263 research outputs found
TeV Gamma Rays Expected from Supernova Remnants in Different Uniform Interstellar Media
Calculations of the expected TeV -ray emission, produced by
accelerated cosmic rays (CRs) in nuclear collisions, from supernova remnants
evolving in a uniform interstellar medium (ISM) are presented. The aim is to
study the sensitivity of -ray production to a physical parameter set.
Apart from its general proportionality to N_H, it is shown that the
-ray production essentially depends upon the ratio of the CR diffusion
coefficient to a critical value , where B_0 and N_H are the magnetic
field and hydrogen number density of the ISM, and denotes the Bohm
diffusion coefficient. If is of the same order or lower than
, then the peak TeV -ray flux in the Sedov evolutionary
phase, normalized to a distance of 1 kpc, is about 10^{-10}(N_H/0.3 {cm}^{-3})
photons cm^{-2} s^{-1}. For a CR diffusion coefficient that is significantly
larger than , the CR cutoff energy is less than 10 TeV and the
expected -ray flux at 1 TeV is considerably below the presently
detectable level of 10^{-12} photons cm^{-2} s^{-1}. The same is of course true
for a SNR in the rarified, so-called hot ISM.Comment: 9 pages, 2 figures, to appear in Astroparticle Physic
Electrostatic Potentials in Supernova Remnant Shocks
Recent advances in the understanding of the properties of supernova remnant
shocks have been precipitated by the Chandra and XMM X-ray Observatories, and
the HESS Atmospheric Cerenkov Telescope in the TeV band. A critical problem for
this field is the understanding of the relative degree of dissipative
heating/energization of electrons and ions in the shock layer. This impacts the
interpretation of X-ray observations, and moreover influences the efficiency of
injection into the acceleration process, which in turn feeds back into the
thermal shock layer energetics and dynamics. This paper outlines the first
stages of our exploration of the role of charge separation potentials in
non-relativistic electron-ion shocks where the inertial gyro-scales are widely
disparate, using results from a Monte Carlo simulation. Charge density spatial
profiles were obtained in the linear regime, sampling the inertial scales for
both ions and electrons, for different magnetic field obliquities. These were
readily integrated to acquire electric field profiles in the absence of
self-consistent, spatial readjustments between the electrons and the ions. It
was found that while diffusion plays little role in modulating the linear field
structure in highly oblique and perpendicular shocks, in quasi-parallel shocks,
where charge separations induced by gyrations are small, and shock-layer
electric fields are predominantly generated on diffusive scales.Comment: 7 pages, 2 embedded figures, Accepted for publication in Astrophysics
and Space Science, as part of the HEDLA 2006 conference proceeding
Particle Acceleration in Cosmic Sites - Astrophysics Issues in our Understanding of Cosmic Rays
Laboratory experiments to explore plasma conditions and stimulated particle
acceleration can illuminate aspects of the cosmic particle acceleration
process. Here we discuss the cosmic-ray candidate source object variety, and
what has been learned about their particle-acceleration characteristics. We
identify open issues as discussed among astrophysicists. -- The cosmic ray
differential intensity spectrum is a rather smooth power-law spectrum, with two
kinks at the "knee" (~10^15 eV) and at the "ankle" (~3 10^18 eV). It is unclear
if these kinks are related to boundaries between different dominating sources,
or rather related to characteristics of cosmic-ray propagation. We believe that
Galactic sources dominate up to 10^17 eV or even above, and the extragalactic
origin of cosmic rays at highest energies merges rather smoothly with Galactic
contributions throughout the 10^15--10^18 eV range. Pulsars and supernova
remnants are among the prime candidates for Galactic cosmic-ray production,
while nuclei of active galaxies are considered best candidates to produce
ultrahigh-energy cosmic rays of extragalactic origin. Acceleration processes
are related to shocks from violent ejections of matter from energetic sources
such as supernova explosions or matter accretion onto black holes. Details of
such acceleration are difficult, as relativistic particles modify the structure
of the shock, and simple approximations or perturbation calculations are
unsatisfactory. This is where laboratory plasma experiments are expected to
contribute, to enlighten the non-linear processes which occur under such
conditions.Comment: accepted for publication in EPJD, topical issue on Fundamental
physics and ultra-high laser fields. From review talk at "Extreme Light
Infrastructure" workshop, Sep 2008. Version-2 May 2009: adjust some wordings
and references at EPJD proofs stag
Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. I: spectrum and chemical composition
In this paper we investigate the effect of stochasticity in the spatial and
temporal distribution of supernova remnants on the spectrum and chemical
composition of cosmic rays observed at Earth. The calculations are carried out
for different choices of the diffusion coefficient D(E) experienced by cosmic
rays during propagation in the Galaxy. In particular, at high energies we
assume that D(E)\sim E^{\delta}, with and being the
reference scenarios. The large scale distribution of supernova remnants in the
Galaxy is modeled following the distribution of pulsars, with and without
accounting for the spiral structure of the Galaxy. We find that the stochastic
fluctuations induced by the spatial and temporal distribution of supernovae,
together with the effect of spallation of nuclei, lead to mild but sensible
violations of the simple, leaky-box-inspired rule that the spectrum observed at
Earth is with , where
is the slope of the cosmic ray injection spectrum at the sources. Spallation of
nuclei, even with the small rates appropriate for He, may account for slight
differences in spectral slopes between different nuclei, providing a possible
explanation for the recent CREAM observations. For we find that
the slope of the proton and helium spectra are and
respectively at energies above 1 TeV (to be compared with the measured values
of and ). For the hardening of the He
spectra is not observed. We also comment on the effect of time dependence of
the escape of cosmic rays from supernova remnants, and of a possible clustering
of the sources in superbubbles. In a second paper we will discuss the
implications of these different scenarios for the anisotropy of cosmic rays.Comment: 28 pages, To appear in JCA
Topical Issues for Particle Acceleration Mechanisms in Astrophysical Shocks
Particle acceleration at plasma shocks appears to be ubiquitous in the
universe, spanning systems in the heliosphere, supernova remnants, and
relativistic jets in distant active galaxies and gamma-ray bursts. This review
addresses some of the key issues for shock acceleration theory that require
resolution in order to propel our understanding of particle energization in
astrophysical environments. These include magnetic field amplification in shock
ramps, the non-linear hydrodynamic interplay between thermal ions and their
extremely energetic counterparts possessing ultrarelativistic energies, and the
ability to inject and accelerate electrons in both non-relativistic and
relativistic shocks. Recent observational developments that impact these issues
are summarized. While these topics are currently being probed by
astrophysicists using numerical simulations, they are also ripe for
investigation in laboratory experiments, which potentially can provide valuable
insights into the physics of cosmic shocks.Comment: 13 pages, no figures. Invited review, accepted for publication in
Astrophysics and Space Science, as part of the HEDLA 2006 conference
proceeding
Magnetic fields in cosmic particle acceleration sources
We review here some magnetic phenomena in astrophysical particle accelerators
associated with collisionless shocks in supernova remnants, radio galaxies and
clusters of galaxies. A specific feature is that the accelerated particles can
play an important role in magnetic field evolution in the objects. We discuss a
number of CR-driven, magnetic field amplification processes that are likely to
operate when diffusive shock acceleration (DSA) becomes efficient and
nonlinear. The turbulent magnetic fields produced by these processes determine
the maximum energies of accelerated particles and result in specific features
in the observed photon radiation of the sources. Equally important, magnetic
field amplification by the CR currents and pressure anisotropies may affect the
shocked gas temperatures and compression, both in the shock precursor and in
the downstream flow, if the shock is an efficient CR accelerator. Strong
fluctuations of the magnetic field on scales above the radiation formation
length in the shock vicinity result in intermittent structures observable in
synchrotron emission images. Resonant and non-resonant CR streaming
instabilities in the shock precursor can generate mesoscale magnetic fields
with scale-sizes comparable to supernova remnants and even superbubbles. This
opens the possibility that magnetic fields in the earliest galaxies were
produced by the first generation Population III supernova remnants and by
clustered supernovae in star forming regions.Comment: 30 pages, Space Science Review
Evidence of Color Coherence Effects in W+jets Events from ppbar Collisions at sqrt(s) = 1.8 TeV
We report the results of a study of color coherence effects in ppbar
collisions based on data collected by the D0 detector during the 1994-1995 run
of the Fermilab Tevatron Collider, at a center of mass energy sqrt(s) = 1.8
TeV. Initial-to-final state color interference effects are studied by examining
particle distribution patterns in events with a W boson and at least one jet.
The data are compared to Monte Carlo simulations with different color coherence
implementations and to an analytic modified-leading-logarithm perturbative
calculation based on the local parton-hadron duality hypothesis.Comment: 13 pages, 6 figures. Submitted to Physics Letters
Search for single top quarks in the tau+jets channel using 4.8 fb of collision data
We present the first direct search for single top quark production using tau
leptons. The search is based on 4.8 fb of integrated luminosity
collected in collisions at =1.96 TeV with the D0 detector
at the Fermilab Tevatron Collider. We select events with a final state
including an isolated tau lepton, missing transverse energy, two or three jets,
one or two of them tagged. We use a multivariate technique to discriminate
signal from background. The number of events observed in data in this final
state is consistent with the signal plus background expectation. We set in the
tau+jets channel an upper limit on the single top quark cross section of
\TauLimObs pb at the 95% C.L. This measurement allows a gain of 4% in expected
sensitivity for the observation of single top production when combining it with
electron+jets and muon+jets channels already published by the D0 collaboration
with 2.3 fb of data. We measure a combined cross section of
\SuperCombineXSall pb, which is the most precise measurement to date.Comment: 12 pages, 5 figure
Measurement of Z/gamma*+jet+X angular distributions in ppbar collisions at sqrt{s}=1.96 TeV
We present the first measurements at a hadron collider of differential cross
sections for Z+jet+X production in delta phi(Z, jet), |delta y(Z, jet)| and
|y_boost(Z, jet)|. Vector boson production in association with jets is an
excellent probe of QCD and constitutes the main background to many small cross
section processes, such as associated Higgs production. These measurements are
crucial tests of the predictions of perturbative QCD and current event
generators, which have varied success in describing the data. Using these
measurements as inputs in tuning event generators will increase the
experimental sensitivity to rare signals.Comment: Published in Physics Letters B 682 (2010), pp. 370-380. 15 pages, 6
figure
Search for the standard model Higgs boson in tau final states
We present a search for the standard model Higgs boson using hadronically
decaying tau leptons, in 1 inverse femtobarn of data collected with the D0
detector at the Fermilab Tevatron ppbar collider. We select two final states:
tau plus missing transverse energy and b jets, and tau+ tau- plus jets. These
final states are sensitive to a combination of associated W/Z boson plus Higgs
boson, vector boson fusion and gluon-gluon fusion production processes. The
observed ratio of the combined limit on the Higgs production cross section at
the 95% C.L. to the standard model expectation is 29 for a Higgs boson mass of
115 GeV.Comment: publication versio
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