The phenomenon of stress-reorientation has been investigated using in situ X-ray diffraction during the thermomechanical cycling of hydrided Zircaloy-4 tensile specimens. Results have shown that ...loading along a sample’s transverse direction (TD) leads to a greater degree of hydride reorientation when compared to rolling direction (RD)-aligned samples. The elastic lattice micro-strains associated with radially oriented hydrides have been revealed to be greater than those oriented circumferentially, a consequence of strain accommodation. Evidence of hydride redistribution after cycling, to α-Zr grains oriented in a more favourable orientation when under an applied stress, has also been observed and its behaviour has been found to be highly dependent on the loading axis. Finally, thermomechanical loading across multiple cycles has been shown to reduce the difference in terminal solid solubility of hydrogen during dissolution (TSSD,H) and precipitation (TSSP,H).
The multiwavelength observation of the nearby radio galaxy M87 provides a unique opportunity to study in detail processes occurring in active galactic nuclei from radio waves to TeV -rays. Here we ...report the detection of -ray emission above 250 GeV from M87 in spring 2007 with the VERITAS atmospheric Cerenkov telescope array and discuss its correlation with the X-ray emission. The -ray emission is measured to be pointlike with an intrinsic source radius less than 4.5 super(image ). The differential energy spectrum is fitted well by a power-law function: image m super(-2) s super(-1) TeV super(-1). We show strong evidence for a year-scale correlation between the -ray flux reported by TeV experiments and the X-ray emission measured by the ASM RXTE observatory, and discuss the possible short-timescale variability. These results imply that the -ray emission from M87 is more likely associated with the core of the galaxy than with other bright X-ray features in the jet.
The VERITAS collaboration reports the detection of very-high-energy gamma-ray emission from the high-frequency-peaked BL Lac object 1ES 1218+304 located at a redshift of z = 0.182. A gamma-ray signal ...was detected with a statistical significance of 10.4 standard deviations (10.4s) for the observations taken during the first three months of 2007, confirming the discovery of this object made by the MAGIC collaboration. The photon spectrum between ~160 GeV and ~1.8 TeV is well described by a power law with an index of = 3.08 ± 0.34stat ± 0.2sys. The integral flux is (E>200GeV) = (12.2 ± 2.6) X 10-12 cm-2 s-1, which corresponds to ~6% of that of the Crab Nebula. The light curve does not show any evidence for very high energy flux variability. Using lower limits on the density of the extragalactic background light in the near to mid-infrared, we are able to limit the range of intrinsic energy spectra for 1ES 1218+304. We show that the intrinsic photon spectrum has an index that is harder than = 2.32 ± 0.37stat. When including constraints from the spectra of 1ES 1101-232 and 1ES 0229+200, the spectrum of 1ES 1218+304 is likely to be harder than = 1.86 ± 0.37stat.
A reanalysis of data taken on the Cygnus region in 1989–90 using the Whipple Observatory atmospheric Cherenkov imaging telescope confirms the existence of the TeV J2032+4130 source reported by the ...Crimean Astrophysical Observatory and published by the HEGRA Collaboration. The significance at the a priori HEGRA position is 3.3σ. The peak signal was found at RA = 20h32m, Dec = +41°33'. This is 0.6° north of Cygnus X-3 which was the original target of the observations. The flux level (12% of the level of the Crab Nebula) is intermediate between the two later observations and suggests that the TeV source is variable.
The Very Energetic Radiation Imaging Telescope Array System (VERITAS) represents an important step forward in the study of extreme astrophysical processes in the universe. It combines the power of ...the atmospheric Cherenkov imaging technique using a large optical reflector with the power of stereoscopic observatories using arrays of separated telescopes looking at the same shower. The seven identical telescopes in VERITAS, each of aperture 10 m, will be deployed in a filled hexagonal pattern of side 80 m; each telescope will have a camera consisting of 499 pixels with a field of view of 3.5°. VERITAS will substantially increase the catalog of very high energy (
E>100 GeV) γ-ray sources and greatly improve measurements of established sources.
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