The minijets-in-a-jet statistical model and the RMS-flux correlation

The flux variability of blazars at very high energies does not have a clear origin. Flux variations on time scales down to the minute suggest that variability originates in the jet, where a relativistic boost can shorten the observed time scale, while the linear relation between the flux and its RMS or the skewness of the flux distribution suggests that the variability stems from multiplicative processes, which are associated in some models with the accretion disk. We study the RMS-flux relation and emphasize its link to Pareto distributions, characterized by a power-law probability density function. Such distributions are naturally generated within a minijets- in-a-jet statistical model, in which boosted emitting regions are isotropically oriented within the bulk relativistic flow of a jet. We prove that, within this model, the flux of a single minijet is proportional to its RMS. This relation still holds when considering a large number of emitting regions, for which the distribution of the total flux is skewed and could be interpreted as being log-normal. The minijets-in-a-jet statistical model reconciles the fast variations and the statistical properties of the flux of blazars at very high energies.Comment: 6 pages, 6 figures, accepted in A

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

Cascading on extragalactic background light

High-energy gamma-rays propagating in the intergalactic medium can interact with background infrared photons to produce e+e- pairs, resulting in the absorption of the intrinsic gamma-ray spectrum. TeV observations of the distant blazar 1ES 1101-232 were thus recently used to put an upper limit on the infrared extragalactic background light density. The created pairs can upscatter background photons to high energies, which in turn may pair produce, thereby initiating a cascade. The pairs diffuse on the extragalactic magnetic field (EMF) and cascade emission has been suggested as a means for measuring its intensity. Limits on the IR background and EMF are reconsidered taking into account cascade emissions. The cascade equations are solved numerically. Assuming a power-law intrinsic spectrum, the observed 100 MeV - 100 TeV spectrum is found as a function of the intrinsic spectral index and the intensity of the EMF. Cascades emit mainly at or below 100 GeV. The observed TeV spectrum appears softer than for pure absorption when cascade emission is taken into account. The upper limit on the IR photon background is found to be robust. Inversely, the intrinsic spectra needed to fit the TeV data are uncomfortably hard when cascade emission makes a significant contribution to the observed spectrum. An EMF intensity around 1e-8 nG leads to a characteristic spectral hump in the GLAST band. Higher EMF intensities divert the pairs away from the line-of-sight and the cascade contribution to the spectrum becomes negligible.Comment: 5 pages, to be published as a research note in A&