350 research outputs found
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
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
Study of the reaction e^{+}e^{-} -->J/psi\pi^{+}\pi^{-} via initial-state radiation at BaBar
We study the process with
initial-state-radiation events produced at the PEP-II asymmetric-energy
collider. The data were recorded with the BaBar detector at center-of-mass
energies 10.58 and 10.54 GeV, and correspond to an integrated luminosity of 454
. We investigate the mass
distribution in the region from 3.5 to 5.5 . Below 3.7
the signal dominates, and above 4
there is a significant peak due to the Y(4260). A fit to
the data in the range 3.74 -- 5.50 yields a mass value
(stat) (syst) and a width value (stat)(syst) for this state. We do not
confirm the report from the Belle collaboration of a broad structure at 4.01
. In addition, we investigate the system
which results from Y(4260) decay
Oxygen-sensitive 3He-MRI in bronchiolitis obliterans after lung transplantation
Oxygen-sensitive 3He-MRI was studied for the detection of differences in intrapulmonary oxygen partial pressure (pO2) between patients with normal lung transplants and those with bronchiolitis obliterans syndrome (BOS). Using software developed in-house, oxygen-sensitive 3He-MRI datasets from patients with normal lung grafts (n = 8) and with BOS (n = 6) were evaluated quantitatively. Datasets were acqiured on a 1.5-T system using a spoiled gradient echo pulse sequence. Underlying diseases were pulmonary emphysema (n = 10 datasets) and fibrosis (n = 4). BOS status was verified by pulmonary function tests. Additionally, 3He-MRI was assessed blindedly for ventilation defects. Median intrapulmonary pO2 in patients with normal lung grafts was 146 mbar compared with 108 mbar in patients with BOS. Homogeneity of pO2 distribution was greater in normal grafts (standard deviation pO2 34 versus 43 mbar). Median oxygen decrease rate during breath hold was higher in unaffected patients (−1.75 mbar/s versus −0.38 mbar/s). Normal grafts showed fewer ventilation defects (5% versus 28%, medians). Oxygen-sensitive 3He-MRI appears capable of demonstrating differences of intrapulmonary pO2 between normal lung grafts and grafts affected by BOS. Oxygen-sensitive 3He-MRI may add helpful regional information to other diagnostic techniques for the assessment and follow-up of lung transplant recipients
Upper Limits from HESS AGN Observations in 2005-2007
AIMS: Very high energy (VHE; E>100 GeV) gamma-ray studies were performed for
18 active galactic nuclei (AGN) from a variety of AGN classes.
METHODS: VHE observations of a sample of 14 AGN, considered candidate VHE
emitters, were made with the High Energy Stereoscopic System (HESS) between
January 2005 and July 2007. Large-zenith-angle observations of three northern
AGN (Mkn 421, Mkn 501, 1ES 1218+304), known to emit VHE gamma rays, were also
performed in order to sample their spectral energy distributions (SEDs) above 1
TeV. In addition, the VHE flux from 1ES 1101-232, previously detected by HESS
in 2004-2005, was monitored during 2006 and 2007.
RESULTS: As significant detections from the HESS observation program are
reported elsewhere, the results reported here are primarily integral flux upper
limits. The average exposure for each of the 14 VHE-candidate AGN is ~7 h live
time, and the observations have an average energy threshold between 230 GeV and
590 GeV. Upper limits for these 14 AGN range from <0.9% to <4.9% of the Crab
Nebula flux, and eight of these are the most constraining ever reported for the
object. The brief (<2.2 h each) large-zenith-angle observations yield upper
limits for Mkn 501 (<20% Crab above 2.5 TeV) and 1ES 1218+304 (<17% Crab above
1.0 TeV), and a marginal detection (3.5 sigma) of Mkn 421 (50% Crab above 2.1
TeV). 1ES 1101-232 was marginally detected (3.6 sigma, 1.7% Crab above 260 GeV)
during the 2006 (13.7 h live time) observations, but not in the 2007 (4.6 h
live time) data. The upper limit in 2007 (<1.9% Crab above 260 GeV) is below
the average flux measured by HESS from 2004-2006.Comment: 8 Pages, 2 Figures; Accepted on Nov 6, 2007 for publication in
Astronomy & Astrophysic
Discovery of very-high-energy gamma-ray emission from the vicinity of PSR J1913+1011 with H.E.S.S
The H.E.S.S. experiment, an array of four Imaging Atmospheric Cherenkov
Telescopes with high sensitivity and large field-of-view, has been used to
search for emitters of very-high-energy (VHE, >100 GeV) gamma-rays along the
Galactic plane, covering the region 30 deg < l < 60 deg, 280 deg < l < 330 deg,
and -3 deg < b < 3 deg. In this continuation of the H.E.S.S. Galactic Plane
Scan, a new extended VHE gamma-ray source was discovered at
alpha(2000)=19h12m49s, delta(2000)=+10d09'06'' (HESS J1912+101). Its integral
flux between 1-10 TeV is ~10% of the Crab Nebula flux in the same energy range.
The measured energy spectrum can be described by a power law with a photon
index Gamma = 2.7+-0.2(stat)+-0.3(sys). HESS J1912+101 is plausibly associated
with the high spin-down luminosity pulsar PSR J1913+1011. We also discuss
associations with an as yet unconfirmed SNR candidate proposed from low
frequency radio observation and/or with molecular clouds found in 13CO data.Comment: 6 pages, 2 figures; Accepted for publication in A&A on February 20,
200
HESS VHE Gamma-Ray Sources Without Identified Counterparts
The detection of gamma rays in the very-high-energy (VHE) energy range (100
GeV--100 TeV) provides a direct view of the parent population of
ultra-relativistic particles found in astrophysical sources. For this reason,
VHE gamma rays are useful for understanding the underlying astrophysical
processes in non-thermal sources. We investigate unidentified VHE gamma-ray
sources that have been discovered with HESS in the most sensitive blind survey
of the Galactic plane at VHE energies conducted so far. The HESS array of
imaging atmospheric Cherenkov telescopes (IACTs) has a high sensitivity
compared with previous instruments(~ 0.01 Crab) in 25 hours observation time
for a 5 sigma point-source detection), and with its large field of view, is
well suited for scan-based observations. The on-going HESS survey of the inner
Galaxy has revealed a large number of new VHE sources, and for each we attempt
to associate the VHE emission with multi-wavelength data in the radio through
X-ray wavebands. For each of the eight unidentified VHE sources considered
here, we present the energy spectra and sky maps of the sources and their
environment. The VHE morphology is compared with available multi-wavelength
data (mainly radio and X-rays). No plausible counterparts are found
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
Spin alignment of leading mesons in hadronic decays
Helicity density matrix elements for inclusive K*(892)^0 mesons from hadronic Z^0 decays have been measured over the full range of K^*0 momentum using data taken with the OPAL experiment at LEP. A preference for occupation of the helicity zero state is observed at all scaled momentum x_p values above 0.3, with the matrix element rho_00 rising to 0.66 +/- 0.11 for x_p > 0.7. The values of the real part of the off-diagonal element rho_1-1 are negative at large x_p, with a weighted average value of -0.09 +/- 0.03 for x_p > 0.3, in agreement with new theoretical predictions based on Standard Model parameters and coherent fragmentation of the qq(bar) system from the Z^0 decay. All other helicity density matrix elements measured are consistent with zero over the entire x_p range. The K^*0 fragmentation function has also been measured and the total rate determined to be 0.74 +/- 0.02 +/- 0.02 K*(892)^0 mesons per hadronic Z^0 decay.Helicity density matrix elements for inclusive K*(892)^0 mesons from hadronic Z^0 decays have been measured over the full range of K^*0 momentum using data taken with the OPAL experiment at LEP. A preference for occupation of the helicity zero state is observed at all scaled momentum x_p values above 0.3, with the matrix element rho_00 rising to 0.66 +/- 0.11 for x_p > 0.7. The values of the real part of the off-diagonal element rho_1-1 are negative at large x_p, with a weighted average value of -0.09 +/- 0.03 for x_p > 0.3, in agreement with new theoretical predictions based on Standard Model parameters and coherent fragmentation of the qq(bar) system from the Z^0 decay. All other helicity density matrix elements measured are consistent with zero over the entire x_p range. The K^*0 fragmentation function has also been measured and the total rate determined to be 0.74 +/- 0.02 +/- 0.02 K*(892)^0 mesons per hadronic Z^0 decay.Helicity density matrix elements for inclusive K*(892)^0 mesons from hadronic Z^0 decays have been measured over the full range of K^*0 momentum using data taken with the OPAL experiment at LEP. A preference for occupation of the helicity zero state is observed at all scaled momentum x_p values above 0.3, with the matrix element rho_00 rising to 0.66 +/- 0.11 for x_p > 0.7. The values of the real part of the off-diagonal element rho_1-1 are negative at large x_p, with a weighted average value of -0.09 +/- 0.03 for x_p > 0.3, in agreement with new theoretical predictions based on Standard Model parameters and coherent fragmentation of the qq(bar) system from the Z^0 decay. All other helicity density matrix elements measured are consistent with zero over the entire x_p range. The K^*0 fragmentation function has also been measured and the total rate determined to be 0.74 +/- 0.02 +/- 0.02 K*(892)^0 mesons per hadronic Z^0 decay.Helicity density matrix elements for inclusive K*(892)^0 mesons from hadronic Z^0 decays have been measured over the full range of K^*0 momentum using data taken with the OPAL experiment at LEP. A preference for occupation of the helicity zero state is observed at all scaled momentum x_p values above 0.3, with the matrix element rho_00 rising to 0.66 +/- 0.11 for x_p > 0.7. The values of the real part of the off-diagonal element rho_1-1 are negative at large x_p, with a weighted average value of -0.09 +/- 0.03 for x_p > 0.3, in agreement with new theoretical predictions based on Standard Model parameters and coherent fragmentation of the qq(bar) system from the Z^0 decay. All other helicity density matrix elements measured are consistent with zero over the entire x_p range. The K^*0 fragmentation function has also been measured and the total rate determined to be 0.74 +/- 0.02 +/- 0.02 K*(892)^0 mesons per hadronic Z^0 decay.Helicity density matrix elements for inclusive K ∗ (892) 0 mesons from hadronic Z 0 decays have been measured over the full range of K ∗ 0 momentum using data taken with the OPAL experiment at LEP. A preference for occupation of the helicity zero state is observed at all scaled momentum x p values above 0.3, with the matrix element ϱ 00 rising to 0.66 ± 0.11 for x p > 0.7. The values of the real part of the off-diagonal element ϱ 1 - 1 are negative at large x p , with a weighted average value of −0.09 ± 0.03 for x p > 0.3, in agreement with new theoretical predictions based on Standard Model parameters and coherent fragmentation of the q q system from the Z 0 decay. All other helicity density matrix elements measured are consistent with zero over the entire x p range. The K ∗ 0 fragmentation function has also been measured and the total rate determined to be 0.74 ± 0.02 ± 0.02 K ∗ (892) 0 mesons per hadronic Z 0 decay
A search for neutral Higgs bosons in the MSSM and models with two scalar field doublets
A search is described for the neutral Higgs bosons h^0 and A^0 predicted by models with two scalar field doublets and, in particular, the Minimal Supersymmetric Standard Model (MSSM). The search in the Z^0 h^0 and h^0 A^0 production channels is based on data corresponding to an integrated luminosity of 25 pb^{-1} from e^+e^- collisions at centre-of-mass energies between 130 and 172GeV collected with the OPAL detector at LEP. The observation of a number of candidates consistent with Standard Model background expectations is used in combination with earlier results from data collected at the Z^0 resonance to set limits on m_h and m_A in general models with two scalar field doublets and in the MSSM. For example, in the MSSM, for tan(beta) > 1, minimal and maximal scalar top quark mixing and soft SUSY-breaking masses of 1 TeV, the 95% confidence level limits m_h > 59.0 GeV and m_A > 59.5 GeV are obtained. For the first time, the MSSM parameter space is explored in a detailed scan.A search is described for the neutral Higgs bosons h^0 and A^0 predicted by models with two scalar field doublets and, in particular, the Minimal Supersymmetric Standard Model (MSSM). The search in the Z^0 h^0 and h^0 A^0 production channels is based on data corresponding to an integrated luminosity of 25 pb^{-1} from e^+e^- collisions at centre-of-mass energies between 130 and 172 GeV collected with the OPAL detector at LEP. The observation of a number of candidates consistent with Standard Model background expectations is used in combination with earlier results from data collected at the Z^0 resonance to set limits on m_h and m_A in general models with two scalar field doublets and in the MSSM. For example, in the MSSM, for tan(beta) > 1, minimal and maximal scalar top quark mixing and soft SUSY-breaking masses of 1 TeV, the 95% confidence level limits m_h > 59.0 GeV and m_A > 59.5 GeV are obtained. For the first time, the MSSM parameter space is explored in a detailed scan
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