The active nuclei are compact regions, but extremely luminous, located in the heart of some galaxies. These objects are believed to be powered by a central supermassive black holes of several millions of solar masses. Some material is formed by stars and clouds of gas that spiral around the black hole while approaching it. A fraction of this matter eventually falls into the black hole, while another part escapes to the infall and is then brutally expelled. This outflow creates two jets of opposite directions, often larger than the whole host galaxy, and are compounded by particles traveling near the speed of light. AGN present very different properties depending on the viewing angle. The brightest active nuclei at VHE, least numerous, are seen with their jets pointing toward our line of sight. These AGN are called blazar, from a contraction of the words "BL Lac" and "quasar". Indeed, the prototype of these objects is BL Lacertae. The much more numerous active nuclei, for which the jets are viewed almost edge-on, are powerful radio emitters. Centaurus A is one of them.
Until now, 25 sources outside our Galaxy are known to emit VHE gamma-rays. The majority of them belongs to the blazar class, which jets are closely aligned to our line of sight. This particular orientation leads to a relativistic boosting of the emission we detect : the source seems brighter and the emitted photons appear at higher energies. The H.E.S.S. discovery of this radio galaxy confirms the possibility to detect VHE gamma-rays from extragalactic objects less luminous than blazars.
Centaurus A, which is located at 12 millions light-years, is one of the closest active galactic nuclei. Its jets cover a region of the sky larger than 100 times the size of the full moon, however they are not visible to the naked eye. Its proximity makes Centaurus A an ideal target to study the properties of active galactic nuclei : individual components, like the jet, can be spatially resolved in observations and have been particularly mapped in radio, optical, and X-rays. Centaurus A has also been discussed as a potential source for charged particles (cosmic rays) of the highest energies, which could be supported by recent observations made with the Pierre Auger Observatory. Even though a copious amount of data from active galactic nuclei exists, many of the basic processes, such as the particle acceleration near the black holes and the formation of the jets, are not yet well understood. Observations of AGNs using VHE gamma-rays yield crucial information on the acceleration of matter to the highest energies. The VHE gamma radiation lies at the most extreme part of the electromagnetic spectrum, far beyond the X-rays. Our atmosphere is opaque to those VHE gamma-rays : they get absorbed and create a short-lived shower of elementary particles. As these fast-moving particles rush through the atmosphere, they produce a faint flash of blue light, the Cherenkov radiation, which has been exploited in astronomy since only quite recently. The H.E.S.S. Experiment, an array of telescopes located in Namibia, captured these flashes from Centaurus A during more than 100 h between 2004 and 2008. The image of the cosmos as seen at VHE is becoming richer with the addition of a new type of extragalactic sources, the radiogalaxies. These observations suggest that the acceleration of particles to very high energies (above 100 GeV, i.e. more than 100 billion electron-Volts) could be a frequent phenomenon in active galactic nuclei and the environment of massive black holes.
Reference
- Astrophysical Journal Letters, sous presse HESS, Aharonian et al.
- L’expérience H.E.S.S.
Contact
- Jean-Philippe Lenain
- Catherine Boisson
- MarÃa Clementina Medina-
- Fabrice Mottez
- Hélène Sol
- Andreas Zech
– _ Observatoire de Paris, LUTH, CNRS