10,799 research outputs found
A Shotgun Model for Gamma Ray Bursts
We propose that gamma ray bursts (GRBs) are produced by a shower of heavy
blobs running into circumstellar material at highly relativistic speeds. The
gamma ray emission is produced in the shocks these bullets drive into the
surrounding medium. The short term variability seen in GRBs is set by the
slowing-down time of the bullets while the overall duration of the burst is set
by the lifetime of the central engine. A requirement of this model is that the
ambient medium be dense, consistent with a strong stellar wind. The efficiency
of the burst can be relatively high.Comment: 4 pages, 2 figures, revised version accepted by ApJ Letter
Modulation of neural cell membrane conductance by the herbal anxiolytic and antiepileptic drug aswal
To evaluate the effects of aswal on ionic fluxes and neuronal excitation, we performed extracellular and whole cell patch clamp recordings on CA1 pyramidal neurons of guinea pigs and Long-Evans rats. Aswal (100-250 mg/l) was administered systemically, and its effects on the rate of synchronized extracellular field potentials (EFP), membrane parameters, action potentials and postsynaptic potentials were recorded. The extracellular results obtained are consistent with calcium antagonistic properties. Intracellular recordings suggest that a direct sodium antagonistic effect as seen in many antiepileptic drugs plays no significant role. Further effects on ligand gated ion channels are discussed controversially. In summary, the cellular action of aswal appears heterogeneous with calcium antagonism playing a prominent role in counteracting excitation which may be a common feature in epilepsy and different psychiatric conditions as mood and anxiety disorder. Copyright (C) 2000 S. Karger AG, Basel
Hadron production from quark coalescence and jet fragmentation in intermediate energy collisions at RHIC
Transverse momentum spectra of pions, protons and antiprotons in Au+Au
collisions at intermediate RHIC energy of GeV are studied in
a model that includes both quark coalescence from the dense partonic matter and
fragmentation of the quenched perturbative minijet partons. The resulting
baryon to meson ratio at intermediate transverse momenta is predicted to be
larger than that seen in experiments at higher center of mass energies.Comment: 6 pages, 2 figures. Figures replaced to differentially address the
high-pT behavior of baryon versus antibaryon to meson ratio
Differential treatment of bipolar disorder with old and new antiepileptic drugs
Although lithium remains the preferred medication for bipolar disorders, new investigations suggest that only 60 to 80% of patients have a good response with a classical presentation. The antiepileptics carbamazepine and valproate are important alternatives. Several studies have shown that lithium, carbamazepine and valproate are effective in pure mania. Mixed mania and rapid cycling respond, however, well to valproate. One disadvantage of carbamazepine is its enzyme inducing property with the consequence of a decrease of plasma levels of other psychotropic medications and a worsening of psychopathology. First data indicate a good antimanic and antidepressive efficacy of the new antiepileptic drug lamotrigine
Chronic Progressive External Ophthalmoplegia Is Associated with a Novel Mutation in the Mitochondrial tRNA(Asn) Gene
Chronic progressive external ophthalmoplegia (CPEO) is caused by a decreased oxidative phosphorylation (OXPHOS) activity due to large-scale deletions of the mitochondrial genome in 50 % of the patients. The deletions encompass structural OXPHOS genes as well as tRNA genes, required for their expression so that the pathogenesis could be due to the deleted OXPHOS subunits or to an impaired mitochondrial translation. We have analyzed the mitochondrial genome of a patient presenting with CPEO for single base substitutions and discovered a novel heteroplasmic mutation in the tRNAAsn gene at position 5692 that converts a highly conserved adenine into a guanine. This mutation is unique because it is located at the transition of the anticodon loop to the anticodon stem and it leads to an additional base pair, thus reducing the number of loop-forming nucleotides from seven to five. Our findings suggest that CPEO can be caused by a single base substition in a mitochondrial tRNA gene so that the mitochondrial protein synthesis becomes the rate limiting step in OXPHOS fidelity
Fast Equilibration of Hadrons in an Expanding Fireball
Due to long chemical equilibration times within standard hadronic reactions
during the hadron gas phase in relativistic heavy ion collisions it has been
suggested that the hadrons are "born" into equilibrium after the quark gluon
plasma phase. Here we develop a dynamical scheme in which possible Hagedorn
states contribute to fast chemical equilibration times of baryon anti-baryon
pairs (as well as kaon anti-kaon pairs) inside a hadron gas and just below the
critical temperature. Within this scheme, we use master equations and derive
various analytical estimates for the chemical equilibration times. Applying a
Bjorken picture to the expanding fireball, the kaons and baryons as well as the
bath of pions and Hagedorn resonances can indeed quickly chemically equilibrate
for both an initial overpopulation or underpopulation of Hagedorn resonances.
Moreover, a comparison of our results to and
ratios at RHIC, indeed, shows a close match.Comment: 4 pages, 5 figure
High-j single-particle neutron states outside the N=82 core
The behaviour of the i13/2 and h9/2 single-neutron strength was studied with
the (4He,3He) reaction on 138Ba, 140Ce, 142Nd and 144Sm targets at a beam
energy of 51 MeV. The separation between the single-neutron states i13/2 and
h9/2 was measured in N =83 nuclei with changing proton number. To this end
spectroscopic factors for states populated in high-l transfer were extracted
from the data. Some mixing of l=5 and 6 strength was observed with states that
are formed by coupling the f7/2 state to the 2+ and 3- vibrational states and
the mixing matrix elements were found to be remarkably constant. The centroids
of the strength indicate a systematic change in the energies of the i13/2 and
h9/2 single-neutron states with increasing proton number that is in
quantitative agreement with the effects expected from the tensor interaction.Comment: 12 pages of text, 3 diagram
Rapidity Dependence of Strange Particle Ratios in Nuclear Collisions
It was recently found that in sulphur-induced nuclear collisions at 200 A GeV
the observed strange hadron abundances can be explained within a thermodynamic
model where baryons and mesons separately are in a state of relative chemical
equilibrium, with overall strangeness being slightly undersaturated, but
distributed among the strange hadron channels according to relative chemical
equilibrium with a vanishing strange quark chemical potential. We develop a
consistent thermodynamic formulation of the concept of relative chemical
equilibrium and show how to introduce into the partition function deviations
from absolute chemical equilibrium, e.~g.~an undersaturation of overall
strangeness or the breaking of chemical equilibrium between mesons and baryons.
We then proceed to test on the available data the hypothesis that the strange
quark chemical potential vanishes everywhere, and that the rapidity
distributions of all the observed hadrons can be explained in terms of one
common, rapidity-dependent function for the baryon chemical
potential only. The aim of this study is to shed light on the observed strong
rapidity dependence of the strange baryon ratios in the NA36 experiment.Comment: uses REVTeX, 14 pages, 17 ps-figures (uuencoded) added with figures
comman
X-ray and Radio Monitoring of GX 339-4 and Cyg X-1
Previous work by Motch et al. (1985) suggested that in the low/hard state of
GX339-4, the soft X-ray power-law extrapolated backward in energy agrees with
the IR flux level. Corbel and Fender (2002) later showed that the typical hard
state radio power-law extrapolated forward in energy meets the backward
extrapolated X-ray power-law at an IR spectral break, which was explicitly
observed twice in GX339-4. This has been cited as further evidence that jet
synchrotron radiation might make a significant contribution to the observed
X-rays in the hard state. We explore this hypothesis with a series of
simultaneous radio/X-ray hard state observations of GX339-4. We fit these
spectra with a simple, but remarkably successful, doubly broken power-law model
that indeed requires a spectral break in the IR. For most of these
observations, the break position as a function of X-ray flux agrees with the
jet model predictions. We then examine the radio flux/X-ray flux correlation in
Cyg X-1 through the use of 15 GHz radio data, obtained with the Ryle radio
telescope, and Rossi X-ray Timing Explorer data, from the All Sky Monitor and
pointed observations. We find evidence of `parallel tracks' in the radio/X-ray
correlation which are associated with `failed transitions' to, or the beginning
of a transition to, the soft state. We also find that for Cyg X-1 the radio
flux is more fundamentally correlated with the hard, rather than the soft,
X-ray flux.Comment: To Appear in the Proceedings of "From X-ray Binaries to Quasars:
Black Hole Accretion on All Mass Scales" (Amsterdam, July 2004). Eds. T
Maccarone, R. Fender, L. H
Metamagnetic phase transition of the antiferromagnetic Heisenberg icosahedron
The observation of hysteresis effects in single molecule magnets like
Mn-acetate has initiated ideas of future applications in storage
technology. The appearance of a hysteresis loop in such compounds is an outcome
of their magnetic anisotropy. In this Letter we report that magnetic hysteresis
occurs in a spin system without any anisotropy, specifically, where spins
mounted on the vertices of an icosahedron are coupled by antiferromagnetic
isotropic nearest-neighbor Heisenberg interaction giving rise to geometric
frustration. At T=0 this system undergoes a first order metamagnetic phase
transition at a critical field \Bcrit between two distinct families of ground
state configurations. The metastable phase of the system is characterized by a
temperature and field dependent survival probability distribution.Comment: 4 pages, 4 figures, submitted to Physical Review Letter
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