Investigating systematics in the energy reconstruction of the H.E.S.S. telescopes

H.E.S.S. is an array of four Imaging Atmospheric Cherenkov Telescopes that aims at exploring the non-thermal universe by means of photons with energies between 100 Ge V and 100 TeV. These very-high-energy (VHE) gamma-rays can be detected and their energy can be reconstructed by observing the Cherenkov light of extensive particle showers the VHE gamma-rays induce in the atmosphere. This work presents systematical studies of the H.E.S.S. energy reconstruction. The differences in the responses of the individual telescopes are tested and it is investigated whether such asymmetries have an effect on the overall accuracy of the energy reconstruction. Therefore the H.E.S.S. Standard Analysis is tested using Monte Carlo simulations and VHE gamma-ray data sets obtained from the observation of the Crab Nebula and the active galactic nucleus PKS 2155-304. Minor differences in the telescope responses at percentage level are found, which have increased slightly during the last five years, mirroring the decay of the optical components of the telescopes. However, the effect of these inter-telescope systematics on the energy reconstruction is negligible, especially when compared to the ≈ 17% overall energy resolution of the experiment

Studies on the interaction of riboflavin 5'-phosphate with proteins with special attention to bacterial bioluminescence

The central theme of this thesis is the interaction of FMN with proteins. For one of the proteins studied, the enzyme luciferase from bacteria, further investigations were done on the process of light emission.In chapter 2 and 3 studies are reported on the binding of FMN with relatively simple proteins, the flavodoxins. Flavodoxins were chosen, because they are small proteins with a molecular weight not higher than 23 000. They contain only one equivalent of FMN and consist of only one polypeptide chain. No other prosthetic group is known for the flavodoxins.I n addition to this they can be obtained in high yields from bacterial cultures. These features make the flavodoxins excellent objects for studies on the binding of flavins to proteins.In chapter 2 studies on the binding of FMN by apoflavodoxin from Peptostreptococcus elsdenii are reported. Conclusions were drawn from the dependence on the pH and the NaCl concentration. The rate constant of dissociation depends on the pH, even when extrapolated to zero ionic strength. The titration curve of this rate constant can be explained, by the assumption of the involvment of two protonations, that act highly cooperatively. It should be realised that these protonations do not influence the rate constant of association. But once the complex is formed, the chance of falling apart, if these sites are protonated, is around 6 times higher than without these sites protonated. Furthermore, it was found that the calculated rate constant of association, when extrapolated to zero ionic strength is independent on the pH. At increasing ionic strength this rate constant of association will change. Depending on the pH 1 value, this change is a decrease or an increase in value. At a pH of 3.8 there is almost no change with increasing ionic strength, but above this pH value the rate constant increases, while below this value it decreases with increasing ionic strength. This explains why the combination of a high salt concentration and a low pH is a very effective way of removing the FMN from flavodoxins. This finding might possibly be extrapolated to other flavoproteins as well. By interpreting the results in terms of the Brönsted theory, a net positive charge between 11 and 12 is found on the apoenzyme at low pH. This finding is in agreement with the number of basic amino acid residues in the polypeptide chain.A series of flavin analogues were synthesised and the kinetic parameters of the interaction with Azotobacter vinelandii apoflavodoxin investigated, These studies are presented in chapter 3. Use was made of a fast kinetic method, the temperature jump relaxation technique. The resolution time of the instrument employed is 11 microseconds. All complexes studied revealed only one relaxation process, indicating that within the time limits studied (11 micro seconds - ca. 10 seconds), the association of the flavin and the apoenzyme is a one-step process. This finding is in contrast with an earlier publication by other authors, who detected two relaxation processes. It is shown that the earlier published traces are instrumental artifacts.In chapter 4 the interaction of FMN with an intermediate in the in vitro bacterial bioluminescence reaction is described. The so called "longlived lntermediate", which has been suggested to be an FMN flavoprotein, hao been separated into an apoprotein and free FMN. Because of the high quantum yields of light with :respect to FMN, measured upon reaction of the apoprotein with aldehyde, in vitro bacterial bioluminescence appears to be a sensitised reaction. At a first consideration only FMN could be a likely candidate as a sensitising agent.However, in chapter 5 it is shown that a novel protein, isolated from the bacteria itself will definitely sensitise the in vitro bacterial bioluminescence reaction. This novel protein (BFP) is efficiently fluorescent (quantum yield of fluorescence 0.45) and has an emission maximum at 476 nm. As a result of these observations, it is called the blue fluoreseence protein. By diluting this protein to a concentration of around 1 μM, a spectral shift of the emission maximum is observed. Actually the fluorescence emission spectrum of the protein changes from a spectrum identical to the In vivo bacterial bioluminescence into an emission spectrum identical to the in vitro bacterial bioluminescence emission. Although this means that the emission of this protein could account for both the in vivo and the in vitro emission spectra, it should be mentioned that investigations learned that the chromophore of this protein is not a product of the in vitro reaction. This blue fluorescence protein is the only one of all the emitters proposed so far, that simulates the bluest of the bacterial emissions exactly. Furthermore, the addition of the blue fluorescence protein to the in vitro reaction affects the light emission kinetics it acts as a catalyst), increases the light yield and induces a shift to shorter wavelengths in the bioluminescence emission. Together with the fact that the protein is isolated from the bacteria themselves, these features are strong evidence that this protein is the in vivo emitter.In chapter 6 the purification procedure of the BFP is given in more detail. Furthermore it is shown that it can be isolated from at least two of the four common species of marine bioluminescent bacteria. This suggests that all the bacteria emit their light via the same kind of chemical mechanism. Although the proteins from these two species of bacteria appear to have similar molecular weights, they differ in that the protein from P.fischeri is more tightly associated with the luciferase during the purification procedure than the one from P.phosphoreum and also that its fluorescence excitation maximum is shifted about 10 nm to shorter wavelength. Further investigation should be done in order to learn what the chemical nature of the fluorophore is.<p/

Investigating systematics in the energy reconstruction of the H.E.S.S. telescopes

H.E.S.S. is an array of four Imaging Atmospheric Cherenkov Telescopes that aims at exploring the non-thermal universe by means of photons with energies between 100 Ge V and 100 TeV. These very-high-energy (VHE) gamma-rays can be detected and their energy can be reconstructed by observing the Cherenkov light of extensive particle showers the VHE gamma-rays induce in the atmosphere. This work presents systematical studies of the H.E.S.S. energy reconstruction. The differences in the responses of the individual telescopes are tested and it is investigated whether such asymmetries have an effect on the overall accuracy of the energy reconstruction. Therefore the H.E.S.S. Standard Analysis is tested using Monte Carlo simulations and VHE gamma-ray data sets obtained from the observation of the Crab Nebula and the active galactic nucleus PKS 2155-304. Minor differences in the telescope responses at percentage level are found, which have increased slightly during the last five years, mirroring the decay of the optical components of the telescopes. However, the effect of these inter-telescope systematics on the energy reconstruction is negligible, especially when compared to the ≈ 17% overall energy resolution of the experiment

Lunar science prior to Apollo 11

Evolutional aspects and geological interpretations in lunar scienc

A high resolution scintillating fiber tracker with SiPM readout for the PEBS experiment

Using thin scintillating fibers with Silicon Photomultiplier (SiPM) readout a mo dular high-resolution charged-particle tracking detector has been designed. The fiber modules consist of 2 x 5 layers of 128 round multiclad scintillating fiber s of 0.250mm diameter. The fibers are read out by four SiPM arrays (8mm x 1mm) e ach on either end of the module.Comment: 6 pages, 5 figures, presented at the ICATPP 1

Charting the TeV Milky Way: H.E.S.S. Galactic plane survey maps, catalog and source populations

Very-high-energy (VHE, E>100 GeV) gamma-rays provide a unique view of the non-thermal universe, tracing the most violent and energetic phenomena at work inside our Galaxy and beyond. The latest results of the H.E.S.S. Galactic Plane Survey (HGPS) undertaken by the High Energy Stereoscopic System (H.E.S.S.), an array of four imaging atmospheric Cherenkov telescopes located in Namibia, are described here. The HGPS aims at the detection of cosmic accelerators with environments suitable for the production of photons at the highest energies and has led to the discovery of an unexpectedly large and diverse population of over 60 sources of TeV gamma rays within its current range of l = 250 to 65 degrees in longitude and |b|<3.5 degrees in latitude. The data set of the HGPS comprises 2800 hours of high-quality data, taken in the years 2004 to 2013. The sensitivity for the detection of point-like sources, assuming a power-law spectrum with a spectral index of 2.3 at a statistical significance of 5 sigma, is now at the level of 2% Crab or better in the core HGPS region. The latest maps of the inner Galaxy at TeV energies are shown alongside an introduction to the first H.E.S.S. Galactic Plane Survey catalog. Finally, in addition to an overview of the H.E.S.S. Galactic source population a few remarkable, recently discovered sources will be highlighted.Comment: 8 pages, 6 figures, in Proceedings of the 48th Rencontres de Moriond (2013), La Thuile (Italy

Mean-field approximations of networks of spiking neurons with short-term synaptic plasticity

Low-dimensional descriptions of neural network dynamics are an effective tool for bridging different scales of organization of brain structure and function. Recent advances in deriving mean-field descriptions for networks of coupled oscillators have sparked the development of a new generation of neural mass models. Of notable interest are mean-field descriptions of all-to-all coupled quadratic integrate-and-fire (QIF) neurons, which have already seen numerous extensions and applications. These extensions include different forms of short-term adaptation (STA) considered to play an important role in generating and sustaining dynamic regimes of interest in the brain. It is an open question, however, whether the incorporation of pre-synaptic forms of synaptic plasticity driven by single neuron activity would still permit the derivation of mean-field equations using the same method. Here, we discuss this problem using an established model of short-term synaptic plasticity at the single neuron level, for which we present two different approaches for the derivation of the mean-field equations. We compare these models with a recently proposed mean-field approximation that assumes stochastic spike timings. In general, the latter fails to accurately reproduce the macroscopic activity in networks of deterministic QIF neurons with distributed parameters. We show that the mean-field models we propose provide a more accurate description of the network dynamics, although they are mathematically more involved. Using bifurcation analysis, we find that QIF networks with pre-synaptic short-term plasticity can express regimes of periodic bursting activity as well as bi-stable regimes. Together, we provide novel insight into the macroscopic effects of short-term synaptic plasticity in spiking neural networks, as well as two different mean-field descriptions for future investigations of such networks

A Scintillating Fiber Tracker With SiPM Readout

We present a prototype for the first tracking detector consisting of 250 micron thin scintillating fibers and silicon photomultiplier (SiPM) arrays. The detector has a modular design, each module consists of a mechanical support structure of 10mm Rohacell foam between two 100 micron thin carbon fiber skins. Five layers of scintillating fibers are glued to both top and bottom of the support structure. SiPM arrays with a channel pitch of 250 micron are placed in front of the fibers. We show the results of the first module prototype using multiclad fibers of types Bicron BCF-20 and Kuraray SCSF-81M that were read out by novel 32-channel SiPM arrays from FBK-irst/INFN Perugia as well as 32-channel SiPM arrays produced by Hamamatsu. A spatial resolution of 88 micron +/- 6 micron at an average yield of 10 detected photons per minimal ionizig particle has been achieved.Comment: 5 pages, 7 figures, submitted as proceedings to the 11th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD08