The first time that superluminal motion had been detected in our own Galaxy was in the Spring of 1994, Felix Mirabel from Saclay, France, and Luis Rodriguez, from the National Autonomous University in Mexico City, were observing an X-ray emitting object called GRS 1915+105, which had just shown an outburst of radio emission. This object was known to be about 40,000 light-years away, within our own Milky Way Galaxy. Their time series of VLA observations showed that a pair of objects ejected from GRS 1915+105 were moving apart at an apparently superluminal speed.
In some black hole jets, blobs of material have been seen to move at apparent speeds greater than that of light -- a phenomenon called superluminal motion. The apparent faster-than-light motion actually is an illusion seen when a jet of material is travelling close to -- but below -- the speed of light and directed toward Earth.
This surprising result showed that the supermassive black holes at the centers of galaxies -- black holes millions of times more massive than the Sun -- have smaller counterparts capable of producing similar jet ejections. GRS 1915+105 is thought to be a double-star system 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 object is pulling material from its stellar companion, which circles the massive object in an accretion disk before being pulled into it. Friction in the accretion disk creates temperatures hot enough that the material emits X-rays, and magnetic processes are believed to accelerate the material in the jets.
Since Mirabel and Rodriguez discovered the superluminal motion in GRS 1915+105, several other Galactic "microquasars" have been discovered and studied with the VLA and the VLBA. In 1999, NRAO astronomer Robert Hjellming turned the VLA toward a bursting microquasar within 24 hours of a reported X-ray outburst. Working with X-ray observers at MIT, Hellming found that this object is a microquasar only 1,600 light-years away, making it the closest black hole to Earth yet discovered.
There is something strange is lurking in the galactic neighborhood and smart money says its a newly discovered micro-quasar. The unknown object in galaxy M82 12 million light-years away has started sending out radio waves, and the emission does not look like anything seen anywhere in the universe before except perhaps by Ford Prefect. M82 is starburst galaxy five times as bright as the Milky Way and one hundred times as bright as our galaxy's center.
"We don't know what it is," says co-discoverer Tom Muxlow of Jodrell Bank Centre for Astrophysics near Macclesfield, UK. But its apparent sideways velocity is four times the speed of light. This "superluminal" motion occurs usually in high-speed jets of material bursting out by black holes.
“The new object, which appeared in May 2009, has left us scratching our heads - we’ve never seen anything quite like this before,” said Dr Muxlow. “The object turned on very rapidly within a few days and shows no sign of decaying in brightness over the first few months of its existence. The new young supernova explosions that we were expecting to see in M82 brighten at radio wavelengths over several weeks and then decay over several months, so that explanation seems unlikely.”
The object was discovered while Muxlow and his colleagues were monitoring an unrelated stellar explosion in M82 using the MERLIN network of radio telescopes in the UK. Unlike supernova emissions, which usually get brighter over a few weeks and then fade away over months, the enigmatic source has hardly changed in brightness over the course of a year, and its spectrum is steady.
Yet it does seem to be moving – and fast: its apparent sideways velocity is four times the speed of light. Such apparent "superluminal" motion has been seen before in high-speed jets of material squirted out by some black holes. The stuff in these jets is moving towards us at a slight angle and travelling at a fair fraction of the speed of light, and the effects of relativity produce a kind of optical illusion that makes the motion appear superluminal.
A microquasar is formed after a very massive star explodes, leaving behind a black hole around 10 to 20 times the mass of the sun, which then starts feeding on gas from a surviving companion star. Microquasars do emit radio waves – but none seen in our galaxy is as bright as the new source in M82. Microquasars also produce plenty of X-rays, whereas no X-rays have been seen from the mystery object. "So that's not right either", Muxlow said in an interview with New Scientist Maybe it's a megamicroquasar?
Muxlow's best guess without having consulted Deep Thought is still that the radio source is some kind of dense object accreting surrounding material, perhaps a large black hole or a black hole in an unusual environment -a phenomena perhaps more common in M82 because it is a "starburst" galaxy – a cosmic cauldron where massive stars are forming and exploding at a much higher rate than in the Milky Way, creating new black holes.
“We have just started processing data from an array of 20 radio telescopes across the Earth were taken for the central nuclear region of M82. These images will allow us to examine the structure of the new radio source in detail. However, processing such huge datasets takes significant amounts of computing effort and painstaking work. Only then will we be able to see if it is some rare form of micro-quasar. Watch this space…!” said Muxlow.
Casey Kazan via Royal Astronomical Society and New Scientist http://www.nrao.edu/pr/2000/vla20/background/superlum/
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