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&

Cosmic-ray driven winds

The theory of Galactic Winds, driven by the cosmic-ray pressure gradient, is reviewed both on the magnetohydrodynamic and on the kinetic level. In this picture the magnetic field of the Galaxy above the dense gas disk is assumed to have a flux tube geometry, the flux tubes rising locally perpendicular out of the disk to become radially directed at large distances, with the cosmic-ray sources located deep within the Galactic disk. At least above the gas disk, the magnetic fluctuations which resonantly scatter the cosmic rays are selfconsistently excited as Alf{`e}n waves by the escaping cosmic rays. The fluctuation amplitudes remain finite through nonlinear wave dissipation. The spatially increasing speed of the resulting outflow results in a diffusion-convection boundary whose position depends on particle momentum. It replaces the escape boundary of static diffusion models. New effects like overall Galactic mass and angular momentum loss as well as gas heating beyond the disk appear. Also particle re-acceleration in the distant wind halo suggests itself. The resulting magnetohydrodynamic flow properties and the cosmic-ray transport properties are compared with observations. On the whole they show remarkable agreement. General limitations and generalisations of the basic model arise due to the expected simultaneous infall of matter from the environment of the Galaxy. On an intergalactic scale the combined winds from the Local Group galaxies should form a ``Local Group Bubble``. Its properties remain to be studied in detail.Comment: 6 pages, 3 figures; H. Voelk, "Cosmic ray driven winds ", in proceedings of "Cosmic Rays and the InterStellar Medium" PoS(CRISM2014)02

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&

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&

Critical self-organization of astrophysical shocks

There are two distinct regimes of the first order Fermi acceleration at shocks. The first is a linear (test particle) regime in which most of the shock energy goes into thermal and bulk motion of the plasma. The second is an efficient regime when it goes into accelerated particles. Although the transition region between them is narrow, we identify the factors that drive the system to a {\it self-organized critical state} between those two. Using an analytic solution, we determine this critical state and calculate the spectra and maximum energy of accelerated particles.Comment: To appear in ApJL, Sec.3 extensively rewritten, 4 pages, Latex, emulateapj.sty, eps

Expected gamma-ray emission of supernova remnant SN 1987A

A nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants is employed to re-examine the nonthermal properties of the remnant of SN 1987A for an extended evolutionary period of 5--100 yr. It is shown that an efficient production of nuclear CRs leads to a strong modification of the outer supernova remnant shock and to a large downstream magnetic field $B_\mathrm{d}\approx 20$ mG. The shock modification and the strong field are required to yield the steep radio emission spectrum observed, as well as to considerable synchrotron cooling of high energy electrons which diminishes their X-ray synchrotron flux. These features are also consistent with the existing X-ray observations. The expected \gr energy flux at TeV-energies at the current epoch is nearly $\epsilon_{\gamma}F_{\gamma}\approx 4\times 10^{-13}$ erg cm$^2$s$^{-1}$ under reasonable assumptions about the overall magnetic field topology and the turbulent perturbations of this field. The general nonthermal strength of the source is expected to increase roughly by a factor of two over the next 15 to 20 yrs; thereafter it should decrease with time in a secular form.Comment: 7 pages, 5 figures, accepted for publication in ApJ, a number of changes have been made, even though these are not changing the main results of the pape