3,353 research outputs found
A wind model for high energy pulses
A solution to the sigma problem - that of finding a mechanism capable of
converting Poynting energy flux to particle-borne energy flux in a pulsar wind
- was proposed several years ago by Coroniti and Michel who considered a
particular prescription for magnetic reconnection in a striped wind. This
prescription was later shown to be ineffective. In this paper, we discuss the
basic microphysics of the reconnection process and conclude that a more rapid
prescription is permissible. Assuming dissipation to set in at some distance
outside the light-cylinder, we compute the resulting radiation signature and
find that the synchrotron emission of heated particles appears periodic, in
general showing both a pulse and an interpulse. The predicted spacing of these
agrees well with observation in the case of the Crab and Vela pulsars. Using
parameters appropriate for the Crab pulsar - magnetization parameter at the
light cylinder sigma_L = 6 x 10^4, Lorentz factor Gamma=250 - reasonable
agreement is found with the observed total pulsed luminosity. This suggest that
the high-energy pulses from young pulsars originate not in the co-rotating
magnetosphere within the light cylinder (as in all other models) but from the
radially directed wind well outside it.Comment: 6 pages, 2 figures. To appear in the Proceedings of the 270.
WE-Heraeus Seminar on Neutron Stars, Pulsars and Supernova Remnants, Jan.
21-25, 2002, Physikzentrum Bad Honnef, eds W. Becker, H. Lesch & J. Truemper.
Proceedings are available as MPE-Report 27
The Monoceros very-high-energy gamma-ray source
The H.E.S.S. telescope array has observed the complex Monoceros Loop
SNR/Rosette Nebula region which contains unidentified high energy EGRET sources
and potential very-high-energy (VHE) gamma-ray source. We announce the
discovery of a new point-like VHE gamma-ray sources, HESS J0632+057. It is
located close to the rim of the Monoceros SNR and has no clear counterpart at
other wavelengths. Data from the NANTEN telescope have been used to investigate
hadronic interactions with nearby molecular clouds. We found no evidence for a
clear association. The VHE gamma-ray emission is possibly associated with the
lower energy gamma-ray source 3EG J0634+0521, a weak X-ray source 1RXS
J063258.3+054857 and the Be-star MWC 148.Comment: 4 pages, 4 figures, Contribution to the 30th ICRC, Merida Mexico,
July 200
Sets Represented as the Length-n Factors of a Word
In this paper we consider the following problems: how many different subsets
of Sigma^n can occur as set of all length-n factors of a finite word? If a
subset is representable, how long a word do we need to represent it? How many
such subsets are represented by words of length t? For the first problem, we
give upper and lower bounds of the form alpha^(2^n) in the binary case. For the
second problem, we give a weak upper bound and some experimental data. For the
third problem, we give a closed-form formula in the case where n <= t < 2n.
Algorithmic variants of these problems have previously been studied under the
name "shortest common superstring"
Observational constraints on energetic particle diffusion in young SNRs: amplified magnetic field and maximum energy
Constraints on the diffusion and acceleration parameters in five young
supernova remnants (SNRs) are derived from the observed thickness of their
X-ray rims, as limited by the synchrotron losses of the highest energy
electrons, assuming uniform and isotropic turbulence. It is shown that the
magnetic field must be amplified up to values between 250 and 500 microGauss in
the case of Cas A, Kepler, and Tycho, or ~ 100 microGauss in the case of SN
1006 and G347.3-0.5. The diffusion coefficient at the highest electron energy
can also be derived from the data, by relating the X-ray energy cutoff to the
acceleration timescale. Values typically between 1 and 10 times the Bohm
diffusion coefficient are found to be required. We also find interesting
constraints on the energy dependence of the diffusion coefficient. This favours
diffusion regimes between the Kraichnan and the Bohm regime, and rejects
turbulence spectrum indices larger than ~ 3/2. Finally, the maximum energy of
the accelerated particles is found to lay between 10^13 and 5 10^13 eV for
electrons, and around Z \times 8 10^14 eV at most for nuclei (or ~ 2.5 times
less if a Bohm diffusion regime is assumed), roughly independently of the
compression ratio assumed at the shock. Even by taking advantage of the
uncertainties on the measured parameters, it appears very difficult for the
considered SNRs in their current stage of evolution to produce protons up to
the knee of the cosmic-ray spectrum, at ~ 3 10^15 eV, and essentially
impossible to accelerate Fe nuclei up to either the ankle at ~ 3 10^18 eV or
the second knee at ~ 5 10^17 eV.Comment: 11 pages, 3 figures, accepted for publication in A&
Microstructure of Mealy and Vitreous Wheat Endosperms (Triticum durum L.) with Special Emphasis on Location and Polymorphic Behaviour of Lipids
Dark inclusions observed in osmiophilic zones, already described in mature wheat endosperm using transmission electron microscopy, were confirmed by freeze-fracture electron microscopy to be lipids. The polar lipids (glycolipids, phospholipids, free fatty acides, monoglycerides) were organized in a liquid crystalline phase. The reversed hexagonal or HII phase should be the main lattice which might arise from the transition of lipids present in membranous structures as a lamellar phase. This transition was caused by dehydration occurring during maturation. It is suggested that the water-dependent lamellar hexagonal phase transitions are of considerable importance in cereal food technology
Functional Subdomains within Human FFA
Cataloged from PDF version of article.The fusiform face area (FFA) is a well-studied human brain region that shows strong activation for faces. In functional MRI studies, FFA is often assumed to be a homogeneous collection of voxels with similar visual tuning. To test this assumption, we used natural movies and a quantitative voxelwise modeling and decoding framework to estimate category tuning profiles for individual voxels within FFA. We find that the responses in most FFA voxels are strongly enhanced by faces, as reported in previous studies. However, we also find that responses of individual voxels are selectively enhanced or suppressed by a wide variety of other categories and that these broader tuning profiles differ across FFA voxels. Cluster analysis of category tuning profiles across voxels reveals three spatially segregated functional subdomains within FFA. These subdomains differ primarily in their responses for nonface categories, such as animals, vehicles, and communication verbs. Furthermore, this segregation does not depend on the statistical threshold used to define FFA from responses to functional localizers. These results suggest that voxels within FFA represent more diverse information about object and action categories than generally assumed. © 2013 the authors
Particle acceleration by ultra-relativistic shocks: theory and simulations
We consider the acceleration of charged particles near ultra-relativistic
shocks, with Lorentz factor Gamma_s >> 1. We present simulations of the
acceleration process and compare these with results from semi-analytical
calculations. We show that the spectrum that results from acceleration near
ultra-relativistic shocks is a power law, N(E) \propto E^{-s}, with a nearly
universal value s \approx 2.2 - 2.3 for the slope of this power law.
We confirm that the ultra-relativistic equivalent of Fermi acceleration at a
shock differs from its non-relativistic counterpart by the occurence of large
anisotropies in the distribution of the accelerated particles near the shock.
In the rest frame of the upstream fluid, particles can only outrun the shock
when their direction of motion lies within a small loss cone of opening angle
theta_c \approx 1/Gamma_s around the shock normal.
We also show that all physically plausible deflection or scattering
mechanisms can change the upstream flight direction of relativistic particles
originating from downstream by only a small amount: Delta theta ~ 1/Gamma_s.
This limits the energy change per shock crossing cycle to Delta E ~ E, except
for the first cycle where particles originate upstream. In that case the
upstream energy is boosted by a factor ~ Gamma_s^2 for those particles that are
scattered back across the shock into the upstream region.Comment: Accepted by MNRAS, 39 pages, 10 figures, uses latex macro mn.st
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