Radio Halo and Relic Candidates from the NRAO VLA Sky Survey

We present the first results of the search of new halo and relic candidates in the NRAO VLA Sky Survey. We have inspected a sample of 205 clusters from the X-ray-brightest Abell-type clusters presented by Ebeling et al (1996), and found 29 candidates. Out of them, 11 clusters are already known from the literature to contain a diffuse cluster-wide source, while in 18 clusters this is the first indication of the existence of this type of sources. We classify these sources as halos or relics according to their location in the cluster center or periphery, respectively. We find that the occurrence of cluster halos and relics is higher in clusters with high X-ray luminosity and high temperature. We also confirm the correlation between the absence of a cooling flow and the presence of a radio halo at the cluster center.Comment: 25 Pages, 4 Figures included in the text, Figures 3a to 3g in separated files. New Astronomy in pres

The reaction of Pseudomonas nitrite reductase and nitrite. A stopped-flow and EPR study.

The reaction between reduced Pseudomonas nitrite reductase and nitrite has been studied by stopped-flow and rapid-freezing EPR spectroscopy. The interpretation of the kinetics at pH 8.0 is consistent with the following reaction mechanism (where k1 and k3 much greater than k2). [formula: see text] The bimolecular step (Step 1) is very fast, being lost in the dead time of a rapid mixing apparatus; the stoichiometry of the complex has been estimated to correspond to one NO2- molecule/heme d1. The final species is the fully reduced enzyme with NO bound to heme d1; and at all concentrations of nitrite, there is no evidence for dissociation of NO or for further reduction of NO to N2O. Step 2 is assigned to an internal electron transfer from heme c to reduced NO-bound heme d1 occurring with a rate constant of 1 s-1; this rate is comparable to the rate of internal electron transfer previously determined when reducing the oxidized enzyme with azurin or cytochrome c551. When heme d1 is NO-bound, the rate at which heme c can accept electrons from ascorbate is remarkably increased as compared to the oxidized enzyme, suggesting an increase in the redox potential of the latter heme

EVN Observations of GRS1915+105

We obtained EVN observations of the X-ray transient source GRS1915+105 in its radio-loud state. In 2 images obtained before a major flare, the source is resolved, showing two opposite asymmetric jets. This indicates that steady jets exist also before a major flare, with an estimated velocity of 0.2-0.6c.Comment: Proceedings of the 5th EVN Symposium, Eds. J. Conway, A. Polatidis, R. Booth, Onsala Observatory, Sweden (June 2000

Illumination system in visible light with variable solar-divergence for the solar orbiter METIS coronagraph

The measurement of the stray-rejection capabilities of METIS is part of the acceptance package of the instrument. The Illumination System in Visible Light (ISVL) has been developed to allow the stray-light rejection measurement down to 1x10-9 and under different operating conditions. The main characteristics of ISVL are outlined and discussed; the activities for the integration and verification of ISVL included the absolute radiometric characterization of the facility, including radiance measurement and radiance spatial and angular distribution. The procedures used to measure the performances of the facility are discussed and the obtained results illustrated