Results 1 - 10 of 3346
Results 1 - 10 of 3346. Search took: 0.029 seconds
|Sort by: date | relevance|
[en] We consider a way of defining quantum Hamiltonians involving particle creation and annihilation based on an interior-boundary condition (IBC) on the wave function, where the wave function is the particle-position representation of a vector in Fock space, and the IBC relates (essentially) the values of the wave function at any two configurations that differ only by the creation of a particle. Here we prove, for a model of particle creation at one or more point sources using the Laplace operator as the free Hamiltonian, that a Hamiltonian can indeed be rigorously defined in this way without the need for any ultraviolet regularization, and that it is self-adjoint. We prove further that introducing an ultraviolet cut-off (thus smearing out particles over a positive radius) and applying a certain known renormalization procedure (taking the limit of removing the cut-off while subtracting a constant that tends to infinity) yields, up to addition of a finite constant, the Hamiltonian defined by the IBC.
[en] We begin by giving correct expressions for the short-time action following the work Makri–Miller. We use these estimates to derive an accurate expression modulo Δt2 for the quantum propagator and we show that the quantum potential is negligible modulo Δt2 for a point source, thus justifying an unfortunately largely ignored observation of Holland made twenty years ago. We finally prove that this implies that the quantum motion is classical for very short times.
[en] In this Letter, we introduce a new method of image stacking to directly study the undetected but possible γ-ray point sources. Applying the method to the Australia Telescope 20 GHz Survey sources which have not been detected by Large Area Telescope on the Fermi Gamma-ray Space Telescope, we find that the sources contribute (10.5 ± 1.1)% and (4.3 ± 0.9)% of the extragalactic gamma-ray background (EGB) and have a very soft spectrum with the photon indices of 3.09 ± 0.23 and 2.61 ± 0.26 in the 1-3 GeV and 3-300 GeV energy ranges, respectively. In the 0.1-1 GeV range, they probably contribute a larger fraction to the EGB, but it is not quite certain. It may not be appropriate to assume that the undetected sources have similar properties to the detected sources.
[en] We have recently completed a large-area, coded-aperture, gamma-ray imager for use in searching for radiation sources. The instrument was constructed to verify that weak point sources can be detected at considerable distances if one uses imaging to overcome fluctuations in the natural background. The instrument uses a rank-19, one-dimensional coded aperture to cast shadow patterns onto a 0.57 m2 NaI(Tl) detector composed of 57 individual cubes each 10 cm on a side. These are arranged in a 19 x 3 array. The mask is composed of four-centimeter thick, one-meter high, 10-cm wide lead blocks. The instrument is mounted in the back of a small truck from which images are obtained as one drives through a region. Results of first measurements obtained with the system are presented
[en] We study the cosmological consequences of codecaying dark matter - a recently proposed mechanism for depleting the density of dark matter through the decay of nearly degenerate particles. A generic prediction of this framework is an early dark matter dominated phase in the history of the Universe, that results in the enhanced growth of dark matter perturbations on small scales. We compute the duration of the early matter dominated phase and show that the perturbations are robust against washout from free streaming. The enhanced small-scale structure is expected to survive today in the form of compact microhalos and can lead to significant boost factors for indirect-detection experiments, such as FERMI, where dark matter would appear as point sources. Authors:
[en] We investigate the methods to optimize the directivity of point source array by using pseudostochastic sequences. Maximum-length sequence (MLS) array and Quadratic-residue sequence (QRS) array are theoretically analysed, and their simulated responses are shown. The results indicate that pseudostochastic sequences can be used to optimize the directivity of point source array
[en] To calibrate pinhole MicroSPECT system, two improved methods using only a single point source were proposed. The uniqueness of calibration result by the proposed methods was proved theoretically. And simulation calibration of a dual-head system was carried out to validate the performance of the proposed methods. The simulation results show that reasonable calibration error is given by both proposed methods.
[en] A group of barchans, crescent sand dunes, exhibit a characteristic flying-geese pattern in deserts on Earth and Mars. This pattern implies that an indirect interaction between barchans, mediated by an inter-dune sand stream, which is released from one barchan's horns and caught by another barchan, plays an important role in the dynamics of barchan fields. We used numerical simulations of a recently proposed cell model to investigate the effects of inter-dune sand streams on barchan fields. We found that a sand stream from a point source moves a downstream barchan laterally until the head of the barchan is finally situated behind the stream. This final configuration was shown to be stable by a linear stability analysis. These results indicate that flying-geese patterns are formed by the lateral motion of barchans mediated by inter-dune sand streams. By using simulations we also found a barchan mono-corridor generation effect, which is another effect of sand streams from point sources.
[en] I present a catalog of point-source objects toward NGC 1333, resolving a wide variety of confusion about source names (and occasionally positions) in the literature. I incorporate data from optical to radio wavelengths, but focus most of the effort on being complete and accurate from J (1.25 μm) to 24 μm.