This image of microquasar SS 433, first discovered forty years ago, located about 18 000 light-years away in the constellation of Aquila (The Eagle), was captured for the very first time at submillimeter wavelengths by the Atacama Large Millimeter/submillimeter Array (ALMA). The image is a game changer because it shows the jets emitted by a hot, swirling disc of material encircling the supermassive black hole at SS 433’s center –these jets have been seen to move at apparent speeds greater than that of light .
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 to speeds approaching that of light. The Very Large Array (VLA) has observed such jets for many years. In some of these 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 traveling close to — but below — the speed of light and directed toward Earth.
The corkscrew shape visible here is created by a phenomenon known as precession; as they move outwards through space, these two jets are slowly tumbling around an axis in a similar way to the motion of a gyroscope or a spinning top slowing down, the orientation of their rotational axes changing as they do so. The scale of this corkscrew is enormous, at 5000 times the size of the Solar System.
One remarkable aspect of this observation is that its detailed shape was entirely predicted from spectroscopic measurements by the Global Jet Watch telescopes in the preceding year before the ALMA observations were made. The sequence of these observations allowed researchers to make and test predictions about the paths the jets would take, representing a new milestone in the study of microquasars.
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 — in our own cosmic neighborhood. Their time series of VLA observations, seen in this image, showed that a pair of objects ejected from GRS 1915+105 were moving apart at an apparently superluminal speed. This was the first time that superluminal motion had been detected in our own Galaxy.
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. The material 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 Very Large Base Array (VLBA). In 1999, National Radio Observatory (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 Donald Smith and Ronald Remillard of 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.
Microquasars within our own Milky Way Galaxy, because they are closer and thus easier to study, have become invaluable “laboratories” for revealing the physical processes that produce super fast jets of material.
Image credit: ALMA (ESO/NAOJ/NRAO)/K. Blundell (University of Oxford, UK), R. Laing, S. Lee & A. Richards, ApJ Letters.