Dynamics of thick discs around Schwarzschild-de Sitter black holes

We consider the effects of a cosmological constant on the dynamics of constant angular momentum discs orbiting Schwarzschild-de Sitter black holes. The motivation behind this study is to investigate whether the presence of a radial force contrasting the black hole's gravitational attraction can influence the occurrence of the runaway instability, a robust feature of the dynamics of constant angular momentum tori in Schwarzschild and Kerr spacetimes. In addition to the inner cusp near the black hole horizon through which matter can accrete onto the black hole, in fact, a positive cosmological constant introduces also an outer cusp through which matter can leave the torus without accreting onto the black hole. To assess the impact of this outflow on the development of the instability we have performed time-dependent and axisymmetric hydrodynamical simulations of equilibrium initial configurations in a sequence of background spacetimes of Schwarzschild-de Sitter black holes with increasing masses. The simulations have been performed with an unrealistic value for the cosmological constant which, however, yields sufficiently small discs to be resolved accurately on numerical grids and thus provides a first qualitative picture of the dynamics. The calculations, carried out for a wide range of initial conditions, show that the mass-loss from the outer cusp can have a considerable impact on the instability, with the latter being rapidly suppressed if the outflow is large enough.Comment: 12 pages; A&A, in pres

GENETIC CONTROL OF THE IMMUNE RESPONSE : FREQUENCY AND CHARACTERISTICS OF ANTIGEN-BINDING CELLS IN HIGH AND LOW RESPONDER MICE

The influence of immunization with (T,G)-A--L on the frequency and characteristics of [125I] (T,G)-A--L-binding cells (ABC) was investigated in high and low responder mice, whose ability to respond to (T,G)-A--L is under control of an H-2-linked immune response gene, Ir-1. Unimmunized high and low responder mice have about the same number of ABC in spleen and lymph nodes (6–12 ABC/104). However, after immunization with (T,G)-A--L in aqueous solution, ABC in high responders increase to a much greater extent than they do in low responders. By inhibition of ABC with class-specific anti-Ig sera, it was demonstrated that in nonimmune and primed mice antigen is bound to IgM receptors, which is in agreement with the exclusive production of 19S anti-(T,G)-A--L antibody in primed animals. In contrast, after secondary challenge with antigen, ABC in high and low responder mice have mainly IgG receptors, although under the conditions used for immunization, low responders are not able to produce detectable amounts of 7S anti-(T,G)-A--L antibody. From these results and from the evidence that low responders very probably have a T cell defect, it is suggested that the switchover from IgM to IgG precursor cells can be induced by antigen itself, without the action of specific T cells. Furthermore, the failure of marked proliferation of ABC in low responders after antigenic stimulation is explained by the lack of stimulation by specific T cells. By independent methods it has been shown that all ABC detected in this study are B cells. Preliminary experiments indicate that purified peripheral T cells bind antigen, but much less per cell than do B cells

Efficient extraction of a collimated ultra-cold neutron beam using diffusive channels

We present a first experimental demonstration of a new method to extract a well-collimated beam of ultra-cold neutrons (UCN) from a storage vessel. Neutrons with too large divergence are not removed from the beam by an absorbing collimation, but a diffuse or semidiffuse channel with high Fermi potential reflects them back into the vessel. This avoids unnecessary losses and keeps the storage time high, which may be beneficial when the vessel is part of a UCN source with long buildup time of a high UCN density

Capillary Filling of Anodized Alumina Nanopore Arrays

The filling behavior of a room temperature solvent, perfluoromethylcyclohexane, in approximately 20 nm nanoporous alumina membranes was investigated in situ with small angle x-ray scattering. Adsorption in the pores was controlled reversibly by varying the chemical potential between the sample and a liquid reservoir via a thermal offset, $\Delta$T. The system exhibited a pronounced hysteretic capillary filling transition as liquid was condensed into the nanopores. These results are compared with Kelvin-Cohan theory, with a modified Derjaguin approximation, as well as with predictions by Cole and Saam.Comment: 4 pages, 3 figures, pre-proof

Relations between concurrent hard X-ray sources in solar flares

Context: Solar flares release a large fraction of their energy into non-thermal electrons, but it is not clear where and how. Bremsstrahlung X-rays are observed from the corona and chromosphere. Aims: We aim to characterize the acceleration process by the coronal source and its leakage toward the footpoints in the chromosphere. The relations between the sources reflect the geometry and constrict the configuration of the flare. Methods: We studied solar flares of GOES class larger than M1 with three or more hard X-ray sources observed simultaneously in the course of the flare. The events were observed with the X-ray satellite RHESSI from February 2002 until July 2005. We used imaging spectroscopy methods to determine the spectral evolution of each source in each event. The images of all of the five events show two sources visible only at high energies (footpoints) and one source only visible at low energies (coronal or looptop source, in two cases situated over the limb). Results: We find soft-hard-soft behavior in both, coronal source and footpoints. The coronal source is nearly always softer than the footpoints. The footpoint spectra differ significantly only in one event out of five. Conclusions: The observations are consistent with acceleration in the coronal source and an intricate connection between the corona and chromosphere.Comment: accepted for publication in A&A, 11 pages, 9 figure

Do solar decimetric spikes originate in coronal X-ray sources?

In the standard solar flare scenario, a large number of particles are accelerated in the corona. Nonthermal electrons emit both X-rays and radio waves. Thus, correlated signatures of the acceleration process are predicted at both wavelengths, coinciding either close to the footpoints of a magnetic loop or near the coronal X-ray source. We attempt to study the spatial connection between coronal X-ray emission and decimetric radio spikes to determine the site and geometry of the acceleration process. The positions of radio-spike sources and coronal X-ray sources are determined and analyzed in a well-observed limb event. Radio spikes are identified in observations from the Phoenix-2 spectrometer. Data from the Nan\c{c}ay radioheliograph are used to determine the position of the radio spikes. RHESSI images in soft and hard X-ray wavelengths are used to determine the X-ray flare geometry. Those observations are complemented by images from GOES/SXI. We find that decimetric spikes do not originate from coronal X-ray flare sources contrary to previous expectations. However, the observations suggest a causal link between the coronal X-ray source, related to the major energy release site, and simultaneous activity in the higher corona.Comment: 4 pages, 3 figures, A&AL accepte