11,287 research outputs found
Simulations of particle acceleration beyond the classical synchrotron burnoff limit in magnetic reconnection: An explanation of the Crab flares
It is generally accepted that astrophysical sources cannot emit synchrotron
radiation above 160 MeV in their rest frame. This limit is given by the balance
between the accelerating electric force and the radiation reaction force acting
on the electrons. The discovery of synchrotron gamma-ray flares in the Crab
Nebula, well above this limit, challenges this classical picture of particle
acceleration. To overcome this limit, particles must accelerate in a region of
high electric field and low magnetic field. This is possible only with a
non-ideal magnetohydrodynamic process, like magnetic reconnection. We present
the first numerical evidence of particle acceleration beyond the synchrotron
burnoff limit, using a set of 2D particle-in-cell simulations of
ultra-relativistic pair plasma reconnection. We use a new code, Zeltron, that
includes self-consistently the radiation reaction force in the equation of
motion of the particles. We demonstrate that the most energetic particles move
back and forth across the reconnection layer, following relativistic Speiser
orbits. These particles then radiate >160 MeV synchrotron radiation rapidly,
within a fraction of a full gyration, after they exit the layer. Our analysis
shows that the high-energy synchrotron flux is highly variable in time because
of the strong anisotropy and inhomogeneity of the energetic particles. We
discover a robust positive correlation between the flux and the cut-off energy
of the emitted radiation, mimicking the effect of relativistic Doppler
amplification. A strong guide field quenches the emission of >160 MeV
synchrotron radiation. Our results are consistent with the observed properties
of the Crab flares, supporting the reconnection scenario.Comment: 15 pages, 16 figures, Accepted for publication in The Astrophysical
Journa
Suppression weakens unwanted memories via a sustained reduction of neural reactivation
Aversive events often turn into intrusive memories. However, prior evidence indicates that these memories can be forgotten via a mechanism of retrieval suppression. Here, we test the hypothesis that suppression weakens memories by deteriorating their neural representations. This deterioration, in turn, would hinder their subsequent reactivation and thus impoverish the vividness with which they can be recalled. In an fMRI study, participants repeatedly suppressed memories of aversive scenes. As predicted, this process rendered the memories less vivid. Using a pattern classifier, we observed that it did diminish the reactivation of scene information both globally across the grey matter and locally in the parahippocampal cortices. Moreover, in the right parahippocampal cortex, a stronger decline in vividness was associated with a greater reduction in generic reactivation of scene information and in the specific reinstatement of unique memory representations. These results support the hypothesis that suppression deteriorates memories by compromising their neural representations
Suppression weakens unwanted memories via a sustained reduction of neural reactivation
Aversive events sometimes turn into intrusive memories. However, prior evidence indicates that such memories can be controlled via a mechanism of retrieval suppression. Here, we test the hypothesis that suppression exerts a sustained influence on memories by deteriorating their neural representations. This deterioration, in turn, would hinder their subsequent reactivation and thus impoverish the vividness with which they can be recalled. In an fMRI study, participants repeatedly suppressed memories of aversive scenes. As predicted, this process rendered the memories less vivid. Using a pattern classifier, we observed that suppression diminished the neural reactivation of scene information both globally across the brain and locally in the parahippocampal cortices. Moreover, the decline in vividness was associated with reduced reinstatement of unique memory representations in right parahippocampal cortex. These results support the hypothesis that suppression weakens memories by causing a sustained reduction in the potential to reactivate their neural representations
Light Neutralinos and WIMP direct searches
The predictions of our previous analyses about possible low-mass (lower than
50 GeV) relic neutralinos are discussed in the light of the most recent results
from WIMP direct detection experiments. It is proved that these light
neutralinos are quite compatible with the new annual-modulation data of the
DAMA Collaboration; our theoretical predictions are also compared with the
upper bounds of the CDMS and EDELWEISS Collaborations.Comment: 4 pages, 1 figures, typeset with ReVTeX4. The paper may also be found
at http://www.to.infn.it/~fornengo/papers/note.ps.gz or through
http://www.to.infn.it/astropart/index.htm
Magnetic structure of the antiferromagnetic half-Heusler compound NdBiPt
We present results of single crystal neutron diffraction experiments on the
rare-earth, half-Heusler antiferromagnet (AFM) NdBiPt. This compound exhibits
an AFM phase transition at ~K with an ordered moment of
~ per Nd atom. The magnetic moments are aligned along
the -direction, arranged in a type-I AFM structure with ferromagnetic
planes, alternating antiferromagnetically along a propagation vector of
. The BiPt (= Ce-Lu) family of materials has been proposed as
candidates of a new family of antiferromagnetic topological insulators (AFTI)
with magnetic space group that corresponds to a type-II AFM structure where
ferromagnetic sheets are stacked along the space diagonal. The resolved
structure makes it unlikely, that NdBiPt qualifies as an AFTI.Comment: As resubmitted to PRB, corrected typos and changed symbols in Fig.
High density InAlAs/GaAlAs quantum dots for non-linear optics in microcavities
Structural and optical properties of InAlAs/GaAlAs quantum dots grown by molecular beam epitaxy are studied using transmission electron microscopy, temperature- and time resolvedphotoluminescence. The control of the recombination lifetime (50 ps – 1.25 ns), and of the dot density (5.10−8 – 2.1011 cm−3) strongly suggest that these material systems can find wide applications in opto-electronic devices as focusing non linear dispersive materials as well as fast saturable absorbers
Optimization of Single-Sided Charge-Sharing Strip Detectors
Simulation of the charge sharing properties of single-sided CZT strip detectors with small anode pads are presented. The effect of initial event size, carrier repulsion, diffusion, drift, trapping and detrapping are considered. These simulations indicate that such a detector with a 150 µm pitch will provide good charge sharing between neighboring pads. This is supported by a comparison of simulations and measurements for a similar detector with a coarser pitch of 225 µm that could not provide sufficient sharing. The performance of such a detector used as a gamma-ray imager is discussed
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