* Astronomy

Royal Astronomical Society Press Notice

BLACK HOLE BLOWS BUBBLE BETWEEN THE STARS

A team of astronomers from The Netherlands and the UK has discovered a vast 'jet-powered bubble' formed in the gas around a black hole in the Milky Way.
The discovery means that for decades scientists have been severely underestimating how much power black holes pump back into the universe instead of merely swallowing material across their event horizons.
Jets of energy and particles flowing outwards at close to the speed of light are a common feature of all accreting black holes, ranging from supermassive black holes at the centres of active galactic nuclei to stellar-mass black holes in X-ray binary systems within our own Galaxy.

However, for the first time European astronomers have now discovered a large bubble surrounding an X-ray binary system. The bubble is approximately 10 light years across, and is predicted to be expanding with a speed of around 100 km per second. It appears to have been formed by the action of a powerful outflow or 'jet' of energy and matter from the black hole over a time scale of about a million years.
The new, detailed radio observations of a black hole called Cygnus X-1 show a ring of radio emission around a bubble in the nearby interstellar gas -- the result of a strong shock that develops at the location where the jet strikes the rarefied gas of the interstellar medium.
The jet that created the bubble seems to be carrying more than 100,000 times the total luminosity of our Sun, and yet the only evidence for this incredible flow of energy is its impact on the tenuous gas between the stars, resulting in the expanding bubble.


wide-field radio image of the environment of the black hole system Cygnus X-1. The cross marks the location of the black hole. The bright region to the left (East) of the black hole is a dense cloud of gas existing in the space between the stars, the interstellar medium. The action of the jet from Cygnus X-1 has 'blown a bubble' in this gas cloud, extending to the north and west of the black hole . The image was made at a frequency of 1.4 GHz with the Westerbork radio telescope in The Netherlands.

"We already knew that supermassive black holes at the centre of other galaxies produce enormous amounts of energy, but this finding proves that something similar is happening in our backyard. Remarkably, it also means that, after a massive star dies and turns into a black hole, it is still capable of energising its surroundings, by means of completely different mechanisms." - Elena Gallo of the University of Amsterdam, lead author of the paper which will appear in this week's issue of Nature.

"The importance of this result is that it demonstrates that black holes such as Cygnus X-1, of which there may be millions within our galaxy alone, do not swallow all of the infalling matter and energy, but rather redirect a considerable fraction of it back into space" - Rob Fender, University of Southampton, second author on the paper.


Schematic showing the a model for how the black hole created the bubble. The black hole has been acting on the interstellar gas for around a million years, via a powerful jet which has only been previously imaged on very small scales (inset). The edges of the shell which we observe in the image correspond to gas from the interstellar medium which is heated as the bubble expands into it with a velocity of around 100 km/sec. This gas is expected to be emitting via the 'bremsstrahlung' (or stopping radiation) process typical of 'warm' (about10, 000 degrees C) gas, whereas the bubble itself is predicted to be filled with a tenuous gas of superheated electrons emitting 'synchrotron' radiation'.


"We knew about jets from black holes and expected to discover some interaction of the jet's energy with the gas in our Milky Way, but the size and energy content of this bubble came as a surprise" - co-author Dr. Christian Kaiser, University of Southampton.

The team has ruled out the possibility that the ring might be the low-luminosity remnant of the supernova that spawned the black hole. Since Cygnus X-1 moves in the sky along a trajectory that is roughly perpendicular to the jet, it cannot possibly have been located in the centre of the ring.
Cygnus X-1 is a binary system which includes a black hole with 10 times the mass of the Sun. It lies at a distance of about 6,000 light years from Earth.
The bubble was discovered in radio observations with the Dutch Westerbork radio telescope, and subsequently found during optical observations with the Anglo-Dutch 2.5 m Isaac Newton Telescope in the Canary Islands.
Cygnus X-1 was the subject of a famous bet between Stephen Hawking and Kip Thorne, in which the former bet against the existence of a black hole in the system as an 'insurance policy', but later conceded.


An optical image of the bubble, obtained with the Anglo-Dutch Isaac Newton Telescope on La Palma. Detection of the bubble in the optical band confirms the prediction of the model that the gas should be warm (around 10, 000 degrees C), and offers the possibility of future spectroscopy with large optical telescopes being able to directly measure the expansion velocity of the gas.

The paper published in the 11 August 2005 issue of Nature is entitled "A dark jet dominates the power output of the stellar black hole Cygnus X-1" by Elena Gallo (University of Amsterdam), Rob Fender, Christian Kaiser, David Russell (all University of Southampton), Raffaela Morganti, Tom Oosterloo (ASTRON, Netherlands) and Sebastian Heinz (MIT).

IMAGES ARE AVAILABLE AT:
http://www.astro.soton.ac.uk/~ rpf/bubble

Press Release (pdf)

In 1979 an X-ray "super bubble" was discovered in the constellation Cygnus, made with an X-ray telescope on HEAO-1. It was also discovered that a corona of 100,000 ° C gas exists around the Milky Way galaxy, as theorized in 1956. The corona perhaps is fed with hot material by super bubbles expanding from the galactic plane.