184 research outputs found
Discovery of Very High Energy gamma-ray emission from the distant BL Lac object H2356-309 with the H.E.S.S. Cherenkov Telescopes
texte intégral disponible sur http://proc.sf2a.asso.fr/2006/2006sf2a.conf..0193S.pdfInternational audienceThe H.E.S.S.(High Energy Stereoscopic System) array of imaging Cherenkov telescopes has recently discovered the most distant extra-galactic Very High Energy (VHE, E>160GeV) gamma-ray emitters. Located at a redshift of z=0.165, the extreme synchrotron BL Lac object H2356-309 was observed with H.E.S.S. in a simultaneous multi-wavelength campaign including radio, optical and X-ray observatories. Using a single zone model, we will describe the constraints on VHE emission parameters such as the maximal Lorentz factors of accelerated electrons, and the magnetic field. We will also show the constraints that this source, as well as the other VHE gamma-ray emitter 1ES1101-232, puts on the density of extragalactic background light, which is an important cosmological topic
Vascular Liver Disorders, Portal Vein Thrombosis, and Procedural Bleeding in Patients With Liver Disease:2020 Practice Guidance by the American Association for the Study of Liver Diseases
An overview of the current understanding of bleeding and thrombosis in cirrhosis. An evidence-based justification for bleeding risk assessment in patients with cirrhosis prior to invasive procedures, including current concepts in preprocedural testing and laboratory analysis and their role in predicting bleeding complications. An outline of established and recently identified risk factors for venous thrombosis in the portal and hepatic venous systems in both patients with and without cirrhosis along with thrombophilia testing recommendations
Relevance of axion-like particles for very-high-energy astrophysics
Many extensions of the Standard Model predict the existence of ALPs, which
are very light spin-zero bosons with a two-photon coupling. Photon-ALP
oscillations occur in the presence of an external magnetic field, and ALPs can
lead to observable effects on the measured photon spectrum of astrophysical
sources. An intriguing situation arises when blazars are observed with the
Cherenkov Telescopes H.E.S.S., MAGIC, CANGAROO III and VERITAS. The
extragalactic background light (EBL) produced by galaxies during cosmic
evolution gives rise to a source dimming which becomes important in the VHE
band. This dimming can be considerably reduced by photon-ALP oscillations in
the large-scale magnetic fields, and the resulting blazar spectra become harder
than expected. We find that for ALPs lighter than 5 x 10^{-10} eV the photon
survival probability is larger than predicted by conventional physics above a
few hundred GeV. This is a clear-cut prediction which can be tested with the
planned Cherenkov Telescope Array and HAWC. Moreover, we offer a new
interpretation of the VHE blazars detected so far, according to which the large
spread in the values of the observed spectral index is mainly due to the wide
spread in the source distances rather than to large variations of their
internal physical properties.Comment: Equations (8), (104) and (134) have been corrected for misprints; see
the erratum published in Phys. Rev. D 87, 109903(E) (2013). Numerical results
in the article are not affected, since they were based on the correct
formula
First detection of a VHE gamma-ray spectral maximum from a Cosmic source: H.E.S.S. discovery of the Vela X nebula
The Vela supernova remnant (SNR) is a complex region containing a number of
sources of non-thermal radiation. The inner section of this SNR, within 2
degrees of the pulsar PSR B0833-45, has been observed by the H.E.S.S. gamma-ray
atmospheric Cherenkov detector in 2004 and 2005. A strong signal is seen from
an extended region to the south of the pulsar, within an integration region of
radius 0.8 deg. around the position (RA = 08h 35m 00s, dec = -45 deg. 36'
J2000.0). The excess coincides with a region of hard X-ray emission seen by the
ROSAT and ASCA satellites. The observed energy spectrum of the source between
550 GeV and 65 TeV is well fit by a power law function with photon index = 1.45
+/- 0.09(stat) +/- 0.2(sys) and an exponential cutoff at an energy of 13.8 +/-
2.3(stat) +/- 4.1(sys) TeV. The integral flux above 1 TeV is (1.28 +/- 0.17
(stat) +/- 0.38(sys)) x 10^{-11} cm^{-2} s^{-1}. This result is the first clear
measurement of a peak in the spectral energy distribution from a VHE gamma-ray
source, likely related to inverse Compton emission. A fit of an Inverse Compton
model to the H.E.S.S. spectral energy distribution gives a total energy in
non-thermal electrons of ~2 x 10^{45} erg between 5 TeV and 100 TeV, assuming a
distance of 290 parsec to the pulsar. The best fit electron power law index is
2.0, with a spectral break at 67 TeV.Comment: 5 pages, 4 figures, accepted for publication in Astronomy and
Astrophysics letter
3.9 day orbital modulation in the TeV gamma-ray flux and spectrum from the X-ray binary LS 5039
New observations of LS 5039, a High Mass X-ray Binary comprising a massive
star and compact object, were carried out with the High Energy Stereoscopic
System of Cherenkov Telescopes (H.E.S.S.) in 2005 at very high energy (VHE)
gamma-ray energies. These observations reveal that its flux and energy spectrum
are modulated with the 3.9 day orbital period of the binary system. This is the
first time in gamma-ray astronomy that orbital modulation has been observed,
and periodicity clearly established using ground-based gamma-ray detectors. The
VHE gamma-ray emission is largely confined to half of the orbit, peaking around
the inferior conjunction epoch of the compact object. For this epoch, there is
also a hardening of the energy spectrum in the energy range between 0.2 TeV and
a few TeV. The flux vs. orbital phase profile provides the first clear
indication of gamma-ray absorption via pair production within an astrophysical
source, a process which is expected to occur if the gamma-ray production site
is situated within ~1 AU of the compact object. Moreover the production region
size must be not significantly greater than the binary separation (~0.15 AU).
Notably, these constraints are also considerably smaller than the collimated
outflows or jets (extending out to ~1000 AU) observed in LS 5039. The spectral
hardening could arise from variations with phase in the maximum electron
energies, and/or the dominant VHE gamma-ray production mechanism.Comment: 8 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
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
Discovery of Very High Energy Gamma-Ray Emission from the BL Lac Object H2356-309 with the H.E.S.S. Cherenkov Telescopes
The extreme synchrotron BL Lac object H2356-309, located at a redshift of z =
0.165, was observed from June to December 2004 with a total exposure of approx.
40 h live-time with the H.E.S.S. (High Energy Stereoscopic System) array of
atmospheric-Cherenkov telescopes (ACTs). Analysis of this data set yields, for
the first time, a strong excess of 453 gamma-rays (10 standard deviations above
background) from H2356-309, corresponding to an observed integral flux above
200 GeV of I(>200GeV) = (4.1+-0.5) 10^12 cm^-2.s^-1 (statistical error only).
The differential energy spectrum of the source between 200 GeV and 1.3 TeV is
well-described by a power law with a normalisation (at 1 TeV) of N_0 = (3.00 +-
0.80_stat +- 0.31_sys) 10^-13 cm^-2.s^-1.TeV^-1 and a photon index of Gamma =
3.09 +- 0.24_stat +- 0.10_sys. H2356-309 is one of the most distant BL Lac
objects detected at very-high-energy gamma-rays so far. Results from
simultaneous observations from ROTSE-III (optical), RXTE (X-rays) and NRT
(radio) are also included and used together with the H.E.S.S. data to constrain
a single-zone homogeneous synchrotron self-Compton (SSC) model. This model
provides an adequate fit to the H.E.S.S. data when using a reasonable set of
model parameters.Comment: 7 pages, 4 figures, accepted for publication in Astronomy and
Astrophysics (05/07/2006
A detailed spectral and morphological study of the gamma-ray supernova remnant RX J1713.7-3946 with H.E.S.S
We present results from deep observations of the Galactic shell-type
supernova remnant (SNR) RX J1713.7-3946 (also known as G347.3-0.5) conducted
with the complete H.E.S.S. array in 2004. Detailed morphological and spatially
resolved spectral studies reveal the very-high-energy (VHE -- Energies E > 100
GeV) gamma-ray aspects of this object with unprecedented precision. Since this
is the first in-depth analysis of an extended VHE gamma-ray source, we present
a thorough discussion of our methodology and investigations of possible sources
of systematic errors. Gamma rays are detected throughout the whole SNR. The
emission is found to resemble a shell structure with increased fluxes from the
western and northwestern parts. The differential gamma-ray spectrum of the
whole SNR is measured over more than two orders of magnitude, from 190 GeV to
40 TeV, and is rather hard with indications for a deviation from a pure power
law at high energies. Spectra have also been determined for spatially separated
regions of RX J1713.7-3946. The flux values vary by more than a factor of two,
but no significant change in spectral shape is found. There is a striking
correlation between the X-ray and the gamma-ray image. Radial profiles in both
wavelength regimes reveal the same shape almost everywhere in the region of the
SNR. The VHE gamma-ray emission of RX J1713.7-3946 is phenomenologically
discussed for two scenarios, one where the gamma rays are produced by VHE
electrons via Inverse Compton scattering and one where the gamma rays are due
to neutral pion decay from proton-proton interactions. In conjunction with
multi-wavelength considerations, the latter case is favoured. However, no
decisive conclusions can yet be drawn regarding the parent particle population
dominantly responsible for the gamma-ray emission from RX J1713.7-3946.Comment: 20 pages, 20 figures (low resolution), Accepted for publication in
A&A - Revision 1: Added reference to Section 4.2. Added sentence to
acknowledgement
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