We report the discovery of an unidentified, extended source of very-high-energy gamma-ray emission, VER J2019+407, within the radio shell of the supernova remnant SNR G78.2+2.1, using 21.4 hr of data ...taken by the VERITAS gamma-ray observatory in 2009. These data confirm the preliminary indications of gamma-ray emission previously seen in a two-year (2007-2009) blind survey of the Cygnus region by VERITAS. VER J2019+407, which is detected at a post-trials significance of 7.5 standard deviations in the 2009 data, is localized to the northwestern rim of the remnant in a region of enhanced radio and X-ray emission. It has an intrinsic extent of (ProQuest: Formulae and/or non-USASCII text omitted)and its spectrum is well-characterized by a differential power law (dN/dE =N sub(0) x (E/TeV) super(-Gamma)) with a photon index of Gamma = 2.37+ or -0.14 sub(stat)+ or -0.20 sub(sys) and a flux normalization of N sub(0) = 1.5+ or -0.2 sub(stat)+ or -0.4 sub(sys) x 10 super(-12) photon TeV super(-1) cm super(-2) s super(-1). This yields an integral flux of 5.2+ or -0.8 sub(stat)+ or -1.4 sub(sys) x 10 super(-12) photon cm super(-2) s super(-1) above 320 GeV, corresponding to 3.7% of the Crab Nebula flux. We consider the relationship of the TeV gamma-ray emission with the GeV gamma-ray emission seen from SNR G78.2+2.1 as well as that seen from a nearby cocoon of freshly accelerated cosmic rays. Multiple scenarios are considered as possible origins for the TeV gamma-ray emission, including hadronic particle acceleration at the SNR shock.
The Klebsiella aerogenes ureE gene product was previously shown to facilitate assembly of the urease metallocenter (Lee, M.H., et al., 1992, J. Bacteriol. 174, 4324‐4330). UreE protein has now been ...purified and characterized. Although it behaves as a soluble protein, UreE is predicted to possess an amphipathic β‐strand and exhibits unusually tight binding to phenyl‐Sepharose resin. Immunogold electron microscopic studies confirm that UreE is a cytoplasmic protein. Each dimeric UreE molecule (Mr = 35, 000) binds 6.05 + 0.25 nickel ions (Kd of 9.6 + 1.3 μM) with high specificity according to equilibrium dialysis measurements. The nickel site in UreE was probed by X‐ray absorption and variable‐temperature magnetic circular dichroism spectroscopies. The data are most consistent with the presence of Ni(II) in pseudo‐octahedral geometry with 3‐5 histidyl imidazole ligands. The remaining ligands are nitrogen or oxygen donors. UreE apoprotein has been crystallized and analyzed by X‐ray diffraction methods. Addition of nickel ion to apoprotein crystals leads to the development of fractures, consistent with a conformational change upon binding nickel ion. We hypothesize that UreE binds intracellular nickel ion and functions as a nickel donor during metallocenter assembly into the urease apoprotein.
Information on two molecular mechanisms of ice nucleation on silver iodide and silver chloroiodide particulates has been gained by a chemical kinetics interpretation of the rates and mechanisms of ...ice crystal formation from experiments conducted in a 1 m3isothermal cloud chamber. The interpretation required the consideration of the Langmuir surface adsorption of water molecules by AgI from the vapor and the chemistry of AgI colloids and suspensions in water with and without the influences of supporting electrolytes. One mechanism is a slow rearrangement of three-dimensional water molecule clusters, adsorbed at water saturation and −20°C and lower on 500 Å and larger substrate particles, to ice embryos. The second occurs rapidly at −5 to −20°C on substrate particles wetted by liquid water. This mechanism is hypothesized to involve the rearrangement of ordered water molecule clusters associated with hydrated Ag+ion patches on the substrate surface. The rate of nucleation is slowed by the presence of alkali and alkaline earth halides. Crystal structure match of the substrate with that of ice may help the first mechanism but, perhaps, has no effect on the second.
We report on TeV Delta *g-ray observations of the blazar Mrk 421 (redshift of 0.031) with the VERITAS observatory and the Whipple 10 m Cherenkov telescope. The excellent sensitivity of VERITAS ...allowed us to sample the TeV Delta *g-ray fluxes and energy spectra with unprecedented accuracy where Mrk 421 was detected in each of the pointings. A total of 47.3 hr of VERITAS and 96 hr of Whipple 10 m data were acquired between 2006 January and 2008 June. We present the results of a study of the TeV Delta *g-ray energy spectra as a function of time and for different flux levels. On 2008 May 2 and 3, bright TeV Delta *g-ray flares were detected with fluxes reaching the level of 10 Crab. The TeV Delta *g-ray data were complemented with radio, optical, and X-ray observations, with flux variability found in all bands except for the radio wave band. The combination of the Rossi X-ray Timing Explorer and Swift X-ray data reveal spectral hardening with increasing flux levels, often correlated with an increase of the source activity in TeV Delta *g-rays. Contemporaneous spectral energy distributions were generated for 18 nights, each of which are reasonably described by a one-zone synchrotron self-Compton model.
Glutamate receptor currents were examined in horizontal cells from cultured human retina using whole-cell recording procedures. Horizontal cells possess both AMPA and kainate receptors and both ...produce significant sustained currents. The kainate-induced current did not show significant desensitization and was not enhanced by concanavalin A. The sustained AMPA current was smaller than the kainate current, but the difference was almost entirely due to pronounced desensitization. The horizontal cell AMPA current was enhanced by cyclothiazide but not by PEPA, indicating the presence of the flip receptor variant. GYKI-52466 blocked the AMPA response (IC50 = 5 microM against 100 microM AMPA) but also blocked the kainate response (IC50 = 45 microM against 100 microM kainate). The diversity of glutamate receptors in human horizontal cells suggests that synaptic input to these neurons may be multiplexed through both kainate and AMPA channels.
At the Desert Research Institute, Reno, NV, USA, we investigated atmospheric ice crystal initiation, growth and interaction. We developed a mechanism of ice crystal growth in mixed clouds that is ...consistent with basic chemistry and physics, observations by others, and numerical studies of ice crystal growth from the vapor phase. The mechanism may provide insight into ice crystal morphology and charge separations in ice crystals growing in mixed clouds.
VERITAS has been monitoring the very-high-energy (VHE; > 100 GeV) gamma-ray activity of the radio galaxy M 87 since 2007. During 2008, flaring activity on a timescale of a few days was observed with ...a peak flux of (0.70 + or - 0.16) x 10 super(-11) cm super(-2) s super(-1) at energies above 350 GeV. In 2010 April, VERITAS detected a flare from M 87 with peak flux of (2.71 + or - 0.68) x 10 super(-11) cm super(-2) s super(-1) for E > 350 GeV. The source was observed for six consecutive nights during the flare, resulting in a total of 21 hr of good-quality data. The most rapid flux variation occurred on the trailing edge of the flare with an exponential flux decay time of 0.90 sub(-0.15) super(+0.22) days. The shortest detected exponential rise time is three times as long, at 2.87 sub(-0.99) super(+1.65) days. The quality of the data sample is such that spectral analysis can be performed for three periods: rising flux, peak flux, and falling flux. The spectra obtained are consistent with power-law forms. The spectral index at the peak of the flare is equal to 2.19 + or - 0.07. There is some indication that the spectrum is softer in the falling phase of the flare than the peak phase, with a confidence level corresponding to 3.6 standard deviations. We discuss the implications of these results for the acceleration and cooling rates of VHE electrons in M 87 and the constraints they provide on the physical size of the emitting region.
The variability of the blazar Markarian 421 in TeV gamma rays over a 14-year time period has been explored with the Whipple 10m telescope. It is shown that the dynamic range of its flux variations is ...large and similar to that in X-rays. A correlation between the X-ray and TeV energy bands is observed during some bright flares and when the complete data sets are binned on long timescales. The main database consists of 878.4h of observation with the Whipple telescope, spread over 783 nights. The peak energy response of the telescope was 400GeV with 20% uncertainty. This is the largest database of any TeV-emitting active galactic nucleus (AGN) and hence was used to explore the variability profile of Markarian 421. The time-averaged flux from Markarian 421 over this period was 0.446±0.008Crab flux units. The flux exceeded 10Crab flux units on three separate occasions. For the 2000–2001 season the average flux reached 1.86Crab units, while in the 1996–1997 season the average flux was only 0.23Crab units.
The spin state of the heme in superoxide (O2*- )-producing cytochrome b558 purified from pig neutrophils was examined by means of room-temperature magnetic circular dichroism (MCD) under ...physiological conditions. Cytochrome b558 with varying amounts of low-spin and high-spin heme was prepared by either pH adjustment or heat treatment, and the O2*−-forming activity in a cell-free system was found to correlate with the low-spin heme content. The possibility that the O2*−-forming activity results from a transient high-spin ferric heme form that is induced during activation by anionic amphophils has also been investigated. EPR spectra of cytochrome b558 activated by either arachidonic acid or myristic acid, showed that a transient high-spin ferric species accounting for approximately 50%of the heme appeared in the presence of arachidonic acid, but not in the presence of myristic acid. Hence the appearance of a transient high-spin ferric heme species on activation with an amphophil does not afford a common activation mechanism in the NADPH oxidase system. The EPR results for cytochrome b558 activated with arachidonic acid showed that the transient high-spin ferric heme can bind cyanide. However, the high-spin ferric heme does not contribute to the O2*− production of cytochrome b558 in cell- free assays in the presence of cyanide.
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