes and goes much like a rotating lighthouse beacon, about once every second. We already know that these phenomena come only from a class of objects called low-mass X-ray binaries, of which SCO X-1 is a member. However, the exact nature of the processes that give rise to these peculiar signals are unknown. The most popular theory is one called shot noise, but the wide variety of effects present in these sources (such as changing spectral properties) makes things difficult (but more interesting!) for any theory to explain. With the Rossi XTE, still more new results are beginning to pour in about these mysterious signals, and it seems certain that Nature will be providing us yet more X-ray surprises. In the future, Chandra's unprecedented sensitivity may yield new and exciting data from these QPO sources. For although SCO X-1 is the brightest X-ray source in the sky (excluding the Sun), we need to look at its variability over very short time scales (about one thousandth of a second) to understand in detail what is happening. During this short period, only a few photons can be collected with current satellites, and the data looks "noisy". Chandra will aid us tremendously with its ability to collect more X-ray light than ever before from SCO X-1 and other low mass X-ray binary stars.  The Rossi XTE, a current X-ray telescope It seems almost impossible to imagine that all of our knowledge of the X-ray sky began with a humble rocket experiment a mere 35 years ago. Cosmic X-ray sources have demonstrated Nature's bounty of mysteries and wonders during that time, showering us with clues about the evolution and make-up of our awe-inspiring universe. And SCO X-1 led the way. P.S. We eventually did discover X-rays reflected from the moon. Here is the image of that finding.  The Moon in X-ray Light |
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Revised: August 29, 2006
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