Tagging single muons and other long-flying relativistic charged particles by ultra-fast timing in air Cherenkov telescopes

Atmospheric air Cherenkov telescopes are successfully used for ground-based, very high-energy (VHE) gamma ray astronomy. Triggers from the so-called single muon and other long-flying relativistic charged particle events are an unwanted background for the Cherenkov telescope. Because of low rate at TeV energies the muon background is unimportant. It is much more intense for telescopes with high photon sensitivity and low energy threshold. Below a few hundred GeV energy, the so-called muon background becomes so intense, that it can deteriorate the sensitivity of telescopes (the so-called muon-wall problem). From general considerations it can be anticipated that the signature of these particles should be a light pulse with a narrow time structure. In fact, simulations show that the pulses from muons have a very narrow time profile that is well below the time resolutions of nearly all currently operating telescopes. In this report we elaborate on the time profile of Cherenkov light from the so-called single muons and show that a telescope with ultra-fast time response can open a new dimension allowing one to tag and to reject those events.Comment: Accepted by Astroparticle Physic

Geosynchronous inclined orbits for high-latitude communications

We present and discuss a solution to the growing demand for satellite telecommunication coverage in the high-latitude geographical regions (beyond 55◦N), where the signal from geostationary satellites is limited or unavailable. We focus on the dynamical issues associated to the design, the coverage, the maintenance and the disposal of a set of orbits selected for the purpose. Specifically, we identify a group of highly inclined, moderately eccentric geosynchronous orbits derived from the Tundra orbit (geosynchronous, eccentric and critically inclined). Continuous coverage can be guaranteed by a constellation of three satellites in equally spaced planes and suitably phased. By means of a highprecision model of the terrestrial gravity field and the relevant environmental perturbations, we study the evolution of these orbits. The effects of the different perturbations on the ground track (which is more important for coverage than the orbital elements themselves) are isolated and analyzed. The physical model and the numerical setup are optimized with respect to computing time and accuracy. We show that, in order to maintain the ground track unchanged, the key parameters are the orbital period and the argument of perigee. Furthermore, corrections to the right ascension of the ascending node are needed in order to preserve the relative orientation of the orbital planes. A station-keeping strategy that minimizes propellant consumption is then devised, and comparisons are made between the cost of a solution based on impulsive maneuvers and one with continuous thrust. Finally, the issue of end-of-life disposal is discussed

Vorticity Budget of Weak Thermal Convection in Keplerian disks

By employing the equations of mean-square vorticity (enstrophy) fluctuations in strong shear flows, we demonstrate that unlike energy production of turbulent vorticity in nonrotating shear flows, the turbulent vorticity of weak convection in Keplerian disks cannot gain energy from vortex stretching/tilting by background shear unless the asscoiated Reynolds stresses are negative. This is because the epicyclic motion is an energy sink of the radial component of mean-square turbulent vorticity in Keplerian disks when Reynolds stresses are positive. Consequently, weak convection cannot be self-sustained in Keplerian flows. This agrees with the results implied from the equations of mean-square velocity fluctuations in strong shear flows. Our analysis also sheds light on the explanation of the simulation result in which positive kinetic helicity is produced by the Balbus-Hawley instability in a vertically stratified Keplerian disk. We also comment on the possibility of outward angular momentum transport by strong convection based on azimuthal pressure perturbations and directions of energy cascade.Comment: 8 pages, 1 figure, emulateapj.sty, revised version in response to referee's comments, accepted by Ap

Surface permeability of natural and engineered porous building materials

Characterization of surface gas permeability measurements on a variety of natural and engineered building materials using two relatively new, non-destructive surface permeameters is presented. Surface gas permeability measurements were consistent for both laboratory and field applications and correlated well with bulk gas permeability measurements. This research indicates that surface permeability measurements could provide reliable estimates of bulk gas permeability; and due to the non-destructive nature and relative sampling ease of both surface gas permeability tools, it is possible to quantify the range of the spatial autocorrelation, heterogeneity, and anisotropy in porous building materials and their degree of degradation from weathering

Sistematización diagnóstica de las laxitudes crónicas de la rodilla

Se expone la sistemática llevada a cabo en nuestro Servicio de Patología del Aparato Locomotor para la clasificación y estudio de las laxitudes crónicas de la rodilla, dividiéndolas en dos grandes grupos: moderadas y graves, y dentro de ellos las distintas modalidades de predominio medial lateral y combinadas, para poder efectuar un correcto planteamiento de la táctica quirúrgica de reparación a seguir, tema que es objeto de otro trabajo

Removal of 4-hydroxyphenylacetic acid from aqueous medium by electrochemical oxidation with a BDD anode: mineralization, kinetics and oxidation products

The degradation of 100 mL of solutions containing 4-hydroxyphenylacetic acid in 0.050 M Na2SO4 at pH 3.0 has been performed by anodic oxidation with electrogenerated H2O2 (AO-H2O2) using a stirred tank reactor equipped with a boron-doped diamond (BDD) anode and an air-diffusion cathode. An almost total mineralization with 95.5% total organic carbon (TOC) removal was achieved for a 1.03 mM substrate solution at 100 mA cm-2. The effect of current density between 16.7 and 100 mA cm-2 and 4-hydroxyphenylacetic acid content between 0.21 and 2.06 mM was examined. Greater current efficiency with lower specific energy consumption and smaller mineralization was found at low current density and high substrate concentration. The TOC abatement as well as the 4-hydroxyphenylacetic acid concentration decay obeyed a pseudo-first-order kinetics. The oxidation role of hydroxyl radical formed from water discharge at the BDD anode is explained on the basis of its electrogeneration rate and competitive wasting reactions. 4-Hydroxybenzenemethanol and its derivative 4-hydroxybenzenealdehyde were identified as primary aromatic by-products by gas chromatography-mass spectrometry. Ion-exclusion HPLC allowed the detection of low amounts of the persistent oxalic acid during the AO-H2O2 process. The remaining TOC in final electrolyzed solutions is related to the presence of a large proportion of unidentified by-products that are even more recalcitrant than common short-linear aliphatic carboxylic acids

Spectrum and Variability of Mrk501 as observed by the CAT Imaging Telescope

The CAT Imaging Telescope has observed the BL Lac object Markarian 501 between March and August 1997. We report here on the variability over this time including several large flares. We present also preliminary spectra for all these data, for the low emission state, and for the largest flare.Comment: 4 pages, 4 figures, Late

Observation of the Crab Nebula Gamma-Ray Emission Above 220 Gev by the Cat Cherenkov Imaging Telescope

The CAT imaging telescope, recently built on the site of the former solar plant Themis (French Pyrenees), observed gamma-rays from the Crab nebula from October 1996 to March 1997. This steady source, often considered as the standard candle of very-high-energy gamma-ray astronomy, is used as a test-beam to probe the performances of the new telescope, particularly its energy threshold (220 GeV at 20 degrees zenith angle) and the stability of its response. Due to the fine-grain camera, an accurate analysis of the longitudinal profiles of shower images is performed, yielding the source position in two dimensions for each individual shower.Comment: 5 pages, 3 figures, Tex, contribution to 25th ICRC Durba

Detection of Vhe Gamma-Rays from MRK 501 with the Cat Imaging Telescope

The CAT imaging telescope on the site on the former solar plant Themis has been observing gamma-rays from Mrk501 above 220 GeV in March and April 1997. This source is shown to be highly variable and the light curve is presented. The detected gamma-ray rate for the most intense flare is in excess of 10 per minute.Comment: 5 pages, 4 figures, Tex, contribution to 25th ICRC Durba