85 research outputs found
Nonthermal X-ray emission from young Supernova Remnants
The cosmic-ray spectrum up to the knee ( eV) is attributed to
acceleration processes taking place at the blastwaves which bound supernova
remnants. Theoretical predictions give a similar estimate for the maximum
energy which can be reached at supernova remnant shocks by particle
acceleration. Electrons with energies of the order eV should
give a nonthermal X-ray component in young supernova remnants. Recent
observations of SN1006 and G347.3-0.5 confirm this prediction. We present a
method which uses hydrodynamical simulations to describe the evolution of a
young remnant. These results are combined with an algorithm which
simultaneously calculates the associated particle acceleration. We use the test
particle approximation, which means that the back-reaction on the dynamics of
the remnant by the energetic particles is neglected. We present synchrotron
maps in the X-ray domain, and present spectra of the energies of the electrons
in the supernova remnant. Some of our results can be compared directly with
earlier semi-analytical work on this subject by Reynolds [1].Comment: 4 pages, 2 figures, contribution proceedings of poster presented at
the 11th Annual Astrophysics Conference in Maryland, to appear in Young
Supernova Remnants, ed. by S. S. Holt and U. Hwang (AIP
Particle Acceleration at Ultra-Relativistic Shocks and the Spectra of Relativistic Fireballs
We examine Fermi-type acceleration at relativistic shocks, and distinguish
between the initial boost of the first shock crossing cycle, where the energy
gain per particle can be very large, and the Fermi process proper with repeated
shock crossings, in which the typical energy gain is of order unity. We
calculate by means of numerical simulations the spectrum and angular
distribution of particles accelerated by this Fermi process, in particular in
the case where particle dynamics can be approximated as small-angle scattering.
We show that synchrotron emission from electrons or positrons accelerated by
this process can account remarkably well for the observed power-law spectra of
GRB afterglows and Crab-like supernova remnants. In the context of a
decelerating relativistic fireball, we calculate the maximum particle energy
attainable by acceleration at the external blast wave, and discuss the minimum
energy for this acceleration process and its consequences for the observed
spectrum.Comment: To appear in Proceedings of the 5th Huntsville Gamma-Ray Burst
Symposium. LaTeX, 6 pages, 2 figures, uses aipproc.sty and epsfi
An eigenfunction method for particle acceleration at ultra-relativistic shocks
We adapt and modify the eigenfunction method of computing the power-law
spectrum of particles accelerated at a relativistic shock front via the
first-order Fermi process (Kirk, J.G., Schneider, P., Astrophysical Journal
315, 425 (1987)) to apply to shocks of arbitrarily high Lorentz factor. The
power-law index of accelerated particles undergoing isotropic small-angle
scattering at an ultrarelativistic, unmagnetized shock is found to be s=4.23
+/- 0.2 (where s=d\ln f/ d\ln p, with f the Lorentz-invariant phase-space
density and p the momentum), in agreement with the results of Monte-Carlo
simulations. We present results for shocks in plasmas with different equations
of state and for Lorentz factors ranging from 5 to infinity.Comment: 4 pages, 2 figures, contribution to the Proceedings of the 5th
Huntsville GRB Symposiu
Gamma-ray burst observations with H.E.S.S
The High Energy Stereoscopic System (H.E.S.S.) consists of four Imaging
Atmospheric Cherenkov Telescopes (IACTs) in Namibia for the detection of cosmic
very-high-energy (VHE) gamma-rays. Gamma-ray bursts (GRBs) are among the
potential VHE gamma-ray sources. VHE gamma-emission from GRBs is predicted by
many GRB models. Because of its generally fast-fading nature in many wavebands,
the time evolution of any VHE gamma-radiation is still unknown. In order to
probe the largely unexplored VHE gamma-ray spectra of GRBs, a GRB observing
program has been set up by the H.E.S.S. collaboration. With the high
sensitivity of the H.E.S.S. array, VHE gamma-ray flux levels predicted by GRB
models are well within reach. We report the H.E.S.S. observations of and
results from some of the reported GRB positions during March 2003 - May 2006.Comment: Proceedings of the meeting "070228 - The Next Decade of GRB
Afterglows'' held March 19-23, 2007 in Amsterda
Associations of Very High Energy Gamma-Ray Sources Discovered by H.E.S.S. with Pulsar Wind Nebulae
The H.E.S.S. array of imaging Cherenkov telescopes has discovered a number of
previously unknown gamma-ray sources in the very high energy (VHE) domain above
100 GeV. The good angular resolution of H.E.S.S. (~0.1 degree per event), as
well as its sensitivity (a few percent of the Crab Nebula flux) and wide
5-degree field of view, allow a much better constrained search for counterparts
in comparison to previous instruments. In several cases, the association of the
VHE sources revealed by H.E.S.S. with pulsar wind nebulae (PWNe) is supported
by a combination of positional and morphological evidence, multi-wavelength
observations, and plausible PWN model parameters. These include the plerions in
the composite supernova remnants G 0.9+0.1 and MSH 15-52, the recently
discovered Vela X nebula, two new sources in the Kookaburra complex, and the
association of HESS J1825-137 with PSR B1823-13. The properties of these
better-established associations are reviewed. A number of other sources
discovered by H.E.S.S. are located near high spin-down power pulsars, but the
evidence for association is less complete. These possible associations are also
discussed, in the context of the available multi-wavelength data and plausible
PWN scenarios.Comment: 5 pages, to appear in Astrophysics and Space Science (proceedings of
"The Multi-Messenger Approach to High-Energy Gamma-Ray Sources"
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