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XTE, named for Rossi, discovers binary system

NASA has named the X-ray Timing Explorer, placed into orbit in December, for an MIT pioneer in the field of X-ray astronomy, the late Institute Professor Bruno B. Rossi. The 6,700-pound observatory is now known as the Bruno B. Rossi X-ray Timing Explorer (RXTE).

Dr. Rossi, who died in 1993, was one of the nation's most honored scientists. He played a leading role in the study of cosmic rays and in the development of space plasma physics. He and his colleagues discovered the first non-solar source of X-rays in a rocket flight in 1962. The source, Scorpius X-1, was the first of many collapsed stars discovered. The RXTE is studying collapsed stars, stellar black holes, neutron stars, quasars and other sources.

Shortly after being placed in orbit, the RXTE detected X-ray bursts that had first been noted in early December by another satellite, the Burst and Transient Source Experiment on board the Gamma-ray Observatory. The RXTE information narrowed the area of the sky from which the bursts were coming. The new source is named GRO J1744-28. The GRO refers to Gamma-ray Observatory.

Professor Walter H.G. Lewin of the Department of Physics, who is the lead author on a paper about the new source, said the object is a binary system somewhat similar to the "Rapid Burster," a source he and his team discovered 20 years ago.

Professor Lewin and his current colleagues (including graduate students Robert E. Rutledge and Jefferson M. Kommers) have submitted a paper to The Astrophysical Journal which will appear in print within a few weeks. The paper gives their reasons why the X-ray bursts from the new source are caused by spasmodic accretion and not to thermonuclear explosions as some scientists think.

In this binary system, Professor Lewin says, matter is transferred from a star less massive than our sun to a neutron star, a process known as accretion. As the matter plunges onto the surface of the neutron star, the surface of this star heats up to millions of degrees and emits large amounts of X-rays.

The neutron star in the newly detected system rotates about its axis once every 0.5 seconds, Professor Lewin said, and since the neutron star has a strong magnetic field-about a million times stronger than the magnetic fields that can be made in Earth laboratories-the X-ray emission is pulsed, that is, it bobs up and down with a period of 0.5 seconds, he said.

What makes the new source unique, Professor Lewin says, is the combination of pulsed X-ray emission and bursts.

Notwithstanding this unique combination, Professor Lewin and his co-workers are convinced that the X-ray bursts from the new source, which occurred as frequently as one per three minutes in early December, are the result of spasmodic accretion.

Professor Rossi, a member of the Department of Physics, contributed to the formulation of public policy in the scientific exploration of space as a member of the Space Science Board of the National Academy of Sciences and other advisory groups. He was the co-recipient of the prestigious Wolf Prize in Physics in 1987. The Bruno Rossi Prize, awarded annually by the American Astronomical Society to a top astrophysicist for achievements in the field, is named in his honor.

A team at MIT's Center for Space Research has a major role in the RXTE. Professor Hale Bradt of the Department of Physics is principal investigator and Alan M. Levine, a CSR principal research scientist, is the project scientist. MIT's contributions include the All Sky Monitor (ASM), an instrument designed to scan a wide section of the sky to detect X-ray events, and a new, powerful flight data system, the Experiment Data System (EDS), which preprocesses X-ray data from the ASM and from one of the other two instruments on board. RXTE is the first mission for which 100 percent of the observing time will be available to the broad scientific community, NASA said. Specific observations will be proposed by scientists from the United States and abroad. Observations are planned by scientists at the XTE Science Operations Center at NASA's Goddard Space Flight Center in Greenbelt, MD. The RXTE was developed by and is managed and operated by Goddard for NASA's Office of Space Science in Washington, DC.

A version of this article appeared in MIT Tech Talk on March 13, 1996.

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