In the news this month... probing the black hole in M87
The inner jet of M87 as imaged by the VLBA at a wavelength of 2cm CREDIT: NRAO/AUI and Y. Y. Kovalev, MPIfR and ASC Lebedev
Among the brightest and most energetic objects in the universe are so-called Active Galactic Nuclei (AGN). These are objects outside the Milky Way, thought to be powered by material falling onto supermassive black holes in the centre of other galaxies. The process can create powerful jets which can accelerate particles up to very high speeds and generate very high energy gamma-ray emission. Although astronomers have known for some time that these AGN are bright sources of gamma-rays, they did not know exactly where in these galaxies the high energy emission originated. But in research published in the online edition of Science magazine on the 2nd July, a collaboration of more than 350 scientists has finally narrowed it down.
High energy emission from the nucleus of the active galaxy M87 was first detected in 1998, and gamma-ray outbursts have since been confirmed by the HESS, VERITAS and MAGIC telescopes located in Namibia, Arizona and La Palma respectively. All of these experiments are gamma-ray detectors which pick up this high energy emission, but do not have sufficient resolving power to accurately determine the location of the outburst within M87's core. Located 50 million light years away, M87 is the largest galaxy in the Virgo cluster and contains a central black hole six billion times as massive as the Sun.
In order to determine the source of the gamma-rays, the HESS, VERITAS and MAGIC collaborations teamed up with astronomers using the Very Long Baseline Array - a collection of ten radio telescopes spread across North America from Hawaii to the Caribbean. By linking widely separated radio telescopes together, astronomers can synthesise a much larger telescope, resulting in images with much finer detail. Using all four instruments, HESS, VERITAS, MAGIC and the VLBA, the team monitored M87 over 50 nights between January and May 2008. As well as detecting gamma ray flares, the observers also detected large flares of radio waves from M87. Using the VLBA's high resolution they determined that these flares were occurring at the same time as the gamma ray flares and were being generated in a region very close to the black hole where material falling in forms a tightly rotating torus known as an accretion disk. The observations show that the high energy emission is coming from an area no larger than 50 times the size of the black hole's event horizon - the area within which matter cannot escape from the black hole. For the black hole in M87, this is a region roughly twice the size of the solar system.