471 research outputs found
GLAST and Very High Energy astrophysics
The pioneering age of high energy gamma-ray astrophysics has come to an end for ground-based observatories as the 3rd generation of Atmospheric ˇCerenkov Telescopes (ACTs) come online. Meanwhile the next generation space-based observatory GLAST is being assembled and is scheduled for launch in 2007. At that point, gamma-ray astrophysics will enter again in a period comparable to the CGRO/EGRET epoch where the combination of ground based and space based observatories reshaped our knowledge in many ways. The instruments have evolved into more performant and efficient machines, making the two techniques very different in their conception, but getting ever closer in their energy ranges. Some aspects and constraints in the two techniques are described, focussing on a few scientific topics that would benefit from a coordinated approach
Lognormal variability in BL Lacertae
X-ray data from the blazar BL Lac are used to investigate the nature of its
variability, and more precisely the flux dependency of the variability and the
distribution of fluxes. The variations in the flux are found to have a
lognormal distribution and the average amplitude of variability is proportional
to the flux level. BL Lac is the first blazar in which lognormal X-ray
variability is clearly detected. Lognormal variability in X-ray light curves,
probably related to accretion disk activity, has been discovered in various
compact systems, such as Seyfert galaxies and X-ray binaries. The light curve
is orders of magnitude less variable than other blazars, with few bursting
episodes. If this defines a specific state of the source, then the lognormality
might be the imprint of the accretion disk on the jet, linking for the first
time accretion and jet properties in a blazar.Comment: Accepted for Astronomy & Astrophysic
Evidence for a cosmological effect in \gamma-ray spectra of BL Lacs
We update the list of GeV-TeV extragalactic \gamma-ray sources using the
2-year catalog from the Fermi-LAT and recent results ground-based \gamma-ray
telescopes. Breaks in the spectra between the high energy (100 MeV <E< 300 GeV)
and the very high energy (E> 200 GeV) ranges, and their dependence on distance,
are discussed in the context of absorption on the extragalactic background
light (EBL). We calculate the size of the expected break using a model for the
EBL and compare it to the data taking into account systematic uncertainties in
the measurements. We develop a novel Bayeasian model to describe this dataset
and use it to constrain two simple models for the EBL-induced breaks.Comment: accepted for publication in A&
New AGNs discovered by H.E.S.S
During the last year, six new Active Galactic Nuclei (AGN) have been
discovered and studied by H.E.S.S. at Very High Energies (VHE). Some of these
recent discoveries have been made thanks to new enhanced analysis methods and
are presented at this conference for the first time. The three blazars 1ES
0414+009, SHBL J001355.9-185406 and 1RXS J101015.9-311909 have been targeted
for observation due to their high levels of radio and X-ray fluxes, while the
Fermi/LAT catalogue of bright sources triggered the observation of PKS 0447-439
and AP Librae. Additionally, the BL Lac 1ES 1312-423 was discovered in the
field-of-view (FoV) of Centaurus A thanks to the large exposure dedicated by
H.E.S.S. to this particularly interesting source. The newly-discovered sources
are presented here and in three companion presentations at this conference.Comment: 8 pages, 3 figures, proceeding from the 25th Texas Symposium on
Relativistic Astrophysics (Heidelberg, Germany, 2010
Cooling Tests of the NectarCAM camera for the Cherenkov Telescope Array
The NectarCAM is a camera proposed for the medium-sized telescopes in the
framework of the Cherenkov Telescope Array (CTA), the next-generation
observatory for very-high-energy gamma-ray astronomy. The cameras are designed
to operate in an open environment and their mechanics must provide protection
for all their components under the conditions defined for the CTA observatory.
In order to operate in a stable environment and ensure the best physics
performance, each NectarCAM will be enclosed in a slightly overpressurized,
nearly air-tight, camera body, to prevent dust and water from entering. The
total power dissipation will be ~7.7 kW for a 1855-pixel camera. The largest
fraction is dissipated by the readout electronics in the modules. We present
the design and implementation of the cooling system together with the test
bench results obtained on the NectarCAM thermal demonstrator.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
The Ross procedure versus repair for treatment of a unicuspid aortic valve in adults
OBJECTIVES
Aortic stenosis or regurgitation in patients with a unicuspid valve morphology requires interventions early in life. We have performed either primary valve repair or the Ross procedure. The goal of this study was to compare the midterm results of repair and pulmonary autograft replacement.
METHODS
Between December 1998 and April 2022, a total of 345 patients (77% male; mean age 34 ± 9.7 years) underwent treatment of a unicuspid aortic valve. Patients were excluded if they were 54 years (n = 3) at the time of the operation. The remaining cohort was divided into 2 groups: 167 (64%) patients underwent valve repair; 91 (36%) patients underwent pulmonary autograft replacement.
The indications for surgery were aortic regurgitation (n = 104), aortic stenosis (n = 45), combined disease (n = 103) and endocarditis (n = 6). Fifty-one patients had root dilatation (>43 mm) with aortic regurgitation (repair n = 23; Ross n = 28). Mean follow-up was 5.9 years (SD: 5 years) [range 0.1–22.3 years].
RESULTS
There were 1 early and 3 late deaths; 47 patients required reintervention. Survival at 10 years was 95% in the Ross group and 97% after valve repair (P = 0.769). Freedom from reintervention at 10 years was 98% in the Ross group and 80% after valve repair (P = 0.012). A receiver operating characteristics curve analysis showed a trend towards better durability in patients < 26 years.
CONCLUSIONS
The ideal treatment of the unicuspid aortic valve remains debatable. Repair of a unicuspid valve can be considered a bridge to pulmonary autograft replacement, at least in younger patients. The appropriate times to replace and to repair require further investigation
A Spectacular VHE Gamma-Ray Outburst from PKS 2155-304 in 2006
Since 2002 the VHE (>100 GeV) gamma-ray flux of the high-frequency peaked BL
Lac PKS 2155-304 has been monitored with the High Energy Stereoscopic System
(HESS). An extreme gamma-ray outburst was detected in the early hours of July
28, 2006 (MJD 53944). The average flux above 200 GeV observed during this
outburst is ~7 times the flux observed from the Crab Nebula above the same
threshold. Peak fluxes are measured with one-minute time scale resolution at
more than twice this average value. Variability is seen up to ~600 s in the
Fourier power spectrum, and well-resolved bursts varying on time scales of ~200
seconds are observed. There are no strong indications for spectral variability
within the data. Assuming the emission region has a size comparable to the
Schwarzschild radius of a ~10^9 solar mass black hole, Doppler factors greater
than 100 are required to accommodate the observed variability time scales.Comment: 4 pages, 3 figures; To appear in the Proceedings of the 30th ICRC
(Merida, Mexico
Which Aortic Valve Can Be Surgically Reconstructed?
Purpose of Review
Preservation or repair of the aortic valve has evolved dynamically in the past 20 years. It leads to a high freedom from valve-related complications if an adequate valve durability can be achieved; it may possibly also improve survival. To date, little structured information is available about which valves can be repaired and which should better be replaced.
Recent Findings
For surgical decision-making, the size of the aortic root is important and the anatomy of the aortic valve must be considered. In the presence of root aneurysm, most tricuspid and bicuspid aortic valves can be preserved. In aortic regurgitation and normal aortic dimensions, the majority of tricuspid and bicuspid aortic valves can be repaired with good long-term durability. In bicuspid aortic valves, the morphologic characteristics must be taken into consideration. Unicuspid and quadricuspid aortic valves can be repaired in selected cases. Generally, cusp calcification is a sign of a poor substrate for repair; the same is true for cusp retraction and cusp destruction due to active endocarditis. They are associated with limited valve durability.
Summary
Using current concepts, many non-calcified aortic valves can be repaired. Modern imaging, in particular three-dimensional transesophageal echocardiography (TEE), should be able to define repairable aortic valves with a high probability
Tipping point: kanteling en terugslag in Rotterdam
Stedelijke ontwikkeling wordt vaak gezien als de uitkomst vanplanmatige en doelgerichte interventies in de fysieke en socialestructuren van een stad. De feitelijke ontwikkeling lijkt echtervaak meer een toevallige samenkomst van omstandigheden daneen vooruitgedacht plan. Rotterdam is hierin geen uitzondering,waar interne sturing en externe dynamiek geregeld zorgen vooronverwachte uitkomsten die de originele situatie ruw kunnen doenkantelen. Deze bijdrage richt zich op de onverwachte wendingen diestedelijke ontwikkeling kenmerken en daardoor onvoorspelbaarmaken. Aan de hand van het begrip tipping point of kantelpuntworden deze wendingen in vijf casussen verkend.info:eu-repo/semantics/publishe
Discovery of VHE gamma-rays from RGB J0152+017
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.
We report recent observations of this source made in late October and
November 2007 with the H.E.S.S. array consisting of four imaging atmospheric
Cerenkov telescopes. Contemporaneous observations were made in X-rays with the
Swift and RXTE satellites, in the optical band with the ATOM telescope, and in
the radio band with the Nancay Radio Telescope.
As a result, RGB J0152+017 is discovered as a source of VHE gamma-rays by
H.E.S.S. A signal of 173 gamma-ray photons corresponding to a statistical
significance of 6.6 sigmas 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 (extended jet and blob in jet) non-thermal
synchrotron self-Compton (SSC) leptonic model, plus a thermal host galaxy
component. The parameters that are found are very close to those found for TeV
blazars in similar SSC studies.
The location of its synchrotron peak, as derived from the SED in Swift data,
allows clear classification as a high-frequency-peaked BL Lac (HBL).Comment: 6 pages, 5 figures; accepted to appear in the proceedings of the
Workshop on Blazar Variability across the Electromagnetic Spectrum, 22-25
April 2008, Palaiseau, Franc
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