Black Hole Emits Massive Microquasar

Combining observations made with the European Southern Observatory’s Very Large Telescope and NASA's Chandra x-ray telescope, astronomers have uncovered the most powerful pair of jets emitting from a stellar black hole that have ever been seen. This object, also known as a microquasar, blows a bubble of hot gas 1000 light-years across — twice as large and tens of times more powerful than other known microquasars. The discovery is reported this week in the journal Nature.

The stellar black hole belongs to a binary system as pictured in this artist's impression. (Image: European Southern University/L. Calçada)

"We have been astonished by how much energy is injected into the gas by the black hole," said lead author Manfred Pakull of the University of Strasbourg in France. "This black hole is just a few solar masses, but is a real miniature version of the most powerful quasars and radio galaxies, which contain black holes with masses of a few million times that of the Sun."

Black holes are known to release a prodigious amount of energy when they swallow matter. It was thought that most of the energy came out in the form of radiation, predominantly x-rays. However, the new findings show that some black holes can release at least as much energy, and perhaps much more, in the form of collimated jets of fast-moving particles. The fast jets slam into the surrounding interstellar gas, heating it and triggering an expansion. The inflating bubble contains a mixture of hot gas and ultrafast particles at different temperatures. Observations in several energy bands (optical, radio and x-ray) help astronomers calculate the total rate at which the black hole is heating its surroundings.

The astronomers observed the spots where the jets smash into the interstellar gas located around the black hole, and revealed that the bubble of hot gas is inflating at a speed of almost one million kilometers per hour.

"The length of the jets in NGC 7793 is amazing, compared to the size of the black hole from which they are launched," said co-author Robert Soria of University College London in the UK. "If the black hole were shrunk to the size of a soccer ball, each jet would extend from the Earth to beyond the orbit of Pluto."

Astronomers do not yet have any means of measuring the size of the black hole itself. The smallest stellar black hole discovered so far has a radius of about 15 km. An average stellar black hole of about 10 solar masses has a radius of about 30 km, while a "big" stellar black hole may have a radius of up to 300 km. This is still much smaller than the jets, which extend out to several hundreds light years on each side of the black hole, or about several thousand million million kilometers.

The research will help astronomers understand the similarity between small black holes formed from exploded stars and the supermassive black holes at the centers of galaxies. Very powerful jets have been seen from supermassive black holes, but are thought to be less frequent in the smaller microquasar variety. The new discovery suggests that many of them may simply have gone unnoticed so far.

The gas-blowing black hole is located 12 million light-years away, in the outskirts of the spiral galaxy NGC 7793 (eso0914b). From the size and expansion velocity of the bubble the astronomers have found that the jet activity must have been ongoing for at least 200,000 years.

For more information, visit: www.eso.org