Hypernovae and light dark matter as possible Galactic positron sources

The electron-positron annihilation source in the Galactic center region has recently been observed with INTEGRAL/SPI, which shows that this 511 keV source is strong and its extension is consistent with the Galactic bulge geometry. The positron production rate, estimated to more than 10$^{43}$ per second, is very high and raises a challenging question about the nature of the Galactic positron source. Commonly considered astrophysical positron injectors, namely type Ia supernovae are rare events and fall short to explain the observed positron production rate. In this paper, we study the possibility of Galactic positron production by hypernovae events, exemplified by the recently observed SN2003dh/GRB030329, an asymmetric explosion of a Wolf-Rayet star associated with a gamma-ray burst. In these kinds of events, the ejected material becomes quickly transparent to positrons, which spread out in the interstellar medium. Non radioactive processes, such as decays of heavy dark matter particles (neutralinos) predicted by most extensions of the standard model of particle physics, could also produce positrons as byproducts. However they are expected to be accompanied by a large flux of high-energy gamma-rays, which were not observed by EGRET and ground based Tcherenkov experiments. In this context we explore the possibility of direct positron production by annihilation of light dark matter particles.Comment: 8 pages, 0 figures, 35th COSPAR, accepted in July 2005 by Elsevier Science for publication in "Advances in Space Research

Integral and Light Dark Matter

The nature of Dark Matter remains one of the outstanding questions of modern astrophysics. The success of the Cold Dark Matter cosmological model argues strongly in favor of a major component of the dark matter being in the form of elementary particles, not yet discovered. Based on earlier theoretical considerations, a possible link between the recent SPI/INTEGRAL measurement of an intense and extended emission of 511 keV photons (positron annihilation) from the central Galaxy, and this mysterious component of the Universe, has been established advocating the existence of a light dark matter particle at variance with the neutralino, in general considered as very heavy. We show that it can explain the 511 keV emission mapped with SPI/INTEGRAL without overproducing undesirable signals like high energy gamma-rays arising from $\pi^\circ$ decays, and radio synchrotron photons emitted by high energy positrons circulating in magnetic fields. Combining the annihilation line constraint with the cosmological one (i.e. that the relic LDM energy density reaches about 23% of the density of the Universe), one can restrict the main properties of the light dark matter particle. Its mass should lie between 1 and 100 MeV, and the required annihilation cross section, velocity dependent, should be significantly larger than for weak interactions, and may be induced by the virtual production of a new light neutral spin 1 boson $U$. On astrophysical grounds, the best target to validate the LDM proposal seems to be the observation by SPI/INTEGRAL and future gamma ray telescopes of the annihilation line from the Sagittarius dwarf galaxy and the Palomar-13 globular cluster, thought to be dominated by dark matter.Comment: 7 pages, 0 figures. To appear in the Proceedings of the 5th INTEGRAL Workshop: "The INTEGRAL Universe", February 16-20, 2004, Munich, German

SVOM pointing strategy: how to optimize the redshift measurements?

The Sino-French SVOM mission (Space-based multi-band astronomical Variable Objects Monitor) has been designed to detect all known types of gamma-ray bursts (GRBs) and to provide fast and reliable GRB positions. In this study we present the SVOM pointing strategy which should ensure the largest number of localized bursts allowing a redshift measurement. The redshift measurement can only be performed by large telescopes located on Earth. The best scientific return will be achieved if we are able to combine constraints from both space segment (platform and payload) and ground telescopes (visibility).Comment: Proceedings of Gamma-Ray Bursts 2007 conference, Santa Fe, USA, 5-9 November 2007. Published in AIP conf. proc. 1000, 585-588 (2008

Structure prediction based on ab initio simulated annealing for boron nitride

Possible crystalline modifications of chemical compounds at low temperatures correspond to local minima of the energy landscape. Determining these minima via simulated annealing is one method for the prediction of crystal structures, where the number of atoms per unit cell is the only information used. It is demonstrated that this method can be applied to covalent systems, at the example of boron nitride, using ab initio energies in all stages of the optimization, i.e. both during the global search and the subsequent local optimization. Ten low lying structure candidates are presented, including both layered structures and 3d-network structures such as the wurtzite and zinc blende types, as well as a structure corresponding to the beta-BeO type

A continuous non-linear shadowing model of columnar growth

We propose the first continuous model with long range screening (shadowing) that described columnar growth in one space dimension, as observed in plasma sputter deposition. It is based on a new continuous partial derivative equation with non-linear diffusion and where the shadowing effects apply on all the different processes.Comment: Fast Track Communicatio

Anisotropic Dirac fermions in a Bi square net of SrMnBi2

We report the highly anisotropic Dirac fermions in a Bi square net of SrMnBi2, based on a first principle calculation, angle resolved photoemission spectroscopy, and quantum oscillations for high-quality single crystals. We found that the Dirac dispersion is generally induced in the (SrBi)+ layer containing a double-sized Bi square net. In contrast to the commonly observed isotropic Dirac cone, the Dirac cone in SrMnBi2 is highly anisotropic with a large momentum-dependent disparity of Fermi velocities of ~ 8. These findings demonstrate that a Bi square net, a common building block of various layered pnictides, provide a new platform that hosts highly anisotropic Dirac fermions.Comment: 5 pages, 4 figure

Te Hononga—Modelling indigenous collaborative enterprise. A research report on Māori enterprise collaboration in Aotearoa New Zealand

This study explores the theory and practice of Māori enterprise collaboration. There exists a strong rationale for Māori enterprise collaboration as it builds on the relationality of a Māori world view, shared values and existing whakapapa (genealogical) relationships. Collaboration is considered integral to Māori development because it is set against a background of self-determination and self-governance. Waiū Dairy and MiHI (Movers in Hemp Innovation) are two Māori enterprise collaborations that have been facilitated by Poutama Trust and are at distinctly different stages of maturity. Interviews were conducted with participants from Waiū Dairy and MiHI to gain insights from those involved in the practice of Māori enterprise collaboration.falsePalmerston North, New ZealandNgā Pae o te Māramatang