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In far-distant quasars and galaxies, millions or even billions of light-years away, the gravitational energy of supermassive black holes is capable of accelerating "jets" of subatomic particles tospeeds approaching that of light. The VLA has observed such jets for manyyears. In some of these jets, blobs of material have been seen tomove at apparent speeds greater than that of light -- a phenomenoncalledsuperluminal motion. The apparent faster-than-light motionactually is an illusion seen when a jet of material is travellingclose to -- but below -- the speed of light and directed towardEarth.
In the Spring of 1994, Felix Mirabel from Saclay, France, and Luis Rodriguez, from the National Autonomous University in MexicoCity, were observing an X-ray emitting object called GRS 1915+105,which had just shown an outburst of radio emission. This object wasknown to be about 40,000 light-years away, within our own Milky WayGalaxy -- in our own cosmic neighborhood. Their time series ofVLA observations, seen in this image, showed that a pair of objects ejected from GRS 1915+105 were movingapart at an apparently superluminal speed. This was the first timethat superluminal motion had been detected in our own Galaxy.
This surprising result showed that the supermassive black holesat the centers of galaxies -- black holes millions of times moremassive than the Sun -- have smaller counterparts capable of producingsimilar jet ejections. GRS 1915+105 is thought to be a double-starsystem in which one of the components is a black hole or neutron star only a few times the mass of the Sun. The more-massive objectis pulling material from its stellar companion. The material circlesthe massive object in an accretion disk before being pulled intoit. Friction in the accretion disk creates temperatures hot enoughthat the material emits X-rays, and magnetic processes are believedto accelerate the material in the jets.
Since Mirabel and Rodriguez discovered the superluminal motion inGRS 1915+105, several other Galactic "microquasars" have been discoveredand studied with the VLA and the VLBA. In 1999, NRAO astronomer RobertHjellming turned the VLA toward a bursting microquasar within 24hours of a reported X-ray outburst. Working with X-ray observersDonald Smith and Ronald Remillard of MIT, Hellming found that thisobject is a microquasar only 1,600 light-years away, making it theclosest black hole to Earth yet discovered.
Microquasars within our own Galaxy, because they are closer andthus easier to study, have become invaluable "laboratories" for revealing the physical processes that produce superfast jets of material. For discovering this new class of celestial object,Mirabel and Rodriguez received the prestigous Bruno Rossi Prize of theAmerican Astronomical Society in 1997.