Teams using imaging atmospheric Cherenkov telescopes to detect very high-energy gamma rays have joined forces with astronomers to reveal the precise location of particle acceleration in the nearby giant radio galaxy Messier 87 (M87). Collaborations on the High Energy Stereoscopic System (HESS), the Major Atmospheric Gamma-Ray Imaging Cherenkov (MAGIC) project and the Very Energetic Radiation Imaging Telescope Array System (VERITAS) have worked together with a team at the Very-Long Baseline Array (VLBA) radio telescope in an unprecedented, co-ordinated, 120-hour observational campaign. Their simultaneous observations at the lowest and highest ends of the electromagnetic spectrum indicate that the active galactic nucleus in M87 accelerates charged particles to very high energies in the immediate vicinity of the central black hole (V A Acciari et al. 2009).
M87 is a giant radio galaxy, 54 million light-years from Earth, with a jet structure – a huge outflow from the central region, which is probably fuelled by accretion of matter onto a massive black hole. In the jet, charged particles can be accelerated to very high velocities, with the inevitable accompanying production of high-energy gamma rays. The first indications for very high-energy gamma radiation from M87 were found in 1998 with the High-Energy Gamma-Ray Astronomy (HEGRA) telescope array – the predecessor of HESS and MAGIC. These observations were confirmed by HESS in 2006 and revealed a fast variability of the gamma-ray flux on a timescale of a few days, implying an exceptionally compact gamma-ray source.
To pinpoint the source more closely, HESS, MAGIC and VERITAS jointly observed M87 from January to May 2008, collecting 120 hours’ worth of data. During this time the galaxy underwent two major outbursts of very high-energy gamma-ray emission. Over the same period, high-resolution radio observations by the 43 GHz M87 Monitoring Team at the VLBA, a system of radio telescopes spanning the US, indicated a strong increase of the flux from the innermost core of M87 in the immediate vicinity of the central black hole. The combination of observations at the two extremes of the electromagnetic spectrum indicate that the site of the high-energy gamma emission, and hence the particle acceleration, in M87 must lie close to the black hole.