739 research outputs found
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 in the resulting shock ensemble can
explain the observable cosmic ray spectrum beyond the "knee" up to energies of
the order of eV. For the reaccelerated particles the Galactic Wind
termination shock acts as a reflecting boundary.Comment: LATEX, 14 pages, 7 figures, accepted to 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
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&
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
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 erg cms 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
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