Galactic Center at Very High Energies Essay -- The Universe, Black Hol

Every time a new ‘‘messenger† (different photon wavelengths or a different particle) has added to the list of observables accessible to astrophysicists, the Universe has appeared under a new light: it has revealed surprising features and triggered new questions, ultimately changing our understanding of fundamental physics and cosmology.Examples include the new elementary particles discovered in cosmic rays in the ‘30s and ‘40s, flavor oscillations from the solar and atmospheric neutrinos, or the revolutions brought by radio or X-ray astronomy. The last decade, a new branch of astronomy was born: high energy and very high energy gamma-ray astronomy. Especially, 2OO4 was a very importand year for the gamma-rays astronomy. Firstly it was the year that marked the 30th aniversary of the discovery of the compact radio source Sgr A* (Balick and Brown 1974) which is now strongly believed to be the revelation of a supermassive black hole of a mass of (3 imes 10^{6} M odot ) that seats in the rotanional center of the Galaxy, according to the measurments of star motions near the Galactic Center (GC). Moreover it was the year that the first detection of gamma-rays from a compact region of size (sim 10') around Sgr A* with the INTEGRAL ( extit{International Gamma-Ray Astrophysics Laboratory } ) observatory in the energy rage from 20 to 100 keV (Belanger et al 2004) and with the HESS (High Energy Stereoscopic System) Cerenkov telescope array between 165 and 10 TeV (Aharonian et al 2004) took place. The detection of a high energy radiation source that appears to be pointlike and coincident with the Galactic Nucleus seems to be the reword of 30 years of observations. The GC is now observed also by the Fermi space observatory. When J.Co... ...i.e. within (sim 100 ) Schwarzchild radii of the black hole). This fact must be explained by any model for the TeV gamma-rays and it seems to support the scenario where the gamma-rays are assosiated with electrons accelarated by the pulsar wind nebula. However, protons may be accelarated close to the black hole, but be converted to gamma-rays only after travelling a significant distance away from the accelaration region (e.g. Atoyan n Dermer 2004; Aharonian n Neronov 2005; Ballantyne et al. 2007a). In the scenario presented by Ballantyne et al. (2007a), proton accelaration was assumed to occur at distances only (sim 20-30 ) Scwarzchild radii from the black hole (e.g. Liu et al. 2006). The particles would then diffuse away from the Sgr A* through the magnetized turbulent ISM ? , until possibly colliding with the dense molecular gas in the circumnuclear disk.