Astrophysics with High Energy Gamma Rays

Recent results, the present status and the perspectives of high energy gamma-ray astronomy are described. Since the satellite observations by the Compton Gamma Ray Observatory and its precursor missions have been reviewed extensively, emphasis is on the results from the ground-based gamma-ray telescopes. They concern the physics of Pulsar Nebulae, Supernova Remnants in their assumed role as the Galactic sources of Cosmic Rays, Jets from Active Galactic Nuclei, and the Extragalactic Background radiation field due to stars and dust in galaxies. Since the gamma-ray emission is nonthermal, this kind of astronomy deals with the pervasive high-energy nonequilibrium states in the Universe. The present build-up of larger and more sensitive instruments, both on the ground and in space, gives fascinating prospects also for observational cosmology and astroparticle physics. Through realistically possible further observational developments at high mountain altitudes a rapid extension of the field is to be expected.Comment: 23 pages, 11 figures. To appear in "Astronomy, Cosmology and Fundamental Physics", ed. P. A. Shaver, L. Di Lella, and A. Gimenez, Proc. ESA-CERN-ESO Symposium, Garching, March 2002. Springer-Verlag, Berlin, Heidelberg, series "ESO Astrophysics Symposia

Cosmic ray acceleration parameters from multi-wavelength observations. The case of SN 1006

The properties of the Galactic supernova remnant SN 1006 are theoretically reanalysed. Nonlinear kinetic theory is used to determine the acceleration efficiency of cosmic rays (CRs) in the supernova remnant SN 1006. The known range of astronomical parameters and the existing measurements of nonthermal emission are examined in order to define the values of the relevant physical parameters which determine the CR acceleration efficiency. It is shown that the parameter values -- proton injection rate, electron to proton ratio and downstream magnetic field strength -- are determined with the appropriate accuracy. In particular also the observed azimuthal variations in the gamma-ray morphology agree with the theoretical expectation. These parameter values, together with the reduction of the gamma-ray flux relative to a spherically symmetric acceleration geometry, allow a good fit to the existing data, including the recently detected TeV emission by H.E.S.S. SN 1006 represents the first example where a high efficiency of nuclear CR production, required for the Galactic CR sources, is consistently established.Comment: 10 pages, 6 figures, accepted for publication in A&

The Galactic Centre - A Laboratory for Starburst Galaxies (?)

The Galactic centre - as the closest galactic nucleus - holds both intrinsic interest and possibly represents a useful analogue to star-burst nuclei which we can observe with orders of magnitude finer detail than these external systems. The environmental conditions in the GC - here taken to mean the inner 200 pc in diameter of the Milky Way - are extreme with respect to those typically encountered in the Galactic disk. The energy densities of the various GC ISM components are typically ~two orders of magnitude larger than those found locally and the star-formation rate density ~three orders of magnitude larger. Unusually within the Galaxy, the Galactic centre exhibits hard-spectrum, diffuse TeV (=10^12 eV) gamma-ray emission spatially coincident with the region's molecular gas. Recently the nuclei of local star-burst galaxies NGC 253 and M82 have also been detected in gamma-rays of such energies. We have embarked on an extended campaign of modelling the broadband (radio continuum to TeV gamma-ray), non- thermal signals received from the inner 200 pc of the Galaxy. On the basis of this modelling we find that star-formation and associated supernova activity is the ultimate driver of the region's non-thermal activity. This activity drives a large-scale wind of hot plasma and cosmic rays out of the GC. The wind advects the locally-accelerated cosmic rays quickly, before they can lose much energy in situ or penetrate into the densest molecular gas cores where star-formation occurs. The cosmic rays can, however, heat/ionize the lower density/warm H2 phase enveloping the cores. On very large scales (~10 kpc) the non-thermal signature of the escaping GC cosmic rays has probably been detected recently as the spectacular 'Fermi bubbles' and corresponding 'WMAP haze'.Comment: Invited talk to appear in Proceedings of IAU Symposium No. 284, 2011 (R.J. Tuffs & C.C. Popescu, eds.) `The Spectral Energy Distribution of Galaxies

Cosmic Ray Acceleration by Spiral Shocks in the Galactic Wind

Cosmic ray acceleration by shocks related with Slipping Interaction Regions (SIRs) in the Galactic Wind is considered. SIRs are similar to Solar Wind Corotating Interaction Regions. The spiral structure of our Galaxy results in a strong nonuniformity of the Galactic Wind flow and in SIR formation at distances of 50 to 100 kpc. SIRs are not corotating with the gas and magnetic field because the angular velocity of the spiral pattern differs from that of the Galactic rotation. It is shown that the collective reacceleration of the cosmic ray particles with charge $Ze$ in the resulting shock ensemble can explain the observable cosmic ray spectrum beyond the "knee" up to energies of the order of $10^{17}Z$ eV. For the reaccelerated particles the Galactic Wind termination shock acts as a reflecting boundary.Comment: LATEX, 14 pages, 7 figures, accepted to A&

Cosmic-Ray Induced Diffuse Emissions from the Milky Way and Local Group Galaxies

Cosmic rays fill up the entire volume of galaxies, providing an important source of heating and ionisation of the interstellar medium, and may play a significant role in the regulation of star formation and galactic evolution. Diffuse emissions from radio to high-energy gamma rays (> 100 MeV) arising from various interactions between cosmic rays and the interstellar medium, interstellar radiation field, and magnetic field, are currently the best way to trace the intensities and spectra of cosmic rays in the Milky Way and other galaxies. In this contribution, I describe our recent work to model the full spectral energy distribution of galaxies like the Milky Way from radio to gamma-ray energies. The application to other galaxies, in particular the Magellanic Clouds and M31 that are detected in high-energy gamma-rays by the Fermi-LAT, is also discussed.Comment: Contribution to "The Spectral Energy Distribution of Galaxies" Proceedings IAU Symposium No. 284, 2011, eds. R.J. Tuffs & C.C.Popescu. 4 pages with 4 figure

Magnetic Field Amplification in Tycho and other Shell-type Supernova Remnants

It is shown that amplification of the magnetic field in supernova remnants (SNRs) occurs in all six objects where morphological measurements are presently available in the hard X-ray continuum at several keV. For the three archetypical objects (SN 1006, Cas A and Tycho's SNR) to which nonlinear time-dependent acceleration theory has been successfully applied up to now, the global theoretical and the local observational field strengths agree very well, suggesting in addition that all young SNRs exhibit the amplification effect as a result of very efficient acceleration of nuclear cosmic rays (CRs) at the outer shock. Since this appears to be empirically the case, we may reverse the argument and consider field amplification as a measure of nuclear CR acceleration and it has indeed been argued that acceleration in the amplified fields allows the CR spectrum from SNRs to reach the knee in the spectrum or, in special objects, even beyond. The above results are furthermore used to investigate the time evolution of field amplification in young SNRs. Although the uncertainties in the data do not allow precise conclusions regarding this point, they rather clearly show that the ratio of the magnetic field energy density and the kinetic energy density of gas flow into the shock is of the order of a few percent if the shock speed is high enough V_s > 10^3 km/s, and this ratio remains nearly constant during the SNR evolution. The escape of the highest energy nuclear particles from their sources becomes progressively important with age, reducing also the cutoff in the \pi^0 -decay gamma-ray emission spectrum with time after the end of the sweep-up phase. Simultaneously the leptonic gamma-ray channels will gain in relative importance with increasing age of the sources.Comment: 13 pages, 8 figures, accepted for publication in A&