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Introduction

In June of 1962, on a short rocket flight over White Sands Missile Range in New Mexico, sensitive instruments detected X-rays from an object outside of the Solar System for the first time . The X-rays were coming from a 12th-magnitude star in the constellation Scorpius, more than 9,000 light-years from Earth, one-third of the way toward the center of our Milky Way Galaxy. The object, now called Scorpius X-1, is a binary system composed of a dense neutron star and a fainter, smaller star, similar to the Sun. The two stars are only one million miles apart and revolve about each other in 18 hours, 53 minutes. The intense gravity from the neutron star strips material from the companion star, which forms a disk as it slowly spirals onto the neutron-star surface. This material is heated to 100 million degrees and produces intense X-rays and thermonuclear reactions which accelerate electrons and protons to high speeds. For reasons that are not yet fully understood, these energetic particles are trapped by strong magnetic fields which force them to escape only in oppositely directed conical-shaped narrow beams, perpendicular to the plane of the disk. This flow of energetic particles often travels hundreds of times the size of the disk before splattering against the interstellar material and producing the radio emission that these VLBI observations detect.

The "movie" of radio images shows the changing radio emission from Sco X-1 in June 1999 over a continuous period of 56 hours. In order to obtain this continuous coverage, the object was observed with three different VLBI arrays around the globe. For the first eight hours, the VLBA and the VLA near Socorro, NM, imaged Sco X-1. When the object set in the USA, it was rising across the Pacific Ocean, so observations continued using telescopes in Australia, Japan, China and South Africa. Eight hours later, the telescopes of the European VLBI network, plus one in South Africa and another in Green Bank WV, continued with the observations. Seven such eight-hour observing cycles produced these continuous observations. The Steward and Braeside Observatories and the NASA X-ray satellite, RXTE, also monitored Sco X-1 over much of this period.

Over the observing period, the radio emission from Sco X-1 changed rapidly with time. The original images were made about 50 minutes apart, but have been interpolated so that each movie frame is about 15 minutes long. There are 231 frames, covering 2 days, 7 hours and 13 minutes. The size scale in the movie frames is in billions of miles. The Earth-Sun distance is about 0.1 billion miles and Pluto-Sun distance is 2.3 billion miles, thus Scorpius X-1 covers a volume much larger than our Solar System. The size of the binary star system is about 0.005 billion miles, located at the (0,0) point. The angular-scale size of these images is incredibly small and only VLBI observations can reach these levels. The size of each image on the sky is about 1/50 of a second of arc, or about 1/100,000 times smaller than the diameter of the full moon.

Light travels at about 0.67 billion miles per hour and the motions in the movie are comparable to the speed of light. Since nothing can move faster than this, events which appear to occur 'simultaneously' in different parts of the object cannot be related to the same cause. Sometimes there are periods of about six hours when very little is occurring, while at other times several phenomona occur at different places nearly at the same time. The text in and below the frames will help to explain the fascinating changes in the radio emission from Sco X-1.