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Gathering data on source of X-rays was a global effort

Gathering data on the source of a burst of X-rays required the work of many people and much machinery.

The All-Sky Monitor, which keeps an eye on most of the sky all the time, was built at MIT under the direction of Professor Hale Bradt of physics. Another instrument on board the RXTE, the Proportional Counter Array, built at NASA's Goddard Space Flight Center (GSFC), has been used daily since April 1 to observe XTE J0421+560. Jean Swank of GSFC is in charge of the RXTE satellite.

The presence of strong x-ray emission also was detected with the orbiting Compton Gamma Ray Observatory. William Paciesas of the University of Alabama at Huntsville and Gerald Fishman of NASA's Marshall Space Flight Center reported that they started detecting X-rays from this transient on March 31.

Y. Ueda of the Institute of Space and Aeronautical Science and his Japanese collaborators observed the source with the high-energy-resolution Japanese observatory ASCA on April 3 and 4. They detected an iron line in the spectrum of XTE J0421+560 that indicates that the X-rays are the emissions of hot gas. The PCA sees the iron line as well, but the higher energy resolution of ASCA better fixes the nature of the emission.

The Italian X-ray observatory BeppoSAX also observed CI Cam at about the same time as ASCA. Those researchers, too, observed the all-important iron line.

Ground-based observations made by several groups using optical and radio telescopes noticed as early as April 2-3 that CI Cam was the likely origin of the X-ray transmission.

Robert Hjellming of the National Radio Astronomy Observatory and his collaborator A. Mioduszewski, using the Very Large Array in Socorro, NM, observed strong radio emission from CI Cam. The VLA consists of 27 radiotelescopes coordinated to operate as one giant radiotelescope. It was seen in the movie Contact.

Mark Wagner of Ohio State University and Sumner Starrfield of Arizona State University discovered that CI Cam had brightened substantially and found evidence in their optical spectra, taken with the Perkins 1.8m telescope, that CI Cam and XTE J0421+560 are almost certainly the same object.

Further support for this came on April 4 when Drs. Hjellming and Mioduszewski noticed that the radio emission from CI Cam was highly variable. The clincher came on April 5 when they observed twin radio jets emerging from CI Cam. The velocities of these jets were at least 15 percent of the speed of light. Similar jets had been observed in two previous black hole transients.

A version of this article appeared in MIT Tech Talk on April 15, 1998.

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