The Energy Spectrum of TeV Gamma-Rays from the Crab Nebula as measured by the HEGRA system of imaging air Cherenkov telescopes

The Crab Nebula has been observed by the HEGRA (High-Energy Gamma-Ray Astronomy) stereoscopic system of imaging air Cherenkov telescopes (IACTs) for a total of about 200 hrs during two observational campaigns: from September 1997 to March 1998 and from August 1998 to April 1999. The recent detailed studies of system performance give an energy threshold and an energy resolution for gamma-rays of 500 GeV and ~ 18%, respectively. The Crab energy spectrum was measured with the HEGRA IACT system in a very broad energy range up to 20 TeV, using observations at zenith angles up to 65 degrees. The Crab data can be fitted in the energy range from 1 to 20 TeV by a simple power-law, which yields dJg/dE = (2.79+/-0.02 +/- 0.5) 10^{-7} E^{-2.59 +/- 0.03 +/- 0.05}, ph m^{-2} s^{-1} TeV^{-1} The Crab Nebula energy spectrum, as measured with the HEGRA IACT system, agrees within 15% in the absolute scale and within 0.1 units in the power law index with the latest measurements by the Whipple, CANGAROO and CAT groups, consistent within the statistical and systematic errors quoted by the experiments. The pure power-law spectrum of TeV gamma-rays from the Crab Nebula constrains the physics parameters of the nebula environment as well as the models of photon emission.Comment: to appear in ApJ, 29 pages, 6 figure

Correlated intense X-ray and TeV activity of Mrk~501 in 1998 June

We present exactly simultaneous X-ray and TeV monitoring with {\it RXTE} and HEGRA of the TeV blazar Mrk 501 during 15 days in 1998 June. After an initial period of very low flux at both wavelengths, the source underwent a remarkable flare in the TeV and X-ray energy bands, lasting for about six days and with a larger amplitude at TeV energies than in the X-ray band. At the peak of the TeV flare, rapid TeV flux variability on sub-hour timescales is found. Large spectral variations are observed at X-rays, with the 3--20 keV photon index of a pure power law continuum flattening from $\Gamma=2.3$ to $\Gamma=1.8$ on a timescale of 2--3 days. This implies that during the maximum of the TeV activity, the synchrotron peak shifted to energies $\gtrsim 50$ keV, a behavior similar to that observed during the longer-lasting, more intense flare in 1997 April. The TeV spectrum during the flare is described by a power law with photon index $\Gamma=1.9$ and an exponential cutoff at $\sim$ 4 TeV; an indication for spectral softening during the flare decay is observed in the TeV hardness ratios. Our results generally support a scenario where the TeV photons are emitted via inverse Compton scattering of ambient seed photons by the same electron population responsible for the synchrotron X-rays. The simultaneous spectral energy distributions (SEDs) can be fit with a one-zone synchrotron-self Compton model assuming a substantial increase of the magnetic field and the electron energy by a factor of 3 and 10, respectively.Comment: Accepted for publication in ApJ, Part

TeV Gamma-ray Observations of the Crab and Mkn 501 during Moonshine and Twilight

TeV Gamma-ray signals from the Crab Nebula and Mkn 501 were detected with the HEGRA CT1 imaging Cerenkov telescope during periods when the moon was shining and during twilight. This was accomplished by lowering the high voltage supply of the photomutipliers in fixed steps up to 13%. No other adjustments were made and no filters were used. Laser runs could not establish any non-linearity in the gain of the individual pixels, and the trigger rate was uniform over the whole camera. The energy threshol was increased by up to a factor of two, depending on the amount of HV reduction. In a series of observations lasting 11.7 hours, a signal with a 3.4 sigma significance was detected from the Crab. During the 1997 multiple flare episode of Mkn 501 a 26 sigma combined excess has been recorded during 134 hours of observations under various moonshine/twilight conditions. The results show that this technique can easily be adapted to increase the exposure of a source, which is important for sources showing rapid time variability such as AGNs or GRBs. Observations can be made up to ~20 deg. angular separation from the moon and until the moon is 85% illuminated (ten to eleven days before and after new moon), as well as during 20 to 40 minutes during twilight, before the commencement of astronomical darkness.Comment: 16 pages, 5 figures, submitted to Astroparticle Physic

Search for point sources of gamma radiation above 15 TeV with the HEGRA AIROBICC array

A search for potential point sources of very high energy gamma rays has been carried out on the data taken simultaneously by the HEGRA AIROBICC and Scintillator arrays from August 1994 to March 2000. The list of sought sources includes supernova remnants, pulsars, AGNs and binary systems. The energy threshold is around 15 TeV. For the Crab Nebula, a modest excess of 2.5 standard deviations above the cosmic ray background has been observed. Flux upper limits (at 90% c.l.) of around 1.3 times the flux of the Crab Nebula are obtained, in average, for the candidate sources. A different search procedure has been used for an all-sky search which yields absolute flux upper limits between 4 and 9 crabs depending on declination, in the band from δ = 0 to δ = 60°.F. Aharonian...G. Rowell...et al

Exploring why quality circles work in primary health care: a realist review protocol

BACKGROUND: Quality circles (QCs) are commonly used in primary health care in Europe to consider and improve standard practice over time. They represent a complex social intervention that occurs within the fast-changing system of primary health care. Numerous controlled trials, reviews, and studies have shown small but unpredictable positive effect sizes on behavior change. Although QCs seem to be effective, stakeholders have difficulty understanding how the results are achieved and in generalizing the results with confidence. They also lack understanding of the active components of QCs which result in changes in the behavior of health care professionals. This protocol for a realist synthesis will examine how configurations of components and the contextual features of QCs influence their performance. METHODS/DESIGN: Stakeholder interviews and a scoping search revealed the processes of QCs and helped to describe their core components and underlying theories. After clarifying their historical and geographical distribution, a purposive and systematic search was developed to identify relevant papers to answer the research questions, which are: understanding why, how, and when QCs work, over what time frame, and in what circumstances. After selecting and abstracting appropriate data, configurations of contexts and mechanisms which influence the outcome of QCs within each study will be identified. Studies will be grouped by similar propositional statements in order to identify patterns and validation from stakeholders sought. Finally, theories will be explored in order to explain these patterns and to help stakeholders maintain and improve QC performance. DISCUSSION: Analyzing context-mechanism-outcome (CMO) patterns will reveal how QCs work and how contextual factors interact to influence their outcome. The aim is to investigate unique configurations that enable them to improve the performance of health care professionals. Using a standardized reporting system, this realist review will allow the research questions to be answered to the satisfaction of key stakeholders and enable on-going critical examination and dissemination of the findings. STUDY REGISTRATION: PROSPERO registration number: CRD42013004826

OG 3.2.24 1 Anisotropies of Cosmic Rays and Search for Intergalactic

Data taken with the HEGRA Scintillator Array within 2.5 years have been used to search for spatial anisotropies in the arrival directions of cosmic rays (E # 20 TeV). For this purpose partial sky survey maps are produced, carefully taking the detector response and changing conditions in the atmosphere into account. In this paper, results on the search for TeV #--emission correlated with the Galactic Plane and the Gould Belt, a local Galactic region with an enhanced concentration of young bright stars, molecular and dust clouds, are presented. The upper limit for the emission from the Galactic disc of # # =#CR # 10 for jb g j#5 (b g denoting the Galactic latitude) and E # 42 TeV imposes constraints on the extrapolation of the #-ray flux measured by EGRET. Furthermore, a search for extended TeV #--emission correlated with the arrival directions of the cosmic rays with energies well beyond the Greisen-Zatsepin-Kuzmin cutoff has been performed. For this purpose, the directions of 10 events with energies beyond 5 # 10 eV including the events belonging to three AGASA clusters and the most energetic event (E = 320 EeV) detected by the Fly's eye detector have been examined for correlated TeV emission. Such a correlation can be expected from sufficiently high energetic nuclei inducing electromagnetic cascades on the diffuse intergalactic (2.7 K) background radiation, eventually producing TeV photons. Interestingly, the direction of the most energetic event yields the highest excess with a chance probability of 1.8%

Neural Networks for

Due to the large amount of data and the complexity of the information, analysis of the HEGRA Geiger tower data is based on neural networks (NN). We explain the NN training and the principal characteristics of the analysis and describe first applications to experimental data, yielding an upper limit on the combined fl-ray flux above 50 TeV of 16 nearby northern blazars. 1 Introduction The success of current and future earth-based air shower arrays in detecting point sources of cosmic rays above some 10 TeV depends crucially on the possibility of finding efficient methods for separating fl-induced air showers from the overwhelming background of hadron-induced showers. As shown in [1], this is successfully done by taking advantage of the Cherenkov light distribution, but fl/hadron separation is also possible using the charged particle component of air showers. Due to their multi-layer structure, the Geiger towers of the HEGRA air shower array [2] are able to contribute to fl/hadro..