7,257 research outputs found
Observations of Mkn 421 with the CELESTE Experiment
The CELESTE experiment uses the heliostats of an old solar farm in the French
Pyrenees to detect gamma ray air showers by the atmospheric Cerenkov technique.
Observations of the TeV blazar Markarian 421 have been made with the fully
instrumented CELESTE experiment since December 1999. The detection of gamma ray
emission from this source at energies greater than 50 GeV is presented here. A
comparison is made with the light curve from the CAT experiment at the same
site which shows correlation between the observed gamma ray fluxes and the
detection of short duration flaring episodes.Comment: 4 pages, 4 figures. To appear in the Proceedings of the Heidelberg
Gamma Ray Symposiu
VERITAS: Status and Performance
VERITAS is an atmospheric Cherenkov telescope array sited in Tucson, Arizona.
The array is nearing completion and consists of four, 12m diameter telescopes.
The first telescope in the array has been operating since February 2005, while
observations with the full array are expected to begin in January, 2007. We
report here in some detail on the performance of the first VERITAS telescope,
and briefly discuss the first stereo observations.Comment: 8 pages. Submitted to Proceedings of "Science with New Generation of
High Energy Gamma-ray Experiments", Elba 200
Substructure Discovery Using Minimum Description Length and Background Knowledge
The ability to identify interesting and repetitive substructures is an
essential component to discovering knowledge in structural data. We describe a
new version of our SUBDUE substructure discovery system based on the minimum
description length principle. The SUBDUE system discovers substructures that
compress the original data and represent structural concepts in the data. By
replacing previously-discovered substructures in the data, multiple passes of
SUBDUE produce a hierarchical description of the structural regularities in the
data. SUBDUE uses a computationally-bounded inexact graph match that identifies
similar, but not identical, instances of a substructure and finds an
approximate measure of closeness of two substructures when under computational
constraints. In addition to the minimum description length principle, other
background knowledge can be used by SUBDUE to guide the search towards more
appropriate substructures. Experiments in a variety of domains demonstrate
SUBDUE's ability to find substructures capable of compressing the original data
and to discover structural concepts important to the domain. Description of
Online Appendix: This is a compressed tar file containing the SUBDUE discovery
system, written in C. The program accepts as input databases represented in
graph form, and will output discovered substructures with their corresponding
value.Comment: See http://www.jair.org/ for an online appendix and other files
accompanying this articl
Magnetic field enhanced structural instability in EuTiO_{3}
EuTiO_{3} undergoes a structural phase transition from cubic to tetragonal at
T_S = 282 K which is not accompanied by any long range magnetic order. However,
it is related to the oxygen ocathedra rotation driven by a zone boundary
acoustic mode softening. Here we show that this displacive second order
structural phase transition can be shifted to higher temperatures by the
application of an external magnetic field (increased by 4 K for mu_{0}H = 9 T).
This observed field dependence is in agreement with theoretical predictions
based on a coupled spin-anharmonic-phonon interaction model.Comment: 4 pages, 4 figure
- …