1,786 research outputs found
High zenith angle observations of PKS 2155-304 with the MAGIC telescope
The high frequency peaked BL Lac PKS 2155-304 with a redshift z=0.116 was
discovered 1997 in the VHE range by the University of Durham Mark 6 gamma-ray
telescope in Australia with a flux corresponding to approx. 0.2 times the Crab
Nebula flux. It was later observed and detected with high significance by the
Southern observatories CANGAROO and H.E.S.S. establishing this source as the
best studied Southern TeV blazar. Detection from the Northern hemisphere was
very difficult due to challenging observation conditions under large zenith
angles. In July 2006, the H.E.S.S. collaboration reported an extraordinary
outburst of VHE gamma-emission. During the outburst, the VHE gamma-ray emission
was found to be variable on the time scales of minutes and at a mean flux of
approx. 7 times the flux observed from the Crab Nebula. The MAGIC collaboration
operates a 17m imaging air Cherenkov Telescope at La Palma (Northern
Hemisphere). Follow up observations of the extraordinary outburst have been
triggered in a Target of Opportunity program by an alert from the H.E.S.S.
collaboration. The measured spectrum and light curve are presented.Comment: Contribution to the 31st ICRC, Lodz, Poland, July 200
Ground-based detectors in very-high-energy gamma-ray astronomy
Following the discovery of the cosmic rays by Victor Hess in 1912, more than
70 years and numerous technological developments were needed before an
unambiguous detection of the first very-high-energy gamma-ray source in 1989
was made. Since this discovery the field on very-high-energy gamma-ray
astronomy experienced a true revolution: A second, then a third generation of
instruments were built, observing the atmospheric cascades from the ground,
either through the atmospheric Cherenkov light they comprise, or via the direct
detection of the charged particles they carry. Present arrays, 100 times more
sensitive than the pioneering experiments, have detected a large number of
astrophysical sources of various types, thus opening a new window on the
non-thermal Universe. New, even more sensitive instruments are currently being
built; these will allow us to explore further this fascinating domain. In this
article we describe the detection techniques, the history of the field and the
prospects for the future of ground-based very-high-energy gamma-ray astronomy.Comment: 21 pages, 13 figure
Competitions in layered ruthenates: ferro- vs. antiferromagnetism and triplet vs. singlet pairing
Ru based perovskites demonstrate an amazing richness in their magnetic
properties, including 3D and quasi-2D ferromagnetism, antiferromagnetism, and
unconventional superconductivity. Tendency to ferromagnetism, stemming from the
unusually large involvement of O in magnetism in ruthenates, leads to
ferromagnetic spin fluctuations in Sr2RuO4 and eventually to p-wave
superconductivity. A related compound Ca2RuO4 was measured to be
antiferromagnetic, suggesting a possibility of antiferromagnetic fluctuations
in Sr2RuO4 as well. Here we report first principles calculations that
demonstrate that in both compounds the ferro- and antiferromagnetic
fluctuations coexist, leading to an actual instability in Ca2RuO4 and to a
close competition between p-wave and d-wave superconducting symmetries in
Sr2RuO4. The antiferromagnetism in this system appears to be mostly related
with the nesting, which is the strongest at Q=(2pi/3,2pi/3,0). Surprisingly,
for the Fermiology of Sr2RuO4 the p-wave state wins over the d-wave one
everywhere except in close vicinity of the antiferromagnetic instability. The
most stable state within the d-wave channel has vanishing order parameter at
one out of three Fermi surfaces in Sr2RuO4, while in the p channel its
amplitude is comparable at all three of them.Comment: 4 Revtex pages with 4 embedded postscript figure. Some figures are
color, but should look OK in B&W as wel
Weighted Density Functionals for Ferroelectric Materials
The weighted density approximation, its implementation and its application to
ferroelectric materials is discussed. Calculations are presented for several
perovskite oxides and related materials. In general the weighted density
approximation is found to be superior to either the local density or
generalized gradient approximation for the ground state. Electronic structures
are little changed. The linear response of the weighted density approximation
is calculated for the homogeneous electron gas, and found to be improved
relative to the local density result, but not in full agreement with existing
Monte Carlo data. It is shown that the agreement can be further improved by a
simple modification. Calculations of the ferroelectric soft mode in KNbO
suggest that the low temperature distortion is approximately 20% smaller than
indicated by existing experiments.Comment: 14 pages, 2 embedded figures, uses aipproc style. Contribution
submitted to the Fifth Williamsburg Workshop on First-Principles Calculations
for Ferroelectric
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