The analysis and combination of data from different gamma-ray instruments involves the use of collaboration proprietary software and case-by-case methods. The effort of defining a common data format ...for high-level data, namely event lists and instrument response functions (IRFs), has recently started for very-high-energy gamma-ray instruments, driven by the upcoming Cherenkov Telescope Array (CTA). In this work we implemented this prototypical data format for a small set of MAGIC, VERITAS, FACT, and H.E.S.S. Crab nebula observations, and we analyzed them with the open-source gammapy software package. By combining data from Fermi-LAT, and from four of the currently operating imaging atmospheric Cherenkov telescopes, we produced a joint maximum likelihood fit of the Crab nebula spectrum. Aspects of the statistical errors and the evaluation of systematic uncertainty are also commented upon, along with the release format of spectral measurements. The results presented in this work are obtained using open-access on-line assets that allow for a long-term reproducibility of the results.
Aims.
In the presence of a sufficient amount of target material,
γ
-rays can be used as a tracer in the search for sources of Galactic cosmic rays (CRs). Here we present deep observations of the ...Galactic center (GC) region with the MAGIC telescopes and use them to infer the underlying CR distribution and to study the alleged PeV proton accelerator at the center of our Galaxy.
Methods.
We used data from ≈100 h observations of the GC region conducted with the MAGIC telescopes over five years (from 2012 to 2017). Those were collected at high zenith angles (58−70 deg), leading to a larger energy threshold, but also an increased effective collection area compared to low zenith observations. Using recently developed software tools, we derived the instrument response and background models required for extracting the diffuse emission in the region. We used existing measurements of the gas distribution in the GC region to derive the underlying distribution of CRs. We present a discussion of the associated biases and limitations of such an approach.
Results.
We obtain a significant detection for all four model components used to fit our data (Sgr A*, “Arc”, G0.9+0.1, and an extended component for the Galactic Ridge). We observe no significant difference between the
γ
-ray spectra of the immediate GC surroundings, which we model as a point source (Sgr A*) and the Galactic Ridge. The latter can be described as a power-law with index 2 and an exponential cut-off at around 20 TeV with the significance of the cut-off being only 2
σ
. The derived cosmic-ray profile hints to a peak at the GC position and with a measured profile index of 1.2 ± 0.3 is consistent with the 1/
r
radial distance scaling law, which supports the hypothesis of a CR accelerator at the GC. We argue that the measurements of this profile are presently limited by our knowledge of the gas distribution in the GC vicinity.
In the present research, a numerical modeling approach of the initial stage of consolidation during spark plasma sintering on the microscopic scale is presented. The solution of a fully coupled ...thermo-electro-mechanical problem also accounting for grain boundary and surface diffusion is found by using a staggered way. The finite-element method is applied for solving the thermo-electro-mechanical problem while the finite-difference method is applied for the diffusion problem. A Lagrange-based non-linear formulation is used to deal with the detailed description of plastic and creep strain accumulation. The numerical model is developed for simulating the structural evolution of the involved particles during sintering of powder compacts taking into account both the free surface diffusion of the particles and the grain boundary diffusion at interparticle contact areas. The numerical results obtained by using the two-particle model—as a representative volume element of the powder—are compared with experimental results for the densification of a copper powder compact. The numerical and experimental results are in excellent agreement.
Contact growth and temperature behavior in time during a single high-current pulse representing the initial stage of field-assisted/spark plasma sintering (FAST/SPS) have been studied experimentally ...and numerically. The measured evolution of the electrical resistance and of the neck formation process in two-particle systems is compared to the results obtained from the fully coupled thermo-electro-mechanical finite element simulations. The results of simulations with various models of inelastic deformation show that the viscoelastic/viscoplastic material model provides a realistic contact growth in initial stage of FAST/SPS. The impact of electrical and mechanical loads, material parameters and particle size on temperature, on inelastic strain distribution and on densification has been studied by finite element simulations for copper, stainless steel and nickel particles.
The complex transport processes contributing to sintering are not yet fully understood, partially because in-situ observations of sintering in three dimensions (3D) are very difficult. Here we report ...a novel experiment in which monocrystalline copper spheres are first marked with microscopic boreholes drilled using a focused ion beam, after which high-resolution synchrotron X-ray tomography is carried out to measure translational, rolling and intrinsic rotation movements of some hundred spheres during sintering. Unlike in 1D and 2D systems, we show that, in 3D, intrinsic rotations are more pronounced than angular rolling rearrangements of the particle centres and become the dominant mechanism of particle movement. We conclude that in addition to the well-known neck growth and centre approach mechanisms, grain boundary sliding caused by the different crystallographic orientations of the individual spheres occurs.
1ES 1959+650 is a bright TeV high-frequency-peaked BL Lac object exhibiting interesting features like “orphan” TeV flares and broad emission in the high-energy regime that are difficult to interpret ...using conventional one-zone Synchrotron Self-Compton (SSC) scenarios. We report the results from the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) observations in 2016 along with the multi-wavelength data from the
Fermi
Large Area Telescope (LAT) and
Swift
instruments. MAGIC observed 1ES 1959+650 with different emission levels in the very-high-energy (VHE,
E
> 100 GeV)
γ
-ray band during 2016. In the long-term data, the X-ray spectrum becomes harder with increasing flux and a hint of a similar trend is also visible in the VHE band. An exceptionally high VHE flux reaching ∼3 times the Crab Nebula flux was measured by MAGIC on the 13 and 14 of June, and 1 July 2016 (the highest flux observed since 2002). During these flares, the high-energy peak of the spectral energy distribution (SED) lies in the VHE domain and extends up to several TeV. The spectrum in the
γ
-ray (both
Fermi
-LAT and VHE bands) and the X-ray bands are quite hard. On 13 June and 1 July 2016, the source showed rapid variations in the VHE flux within timescales of less than an hour. A simple one-zone SSC model can describe the data during the flares requiring moderate to large values of the Doppler factors (
δ
≥ 30−60). Alternatively, the high-energy peak of the SED can be explained by a purely hadronic model attributed to proton-synchrotron radiation with jet power
L
jet
∼ 10
46
erg s
−1
and under high values of the magnetic field strength (∼100 G) and maximum proton energy (∼few EeV). Mixed lepto-hadronic models require super-Eddington values of the jet power. We conclude that it is difficult to get detectable neutrino emission from the source during the extreme VHE flaring period of 2016.
The First G-APD Cherenkov Telescope (FACT) demonstrates the usability of novel Geiger-mode operated Avalanche Photo Diodes (G-APD, often called SiPM) for Imaging Atmospheric Cherenkov Telescopes ...(IACT). The camera consists of 1440pixels with dedicated electronics operating at 2 Giga samples per second. It is installed on the refurbished HEGRA telescope with a mirror area of ≈9.5m2 on the Canary Island La Palma. FACT is taking data almost every night since the camera was installed in October 2011. It was possible to improve the data taking efficiency to very high values due to the very stable and reliable operation. This also allows to operate FACT remotely without any need for operators on site. Even remote human intervention became less and less frequent over the years, allowing operation to become mostly automatic. FACT is monitoring the long-term behavior of some very-high energy variable extra-galactic sources with unparalleled sampling density as well as testing the behavior of the sensors under severe weather conditions. Due to the long exposure of FACT's G-APDs under strong moonlight conditions it was possible to evaluate the aging effects of G-APDs due to collected charge. No indication of aging was found. No external calibration device is needed to operate FACT since the properties of the sensors themselves allow for a high precision self-calibration of the camera.
•SiPMs show no sign of ageing in 5years of operation.•High precision self calibration is possible without external calibration device.•Very high data taking efficiency has been reach due to excessive automation.
In materials science X‐ray microtomography has evolved as an increasingly utilized technique for characterizing the 3D microstructure of materials. The fundamentals of X‐ray microtomography ...experimental methods and the reconstruction and data evaluation processes are briefly described. A review of in‐situ synchrotron X‐ray microtomography studies in literature is given. Examples of recent work include in‐situ microtomography investiagtions of metallic foams, in‐situ studies of the sintering of copper particles, and in‐situ investigations of creep damage evolution in composites. Future perspectives of in‐situ X‐ray microtomography studies in materials science are outlined.
In materials science X‐ray microtomography has evolved as an increasingly utilized technique for characterizing the 3D microstructure of materials. The fundamentals of X‐ray microtomography experimental methods and the reconstruction and data evaluation processes are briefly described. A review of in‐situ synchrotron X‐ray microtomography studies in literature is given. Examples of recent work include in‐situ microtomography investiagtions of metallic foams, in‐situ studies of the sintering of copper particles, and in‐situ investigations of creep damage evolution in composites. Future perspectives of in‐situ X‐ray microtomography studies in materials science are outlined.
The initial stage of contact formation during field-activated/assisted sintering of powders is investigated experimentally and numerically for spherical particles of nickel, steel, and copper. The ...results of experimental studies of the contact resistance and the neck formation process due to melting/sintering in two- to three-particle systems under a single high-current pulse from a capacitor discharge are compared with results of fully coupled thermo-electrical finite-element simulations. The impact of particle size, contact geometry, and electrical load on the temperature and temperature gradient distribution during the pulse discharge is analyzed numerically for nickel spheres. The influence of the material itself is compared for nickel, steel, and copper systems.
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