Context. The wealth of recent data from Imaging Air Cherenkov telescopes (IACTs), ultra-high energy cosmic-ray experiments and neutrino telescopes have fuelled a renewed interest in hadronic emission ...models for γ-loud blazars. Aims. We explore physically plausible solutions for a lepto-hadronic interpretation of the stationary emission from high-frequency peaked BL Lac objects (HBLs). The modelled spectral energy distributions are then searched for specific signatures at very high energies that could help to distinguish the hadronic origin of the emission from a standard leptonic scenario. Methods. By introducing a few basic constraints on parameters of the model, such as assuming the co-acceleration of electrons and protons, we significantly reduced the number of free parameters. We then systematically explored the parameter space of the size of the emission region and its magnetic field for two bright γ-loud HBLs, PKS 2155-304 and Mrk 421. For all solutions close to equipartition between the energy densities of protons and of the magnetic field, and with acceptable jet power and light-crossing timescales, we inspected the spectral hardening in the multi-TeV domain from proton-photon induced cascades and muon-synchrotron emission inside the source. Very-high-energy spectra simulated with the available instrument functions from the future Cherenkov Telescope Array (CTA) were evaluated for detectable features as a function of exposure time, source redshift, and flux level. Results. A range of hadronic scenarios are found to provide satisfactory solutions for the broad band emission of the sources under study. The TeV spectrum can be dominated either by proton-synchrotron emission or by muon-synchrotron emission. The solutions for HBLs cover a parameter space that is distinct from the one found for the most extreme BL Lac objects in an earlier study. Over a large range of model parameters, the spectral hardening due to internal synchrotron-pair cascades, the “cascade bump”, should be detectable for acceptable exposure times with the future CTA for a few nearby and bright HBLs.
Aims.The extragalactic background light (EBL) in the ultraviolet to far-infrared wavelength region carries important information about galaxy and star formation history. Direct measurements are ...difficult, especially in the mid infrared region. We derive limits on the EBL density from the energy spectra of distant sources of very high energetic γ-rays (VHE γ-rays). Methods.The VHE γ-rays are attenuated by the photons of the EBL via pair production, which leaves an imprint on the measured spectra from distant sources. So far, there are 14 detected extragalactic sources of VHE γ-rays, 13 of which are TeV blazars. With physical assumptions about the intrinsic spectra of these sources, limits on the EBL can be derived. In this paper we present a new method of deriving constraints on the EBL. Here, we use only very basic assumptions about TeV blazar physics and no pre-defined EBL model, but instead a large number of generic shapes constructed from a grid in EBL density vs. wavelength. In our study we utilize spectral data from all known TeV blazars, making this the most complete study so far. Results.We derive limits on the EBL for three individual TeV blazar spectra (Mkn 501, H 1426+428, 1ES 1101-232) and for all spectra combined. Combining the results from individual spectra leads to significantly stronger constraints over a wide wavelength range from the optical (~$1\,\mu$m) to the far-infrared (∼$80\,\mu$m). The limits are only a factor of 2 to 3 above the absolute lower limits derived from source counts. In the mid-infrared our limits are the strongest constraints derived from TeV blazar spectra so far over an extended wavelength range. A high density of the EBL around 1$\,\mu$m, reported by direct detection experiments, can be excluded. Conclusions.Our results can be interpreted in two ways. (i) The EBL is almost resolved by source counts, leaving only a little room for additional components, such as the first stars, or (ii) the assumptions about the underlying physics are not valid, which would require substantial changes in the standard emission models of TeV blazars.
Very high energy (VHE, energy E ≲ 100 GeV) γ-rays from cosmological sources are attenuated due to the interaction with photons of the extragalactic background light (EBL) in the ultraviolet to ...infrared wavelength band. The EBL, thus, leaves an imprint on the observed energy spectra of these objects. In the last four years, the number of extragalactic VHE sources discovered with imaging atmospheric Cherenkov telescopes (IACTs), such as MAGIC, H.E.S.S., and VERITAS, has doubled. Furthermore, the measurements of the Fermi satellite brought new insights into the intrinsic spectra of the sources at GeV energies. In this paper, upper limits on the EBL intensity are derived by considering the most extensive VHE source sample ever used in this context. This is accomplished by constructing a large number of generic EBL shapes and combining spectral informations from Fermi and IACTs together with minimal assumptions about the source physics at high and very high γ-ray energies. The evolution of the EBL with redshift is accounted for and the possibility of the formation of an electromagnetic cascade and the implications on the upper limits are explored. The EBL density at z = 0 is constrained over a broad wavelength range between 0.4 and 100 μm. At optical wavelengths, the EBL density is constrained below 24 nW m-2 sr-1 and below 5 nW m-2 sr-1 between 8 μm and 31 μm.
The two MAGIC imaging atmospheric Cherenkov telescopes (IACTs) are located on the Canary island of La Palma. Their cameras are currently equipped with more than 1000 photomultiplier tubes (PMTs). We ...developed a silicon photomultiplier (SiPM) based detector module to evaluate the feasibility of a camera upgrade using only solid state detectors. We combined SiPMs to a composite matrix in order to achieve the same active area as the 1inch PMTs currently in use. This first prototype was installed in 2015 and is operated alongside the PMTs. Utilizing the collected data, we developed two more SiPM based detector modules. They are equipped with SiPMs from two different suppliers. These three different SiPM based detector modules will enable us to compare three different manufacturers of SiPMs under real observational conditions. Most important is the comparison to the neighbouring PMT modules.
•The performance expectations at high levels of background light are IACTs described.•The operational stability of DC and temperature is presented.•A calibration procedure for gain and cross talk is demonstrated.•A comparison to PMTs is shown utilizing the described calibration method.
Vector representations of physical quantities in pseudo-Euclidean space are used to discuss several options for an appropriate coordinate system used to express a system of units. The SI system is ...found to have several issues causing the system of units to be overdetermined and underdetermined. We propose a version of the SI system that takes into account the three-dimensional nature of physical space and eliminates “dimensional isotopes,” i.e., quantities with different physical meaning but identical dimensions in the SI system. We demonstrate that the mathematical expression for the vector norm of a measurement unit, referred to as a “system unit,” can be used to determine whether a unit is within the range of variation in such a system unit.
Context. The formation of the first stars (Population III; PopIII) marks the end of the dark ages of the universe, a subject of lively scientific debate. Not (yet) accessible to direct observations, ...this early stage of the universe is mostly studied via theoretical calculations and numerical simulations. An indirect window is provided by integrated present day observables such as the metal abundance or the diffuse extragalactic photon fields. Aims. We aim to derive constraints on the properties of the PopIII and low metallicity Population II (LM PopII) stars utilizing limits on the density of the extragalactic background light (EBL), recently derived from very-high-energy ($E>100$ GeV; VHE) observations. Methods. A model calculation for the evolving EBL density produced by PopIII/LM PopII stars is presented. The model utilizes stellar population spectra (SPS) for zero and low metallicity stars and accounts for the changing emission of an aging stellar population. Emission from the dense HII regions surrounding the stars (nebula) is included. The resulting EBL density for different scenarios (metallicity, star formation rate, initial mass function) is compared to the limit on the EBL density. The potential for detecting a cut-off in HE/VHE spectra is discussed. Results. Assuming a maximum contribution from PopIII/LM PopII stars to the EBL density of 5 nW m-2 s-1 at 2 μm, a limit on the star formation rate (SFR) of the first stars of 0.3 to 3 $M_\odot$ Mpc-3 yr-1 in the redshift range $7{-}14$ is derived. The limit depends on the assumed shape of the SFR and metallicity. Conclusions. The EBL can be used as a probe to investigate the properties of PopIII/LM PopII stars. Limits on the EBL density derived from VHE observations can provide constraints on the parameters of the these stars, in particular the star formation rate.
Girders with corrugated steel web are preferred and widely used in recently constructed bridges and industrial buildings. Sandwich core girders with corrugated web are constructed by adding two ...plates (skins) to the corrugated web. This study aims to investigate the shear span-to-depth ratio impact on the performance of sandwich core steel girders with corrugated web. Three span-to-depth ratios (a/d): 1.0, 1.833 and 2.5, were examined. The test includes three girders with sandwich web thickness of 30 mm, three girders with 60 mm sandwich web thickness, and three girders with conventional flat webs. A total of nine simply supported steel girders subjected to a concentrated load were fabricated and tested up to failure. The responses of the examined girders are presented in term of the load deflection curves, the ultimate load, and the maximum displacement. Among the conclusions drawn in this study that girders with sandwich core thickness of 30 mm demonstrate higher ultimate load capacity than girders with sandwich core of 60 mm, the maximum difference in the ultimate load capacity was about 20% and can be seen at a/d equals to 1.0. The results also pointed out that the behavior of the beams was noticeably impacted by the shear span-to-depth ratio.
Abstract
The Cherenkov Telescope Array (CTA) is the next-generation ground-based very-high-energy gamma-ray observatory. By using three types of telescopes CTA can cover a wide energy range (20 ...GeV–300 TeV) with an order of magnitude higher sensitivity than the current telescopes. The Large-Sized Telescope (LST) is designed to detect 20 GeV–1 TeV gamma rays thanks to the large light collection area, sensitive photosensors, a fast trigger system, and readout electronics. The camera readout system must have a high signal-to-noise ratio and a linear signal sampling with a large dynamic range in order to efficiently detect dim and low-energy atmospheric showers. To meet this requirement we use the Domino Ring Sampler version 4 (DRS4), which also enables ultra-fast sampling with low power consumption. Some of the intrinsic characteristics of DRS4 chips require software corrections. These procedures lower the effect of non-Gaussian noise contribution and improve the timing resolution of the system. In this contribution we discuss the calibration algorithms and the resulting performance.
The MAGIC collaboration operates two 17m diameter Imaging Atmospheric Cherenkov Telescopes (IACTs) on the Canary Island of La Palma. Each of the two telescopes is currently equipped with a ...photomultiplier tube (PMT) based imaging camera. Due to the advances in the development of Silicon Photomultipliers (SiPMs), they are becoming a widely used alternative to PMTs in many research fields including gamma-ray astronomy. Within the Otto-Hahn group at the Max Planck Institute for Physics, Munich, we are developing a SiPM based detector module for a possible upgrade of the MAGIC cameras and also for future experiments as, e.g., the Large Size Telescopes (LST) of the Cherenkov Telescope Array (CTA). Because of the small size of individual SiPM sensors (6mm×6mm) with respect to the 1-inch diameter PMTs currently used in MAGIC, we use a custom-made matrix of SiPMs to cover the same detection area. We developed an electronic circuit to actively sum up and amplify the SiPM signals. Existing non-imaging hexagonal light concentrators (Winston cones) used in MAGIC have been modified for the angular acceptance of the SiPMs by using C++ based ray tracing simulations. The first prototype based detector module includes seven channels and was installed into the MAGIC camera in May 2015. We present the results of the first prototype and its performance as well as the status of the project and discuss its challenges.
•The design of the first SiPM large-size IACT pixel is described.•The simulation of the light concentrators is presented.•The temperature stability of the detector module is demonstrated.•The calibration procedure of SiPM device in the field is described.
Detection of low-intensity light relies on the conversion of photons to photoelectrons, which are then multiplied and detected as an electrical signal. To measure the actual intensity of the light, ...one must know the factor by which the photoelectrons have been multiplied. To obtain this amplification factor, we have developed a procedure for estimating precisely the signal caused by a single photoelectron. The method utilizes the fact that the photoelectrons conform to a Poisson distribution. The average signal produced by a single photoelectron can then be estimated from the number of noise events, without requiring analysis of the distribution of the signal produced by a single photoelectron. The signal produced by one or more photoelectrons can be estimated experimentally without any assumptions. This technique, and an example of the analysis of a signal from a photomultiplier tube, are described in this study.
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