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The prevalence of falls among neurological patients is unknown, although disturbances of gait and posture are common. Falls may lead to burdens for the patient, the caregivers and the health system. We designed a prospective study and investigated all patients for a history of falls admitted to a neurological hospital during a 100-day period. Clinical investigation was carried out and several disease specific rating scales were applied. A total of 548 patients were investigated. Of all patients 34% had fallen once or more often during the last twelve months. A disturbance of gait was blamed for the fall in 55%, epileptic seizures in 12%, syncope in 10 % and stroke in 7%. Intrinsic risk factors for falls were high age, disturbed gait, poor balance and a fear of falling. As extrinsic factors we identified the treatment with antidepressants, neuroleptics and different cardiovascular medications, adverse environmental factors in the patients' home and the use of walking aids. Within the diagnoses, falls were most frequent in Parkinson's disease (62 %), syncope (57%) and polyneuropathy (48 %). According to these findings falls in neurological in-patients are twice as frequent as in an age-matched population living in the community. Falls in neurological patients are particularly linked to medication and disorders affecting gait and balance.

Photon counts statistics have recently been proven to provide a sensitive observable for characterizing gamma-ray source populations and for measuring the composition of the gamma-ray sky. In this work, we generalize the use of the standard 1-point probability distribution function (1pPDF) to decompose the high-latitude gamma-ray emission observed with Fermi-LAT into: (i) point-source contributions, (ii) the Galactic foreground contribution, and (iii) a diffuse isotropic background contribution. We analyze gamma-ray data in five adjacent energy bands between 1 and 171 GeV. We measure the source-count distribution dN/dS as a function of energy, and demonstrate that our results extend current measurements from source catalogs to the regime of so far undetected sources. Our method improves the sensitivity for resolving point-source populations by about one order of magnitude in flux. The dN/dS distribution as a function of flux is found to be compatible with a broken power law. We derive upper limits on further possible breaks as well as the angular power of unresolved sources. We discuss the composition of the gamma-ray sky and capabilities of the 1pPDF method.

The continuum high-energy gamma-ray emission between 1 GeV and 100 TeV from the Crab Nebula has been measured for the first time in overlapping energy bands by the Fermi large-area telescope (Fermi/LAT) below ~ 100 GeV and by ground-based imaging air Cherenkov telescopes (IACTs) above ~ 60 GeV. To follow up on the phenomenological approach suggested by Hillas et al. (1998), the broad band spectral and spatial measurement (from radio to low-energy gamma-rays < 1 GeV) is used to extract the shape of the electron spectrum. While this model per construction provides an excellent description of the data at energies < 1 GeV, the predicted inverse Compton component matches the combined Fermi/LAT and IACT measurements remarkably well after including all relevant seed photon fields and fitting the average magnetic field to B = (124 +/- 6 (stat.) +15 / -6 (sys.)) {\\mu}G. The close match of the resulting broad band inverse Compton component with the combined Fermi/LAT and IACTs data is used to derive instrument specific energy-calibration factors. These factors can be used to combine data from Fermi/LAT and IACTs without suffering from systematic uncertainties on the common energy scale. As a first application of the cross calibration, we derive an upper limit to the diffuse gamma-ray emission between 250 GeV and 1 TeV based upon the combined measurements of Fermi/LAT and the H.E.S.S. ground-based Cherenkov telescopes. Finally, the predictions of the magneto-hydrodynamic flow model of Kennel & Coroniti (1984) are compared to the measured SED.

Common extensions of the Standard Model of particle physics predict the existence of a \"hidden\" sector that comprises particles with a vanishing or very weak coupling to particles of the Standard Model (visible sector). For very light (m < 10^-14 eV) hidden U(1) gauge bosons (hidden photons), broad-band radio spectra of compact radio sources could be modified due to weak kinetic mixing with radio photons. Here, search methods are developed and their sensitivity discussed, with specific emphasis on the effect of the coherence length of the signal, instrumental bandwidth, and spectral resolution. We conclude that radio observations in the frequency range of 0.03--1400 GHz probe kinetic mixing of ~10^-3 of hidden photons with masses down to ~10^-17 eV. Prospects for improving the sensitivity with future radio astronomical facilities as well as by stacking data from multiple objects are discussed.

The standard paradigm of hierarchical structure formation in a LambdaCDM universe predicts the presence of dark matter subhaloes, hosted by Milky Way-sized galaxies. Anticipated subhalo masses range from 10^{10} down to a cut-off mass between 10^{-3} and 10^{-11} solar masses. If dark matter is composed of heavy self-annihilating or decaying particles, these subhaloes could be visible in the gamma-ray band as faint and temporally constant sources without astrophysical counterparts. Based upon realistic subhalo models and current observational constraints on annihilating dark matter scenarios, we predict that one massive Galactic subhalo between 10^6 and 10^8 solar masses may already be present in the 11-month catalogue of Fermi-LAT. Indeed, at least twelve objects in the first Fermi catalogue qualify as candidates. The most promising object, 1FGL J0030.7+0724, is investigated in detail using a dedicated Swift X-ray follow-up observation and a refined positional analysis of the 24-month Fermi-LAT data. With the new observations, seven point-like X-ray sources have been discovered, of which SWIFT J003119.8+072454, which coincides with a faint radio source (12 mJy at 1.4 GHz), serves as a counterpart candidate of 1FGL J0030.7+0724. The broad-band spectral energy distribution is consistent with a high-energy-peaked blazar. However, flux and extent of 1FGL J0030.7+0724 may also be compatible with a dark matter subhalo. Detection of temporal variability or improved astrometry of 1FGL J0030.7+0724 are necessary to rule out or confirm an astrophysical origin. We discuss strategies to identify gamma-ray sources that are associated with self-annihilating dark matter subhaloes.

Very high-energy gamma-rays (VHE; E>100 GeV) have been detected from the direction of the Galactic Centre up to energies E>10 TeV. Up to now, the origin of this emission is unknown due to the limited positional accuracy of the observing instruments. One of the counterpart candidates is the super-massive black hole (SMBH) Sgr A*. If the VHE emission is produced within ~10^{15} cm ~1000 r_G (r_G=G M/c^2 is the Schwarzschild radius) of the SMBH, a decrease of the VHE photon flux in the energy range 100--300 GeV is expected whenever an early type or giant star approaches the line of sight within ~ milli-arcseconds (mas). The dimming of the flux is due to absorption by pair-production of the VHE photons in the soft photon field of the star, an effect we refer to as pair-production eclipse (PPE). Based upon the currently known orbits of stars in the inner arcsecond of the Galaxy we find that PPEs lead to a systematic dimming in the 100--300 GeV band at the level of a few per cent and lasts for several weeks. Since the PPE affects only a narrow energy band and is well correlated with the passage of the star, it can be clearly discriminated against other systematic or even source-intrinsic effects. While the effect is too small to be observable with the current generation of VHE detectors, upcoming high count-rate experiments like the Cherenkov telescope array (CTA) will be sufficiently sensitive. Measuring the temporal signature of the PPE bears the potential to locate the position and size of the VHE emitting region within the inner 1000 r_G or in the case of a non-detection exclude the immediate environment of the SMBH as the site of gamma-ray production altogether.

The Cherenkov Telescope Array (CTA) is a project for a next-generation observatory for very high energy (GeV-TeV) ground-based gamma-ray astronomy, currently in its design phase, and foreseen to be operative a few years from now. Several tens of telescopes of 2-3 different sizes, distributed over a large area, will allow for a sensitivity about a factor 10 better than current instruments such as H.E.S.S, MAGIC and VERITAS, an energy coverage from a few tens of GeV to several tens of TeV, and a field of view of up to 10 deg. In the following study, we investigate the prospects for CTA to study several science questions that influence our current knowledge of fundamental physics. Based on conservative assumptions for the performance of the different CTA telescope configurations, we employ a Monte Carlo based approach to evaluate the prospects for detection. First, we discuss CTA prospects for cold dark matter searches, following different observational strategies: in dwarf satellite galaxies of the Milky Way, in the region close to the Galactic Centre, and in clusters of galaxies. The possible search for spatial signatures, facilitated by the larger field of view of CTA, is also discussed. Next we consider searches for axion-like particles which, besides being possible candidates for dark matter may also explain the unexpectedly low absorption by extragalactic background light of gamma rays from very distant blazars. Simulated light-curves of flaring sources are also used to determine the sensitivity to violations of Lorentz Invariance by detection of the possible delay between the arrival times of photons at different energies. Finally, we mention searches for other exotic physics with CTA.

An updated model for the synchrotron and inverse Compton emission from a population of high energy electrons of the Crab Nebula is used to reproduce the measured spectral energy distribution from radio to high energy gamma-rays. By comparing the predicted inverse Compton component with recent Fermi measurements of the nebula's emission, it is possible to determine the average magnetic field in the nebula and to derive the underlying electron energy distribution. The model calculation can then be used to cross calibrate the Fermi observations with ground based air shower measurements. The resulting energy calibration factors are derived and can be used for combining broad energy measurements taken with Fermi in conjunction with ground based measurements.

Predicted by hierarchical structure formation, Milky Way-type galaxies are anticipated to host numerous dark matter subhalos with masses between 10^{10} and a cut-off of 10^{-6} solar masses, or even less. In self-annihilating dark matter scenarios, these objects could be visible in the gamma-ray band as faint and non-variable sources without astrophysical counterpart. In accordance with realistic subhalo models and current observational constraints on self-annihilating dark matter, we predict that about one massive Galactic subhalo may have already been detected in the 11-months catalog of Fermi-LAT data (1FGL). Selection cuts applied to the 1FGL reveal twelve possible candidates, and in-depth studies of the most promising object, 1FGL J0030.7+0724, are presented. In a dedicated X-ray follow-up observation with the Swift XRT, seven point-like X-ray sources have been discovered. Within the positional uncertainty derived from the 24-months Fermi-LAT data, we consider the unidentified radio source NVSS J003119+072456, coinciding with one of the discovered Swift sources, as the most promising counterpart candidate for 1FGL J0030.7+0724. The broad-band spectral energy distribution is consistent with a high-energy-peaked blazar. However, flux and extent of the Fermi source may also be compatible with a dark matter subhalo. A discrimination between the two scenarios requires further multi-wavelength observations. Strategies for identifying gamma-ray sources associated with self-annihilating DM subhalos are discussed.

Very high energy (VHE, \\(E>\\)100 GeV) \\(\\gamma\\)-ray flaring activity of the high-frequency peaked BL Lac object \\pg\\ has been detected by the \\hess\\ telescopes. The flux of the source increased by a factor of 3 during the nights of 2012 April 26 and 27 with respect to the archival measurements with hint of intra-night variability. No counterpart of this event has been detected in the \\fla\\ data. This pattern is consistent with VHE \\(\\gamma\\) ray flaring being caused by the injection of ultrarelativistic particles, emitting \\(\\gamma\\) rays at the highest energies. The dataset offers a unique opportunity to constrain the redshift of this source at \\bestz\\ using a novel method based on Bayesian statistics. The indication of intra-night variability is used to introduce a novel method to probe for a possible Lorentz Invariance Violation (LIV), and to set limits on the energy scale at which Quantum Gravity (QG) effects causing LIV may arise. For the subluminal case, the derived limits are \\(\\textrm{E}_{\\rm QG,1}>4.10\\times 10^{17}\\) GeV and \\(\\textrm{E}_{\\rm QG,2}>2.10\\times 10^{10}\\) GeV for linear and quadratic LIV effects, respectively.