13,407 research outputs found
Very-High Energy Gamma-Ray Flux Limits for Nearby Active Galactic Nuclei
Combining the results of targeted observations, H.E.S.S. has accumulated a
large amount of extra-galactic exposure at TeV energies. Due to its large field
of view a considerable part of the sky (0.6 sr) has been observed with high
sensitivity outside the targeted observation positions. Since this exposure
region contains little inherent bias, it is well suited for studies of
extra-galactic source populations. Given the correlation between ultra-high
energy cosmic rays and nearby extra-galactic objects recently claimed by the
Auger collaboration, this unbiased sky sample by H.E.S.S. is of interest since
it includes (besides the targeted sources) 63 AGN within 100 Mpc, for which
very-high energy gamma-ray flux limits are derived.Comment: 4 pages, 5 figures, Submitted to Proceedings of "4th International
Symposium on High Energy Gamma-Ray Astronomy 2008
Monte Carlo studies on the sensitivity of the HEGRA imaging atmospheric Cerenkov telescope system in observations of extended gamma-ray sources
In this paper, we present the results of Monte Carlo simulations of
atmospheric showers induced by diffuse gamma rays as detected by the
high-energy gamma ray astronomy (HEGRA) system of five imaging atmospheric
Cerenkov telescopes (IACTs). We have investigated the sensitivity of
observations on extended gamma ray emission over the entire field of view of
the instrument. We discuss a technique to search for extended gamma ray sources
within the field of view of the instrument. We give estimates for HEGRA
sensitivity of observations on extended TeV gamma ray sources.Comment: 21 pages, 7 figures, accepted for publication in "Journal of Physics
G: Nuclear and Particle Physics
Multi-wavelength Observations of LS I +61 303 with VERITAS, Swift and RXT E: 2006-2008
A long term, multi-wavelength monitoring campaign on the TeV binary LS I +61
303 has been performed utilizing >300 GeV observations with VERITAS along with
monitoring in the 0.2-10 keV band by RXTE and Swift between September 2006 and
February 2008. The source was detected by VERITAS as a variable TeV source with
flux values ranging from 5-20% of the Crab Nebula flux with the strongest flux
levels appearing around apastron. X-ray observations by RXTE and Swift show the
source as a highly variable hard X-ray source with flux values varying in the
range of 0.5-3*10^-11 ergs cm^-2 s^-1 over a single orbital cycle. The
2007-2008 RXTE data set also shows the presence of several extremely large
flaring episodes presenting a flux of up to 7.2*10^-11 ergs cm^-2 s^-1, the
largest such flare recorded from this source. Comparison of the contemporaneous
TeV and X-ray data does not show a correlation at this time, however, the
sparsity of data sets do not preclude the existence of such a correlation.Comment: Submitted to Proceedings of "4th Heidelberg International Symposium
on High Energy Gamma-Ray Astronomy 2008
Capability of Cherenkov Telescopes to Observe Ultra-fast Optical Flares
The large optical reflector (~ 100 m^2) of a H.E.S.S. Cherenkov telescope was
used to search for very fast optical transients of astrophysical origin. 43
hours of observations targeting stellar-mass black holes and neutron stars were
obtained using a dedicated photometer with microsecond time resolution. The
photometer consists of seven photomultiplier tube pixels: a central one to
monitor the target and a surrounding ring of six pixels to veto background
events. The light curves of all pixels were recorded continuously and were
searched offline with a matched-filtering technique for flares with a duration
of 2 us to 100 ms. As expected, many unresolved (500 us)
background events originating in the earth's atmosphere were detected. In the
time range 3 to 500 us the measurement is essentially background-free, with
only eight events detected in 43 h; five from lightning and three presumably
from a piece of space debris. The detection of flashes of brightness ~ 0.1 Jy
and only 20 us duration from the space debris shows the potential of this setup
to find rare optical flares on timescales of tens of microseconds. This
timescale corresponds to the light crossing time of stellar-mass black holes
and neutron stars.Comment: Accepted for publication in Astroparticle Physics, 8 pages, 9
figures, 1 tabl
Dynamical Mass Generation and Confinement in Maxwell-Chern-Simons Planar Quantum Electrodynamics
We study the non-perturbative phenomena of Dynamical Mass Generation and
Confinement by truncating at the non-perturbative level the Schwinger-Dyson
equations in Maxwell-Chern-Simons planar quantum electrodynamics. We obtain
numerical solutions for the fermion propagator in Landau gauge within the
so-called rainbow approximation. A comparison with the ordinary theory without
the Chern-Simons term is presented.Comment: 9 pages, 9 figures; prepared for the XIV Mexican School of Particles
and Fields, 4-12 November 2010, Morelia, Michoacan, Mexic
Comparison of techniques to reconstruct VHE gamma-ray showers from multiple stereoscopic Cherenkov images
For air showers observed simultaneously by more than two imaging atmospheric
Cherenkov telescopes, the shower geometry is overconstrained by the images and
image information should be combined taking into account the quality of the
images. Different algorithms are discussed and tested experimentally using data
obtained from observations of Mkn 501 with the HEGRA IACT system. Most of these
algorithms provide an estimate of the accuracy of the reconstruction of shower
geometry on an event-by-event basis, allowing, e.g., to select higher-quality
subsamples for precision measurements.Comment: 14 Pages, 6 figures, Late
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