112 research outputs found
Morphology and hardness ratio exploitation under limited statistics
Gamma-ray astronomy has produced for several years now sky maps for low
photon statistics, non-negligible background and comparatively poor angular
resolution. Quantifying the significance of spatial features remains difficult.
Besides, spectrum extraction requires regions with large statistics while maps
in energy bands allow only qualitative interpretation. The two main competing
mechanisms in the VHE domain are the Inverse-Compton emission from accelerated
electrons radiating through synchrotron in the X-ray domain and the
interactions between accelerated hadrons and the surrounding medium, leading to
the production and subsequent decay of Pi0 mesons. The spectrum of the VHE
emission from leptons is predicted to steepen with increasing distance from the
acceleration zone, owing to synchrotron losses (i.e. cooled population). It
would remain approximately constant for hadrons. Ideally, spectro-imaging
analysis would have the same spatial scale in the TeV and X-ray domains, to
distinguish the local emission mechanisms. More realistically, we investigate
here the possibility of improving upon the currently published HESS results by
using more sophisticated tools.Comment: 4 pages, 6 figures, Proceeding for a poster at the GAMMA08 Heidelberg
Symposiu
H.E.S.S. observations of gamma-ray bursts in 2003-2007
Very-high-energy (VHE; >~100 GeV) gamma-rays are expected from gamma-ray
bursts (GRBs) in some scenarios. Exploring this photon energy regime is
necessary for understanding the energetics and properties of GRBs. GRBs have
been one of the prime targets for the H.E.S.S. experiment, which makes use of
four Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect VHE gamma-rays.
Dedicated observations of 32 GRB positions were made in the years 2003-2007 and
a search for VHE gamma-ray counterparts of these GRBs was made. Depending on
the visibility and observing conditions, the observations mostly start minutes
to hours after the burst and typically last two hours. Results from
observations of 22 GRB positions are presented and evidence of a VHE signal was
found neither in observations of any individual GRBs, nor from stacking data
from subsets of GRBs with higher expected VHE flux according to a
model-independent ranking scheme. Upper limits for the VHE gamma-ray flux from
the GRB positions were derived. For those GRBs with measured redshifts,
differential upper limits at the energy threshold after correcting for
absorption due to extra-galactic background light are also presented.Comment: 9 pages, 4 tables, 3 figure
Discovery of VHE gamma-rays from the high-frequency-peaked BL Lac object RGB J0152+017
Aims: The BL Lac object RGB J0152+017 (z=0.080) was predicted to be a very
high-energy (VHE; > 100 GeV) gamma-ray source, due to its high X-ray and radio
fluxes. Our aim is to understand the radiative processes by investigating the
observed emission and its production mechanism using the High Energy
Stereoscopic System (H.E.S.S.) experiment. Methods: We report recent
observations of the BL Lac source RGB J0152+017 made in late October and
November 2007 with the H.E.S.S. array consisting of four imaging atmospheric
Cherenkov telescopes. Contemporaneous observations were made in X-rays by the
Swift and RXTE satellites, in the optical band with the ATOM telescope, and in
the radio band with the Nancay Radio Telescope. Results: A signal of 173
gamma-ray photons corresponding to a statistical significance of 6.6 sigma was
found in the data. The energy spectrum of the source can be described by a
powerlaw with a spectral index of 2.95+/-0.36stat+/-0.20syst. The integral flux
above 300 GeV corresponds to ~2% of the flux of the Crab nebula. The source
spectral energy distribution (SED) can be described using a two-component
non-thermal synchrotron self-Compton (SSC) leptonic model, except in the
optical band, which is dominated by a thermal host galaxy component. The
parameters that are found are very close to those found in similar SSC studies
in TeV blazars. Conclusions: RGB J0152+017 is discovered as a source of VHE
gamma-rays by H.E.S.S. The location of its synchrotron peak, as derived from
the SED in Swift data, allows clearly classification it as a
high-frequency-peaked BL Lac (HBL).Comment: Accepted for publication in A&A Letters (5 pages, 4 figures
Exploring a SNR/Molecular Cloud Association Within HESS J1745-303
HESS J1745-303 is an extended, unidentified VHE (very high energy) gamma-ray
source discovered using HESS in the Galactic Plane Survey. Since no obvious
counterpart has previously been found in longer-wavelength data, the processes
that power the VHE emission are not well understood. Combining the latest VHE
data with recent XMM-Newton observations and a variety of source catalogs and
lower-energy survey data, we attempt to match (from an energetic and positional
standpoint) the various parts of the emission of HESS J1745-303 with possible
candidates. Though no single counterpart is found to fully explain the VHE
emission, we postulate that at least a fraction of the VHE source may be
explained by a supernova-remnant/molecular-cloud association and/or a
high-spin-down-flux pulsar.Comment: 11 pages, 4 figures, Accepted for publication in Astronomy &
Astrophysic
Simultaneous observations of PKS 2155-304 with H.E.S.S., Fermi, RXTE and ATOM: spectral energy distributions and variability in a low state
We report on the first simultaneous observations that cover the optical,
X-ray, and high energy gamma-ray bands of the BL Lac object PKS 2155-304. The
gamma-ray bands were observed for 11 days, between 25 August and 6 September
2008, jointly with the Fermi Gamma-ray Space Telescope and the H.E.S.S.
atmospheric Cherenkov array, providing the first simultaneous MeV-TeV spectral
energy distribution with the new generation of gamma-ray telescopes. The ATOM
telescope and the RXTE and Swift observatories provided optical and X-ray
coverage of the low-energy component over the same time period. The object was
close to the lowest archival X-ray and Very High Energy state, whereas the
optical flux was much higher. The light curves show relatively little (~30%$)
variability overall when compared to past flaring episodes, but we find a clear
optical/VHE correlation and evidence for a correlation of the X-rays with the
high energy spectral index. Contrary to previous observations in the flaring
state, we do not find any correlation between the X-ray and VHE components.
Although synchrotron self-Compton models are often invoked to explain the SEDs
of BL Lac objects, the most common versions of these models are at odds with
the correlated variability we find in the various bands for PKS 2155-304.Comment: Accepted for publication in the Astrophysical Journa
The 2010 very high energy gamma-ray flare & 10 years of multi-wavelength observations of M 87
Abridged: The giant radio galaxy M 87 with its proximity, famous jet, and
very massive black hole provides a unique opportunity to investigate the origin
of very high energy (VHE; E>100 GeV) gamma-ray emission generated in
relativistic outflows and the surroundings of super-massive black holes. M 87
has been established as a VHE gamma-ray emitter since 2006. The VHE gamma-ray
emission displays strong variability on timescales as short as a day. In this
paper, results from a joint VHE monitoring campaign on M 87 by the MAGIC and
VERITAS instruments in 2010 are reported. During the campaign, a flare at VHE
was detected triggering further observations at VHE (H.E.S.S.), X-rays
(Chandra), and radio (43 GHz VLBA). The excellent sampling of the VHE gamma-ray
light curve enables one to derive a precise temporal characterization of the
flare: the single, isolated flare is well described by a two-sided exponential
function with significantly different flux rise and decay times. While the
overall variability pattern of the 2010 flare appears somewhat different from
that of previous VHE flares in 2005 and 2008, they share very similar
timescales (~day), peak fluxes (Phi(>0.35 TeV) ~= (1-3) x 10^-11 ph cm^-2
s^-1), and VHE spectra. 43 GHz VLBA radio observations of the inner jet regions
indicate no enhanced flux in 2010 in contrast to observations in 2008, where an
increase of the radio flux of the innermost core regions coincided with a VHE
flare. On the other hand, Chandra X-ray observations taken ~3 days after the
peak of the VHE gamma-ray emission reveal an enhanced flux from the core. The
long-term (2001-2010) multi-wavelength light curve of M 87, spanning from radio
to VHE and including data from HST, LT, VLA and EVN, is used to further
investigate the origin of the VHE gamma-ray emission. No unique, common MWL
signature of the three VHE flares has been identified.Comment: 19 pages, 5 figures; Corresponding authors: M. Raue, L. Stawarz, D.
Mazin, P. Colin, C. M. Hui, M. Beilicke; Fig. 1 lightcurve data available
online: http://www.desy.de/~mraue/m87
Very high energy gamma-ray observations of the galaxy clusters Abell 496 and Abell 85 with HESS
Aims. The nearby galaxy clusters Abell 496 and Abell 85 are studied in the very high-energy (VHE, E > 100 GeV) band to investigate VHE cosmic rays (CRs) in this class of objects which are the largest gravitationally bound systems in the Universe. Methods. HESS, an array of four imaging atmospheric cherenkov telescopes (IACT), was used to observe the targets in the range of VHE gamma rays. Results. No significant gamma-ray signal is found at the respective position of the two clusters with several different source size assumptions for each target. In particular, emission regions corresponding to the high-density core, to the extension of the entire X-ray emission in these clusters, and to the very extended region where the accretion shock is expected are investigated. Upper limits are derived for the gamma-ray flux at energies E > 570 GeV for Abell 496 and E > 460 GeV for Abell 85. Conclusions. From the non-detection in VHE gamma rays, upper limits on the total energy of hadronic CRs in the clusters are calculated. If the cosmic-ray energy density follows the large-scale gas density profile, the limit on the fraction of energy in these non-thermal particles with respect to the total thermal energy of the intra-cluster medium (ICM) is 51% for Abell 496 and only 8% for Abell 85 due to its higher mass and higher gas density. These upper limits are compared with theoretical estimates. They predict about ~10% of the thermal energy of the ICM in non-thermal particles. The observations presented here can constrain these predictions especially for the case of the Abell 85 cluster.F. Aharonian...G. Rowell...A. Zech, et a
VITESS 2.10 Virtual Instrumentation Tool for the European Spallation Source
The VITESS software package is a useful tool for the development of neutron scattering instruments. Although initially intended to assist in designing new instruments for the European Spallation Source, it can also be used effectively to simulate complex instrument set ups at continuous, short pulsed or long pulsed sources. This article outlines the conceptual design of the VITESS program and describes new features included in the most recent version of the package, VITESS 2.10. These include the possibility to split parts of the simulation into several threads which improves efficiency on multi processor computers. New components for elliptic mirrors, lenses and beam stops have been added. The neutron guide module now permits the visualization of neutron trajectories. Results of the simulation of the EXED instrument at Helmholtz Zentrum Berlin are presented as an example of the program s use. New features implemented in release 2.11 or planned for release 3 are describe
Measurement of the sea surface wind speed and direction by an airborne microwave radar altimeter
A pilot needs operational information about wind over sea as well as wave height to provide safety of a hydroplane landing on water. Near-surface wind speed and direction can be obtained with an airborne microwave scatterometer, radar designed for measuring the scatter characteristics of a surface. Mostly narrow-beam antennas are applied for such wind measurement. Unfortunately, a microwave narrow-beam antenna has considerable size that hampers its placing on flying apparatus. In this connection, a possibility to apply a conventional airborne radar altimeter as a scatterometer with a nadir-looking wide-beam antenna in conjunction with Doppler filtering for recovering the wind vector over sea is discussed, and measuring algorithms of sea surface wind speed and direction are proposed. The obtained results can be used for creation of an airborne radar system for operational measurement of the sea roughness characteristics and for safe landing of a hydroplane on water. (orig.)Available from TIB Hannover: RA 3251(2001/38) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman
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