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$_3$ 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