Probing the radio emission from air showers with polarization measurements
The emission of radio waves from air showers has been attributed to the so-called geomagnetic emission process. At frequencies around 50 MHz this process leads to coherent radiation which can be observed with rather simple setups. The direction of the electric field induced by this emission process depends only on the local magnetic field vector and on the incoming direction of the air shower. We report on measurements of the electric field vector where, in addition to this geomagnetic component, another component has been observed that cannot be described by the geomagnetic emission process. The data provide strong evidence that the other electric field component is polarized radially with respect to the shower axis, in agreement with predictions made by Askaryan who described radio emission from particle showers due to a negative charge excess in the front of the shower. Our results are compared to calculations which include the radiation mechanism induced by this charge-excess process.

title: The Pierre Auger Observatory: Contributions to the 33rd International Cosmic Ray Conference (ICRC 2013)
abstract: Contributions of the Pierre Auger Collaboration to the 33rd International Cosmic Ray Conference, Rio de Janeiro, Brazil, July 2013

title: Time Calibration of the Radio Air Shower Array LOPES
abstract: LOPES is a digitally read out antenna array consisting of 30 calibrated dipole antennas. It is located at the site of the KASCADE-Grande experiment at Forschungszentrum Karlsruhe and measures the radio emission of cosmic ray air showers in the frequency band from 40 to 80 MHz. LOPES is triggered by KASCADE and uses the KASCADE reconstruction of the shower axis as an input for the analysis of the radio pulses. Thereby LOPES works as an interferometer when the signal of all antennas is digitally merged to form a beam into the shower direction. To be sensitive to the coherence of the radio signal, a precise time calibration with an accuracy in the order of 1 ns is required. Thus, it is necessary to know the delay of each antenna which is time and frequency dependent. Several calibration measurements are performed to correct for this delay in the analysis: The group delay of every antenna is measured regularly (roughly once per year) by recording a test pulse which is emitted at a known time. Furthermore, the delay is monitored continuously by the so called phase calibration method: A beacon (a dipole antenna) emits continuously two sine waves at 63.5 MHz and 68.1 MHz. By that a variation of the delay can be detected in a subsequent analysis of the radio events as a change of the phase at these frequencies. Finally, the dispersion of the analog electronics has been measured to account for the frequency dependence of the delay.

title: The KASCADE-Grande energy spectrum of cosmic rays and the role of hadronic interaction models
abstract: Previous results obtained by KASCADE-Grande using the QGSjetII-02 hadronic interaction model have shown that the energy spectrum of cosmic rays between 10 16  eV and 10 18  eV exhibits a significant hardening at approximately 2 × 10 16  eV and a slight but statistically significant steepening close to 10 17  eV. Moreover, the analysis with QGSjetII-02 suggests that the break observed around 10 17  eV is caused by the heavy component of primary cosmic rays. In this paper, we report on the results of similar analyses performed using the SIBYLL 2.1 and EPOS 1.99 hadronic interaction models to interpret the data. The present results confirm qualitatively the previous findings. However, the intensity of the all-particle spectrum, the positions of the hardening and steepening of the spectrum, as well as the relative abundance of the heavy and light mass groups depend on the hadronic interaction model used to interpret the data.

title: The Tunka Radio Extension (Tunka-Rex): Status and First Results (ICRC 2013)
abstract: Tunka-Rex is a new radio antenna array which extends the Tunka experiment in Siberia close to lake Baikal. It consists of 20 antennas on an area of 1 km^2 which measure the radio emission of high-energy air showers. Tunka-Rex is triggered by the photomultiplier array of Tunka measuring air-Cherenkov light of air showers in the energy range from about 10 PeV to 1 EeV. This configuration allows for the worldwide first hybrid measurements of the radio and air-Cherenkov signal for the same events: an ideal situation to perform a cross-calibration between both methods. Consequently, the main goal of Tunka-Rex is to determine the achievable energy and Xmax precision of radio measurements by comparing them to the reconstruction of the air-Cherenkov measurements. Tunka-Rex started operation in autumn 2012, and already detected air-shower events. In this paper we present the status of Tunka-Rex and first results which indicate that Tunka-Rex measures indeed the radio emission by air showers and that is is sensitive to their energy.

title: New method for the time calibration of an interferometric radio antenna array
abstract: Digital radio antenna arrays, like LOPES (LOFAR PrototypE Station), detect high-energy cosmic rays via the radio emission from atmospheric extensive air showers. LOPES is an array of dipole antennas placed within and triggered by the KASCADE-Grande experiment on site of the Karlsruhe Institute of Technology, Germany. The antennas are digitally combined to build a radio interferometer by forming a beam into the air shower arrival direction which allows measurements even at low signal-to-noise ratios in individual antennas. This technique requires a precise time calibration. A combination of several calibration steps is used to achieve the necessary timing accuracy of about 1 ns. The group delays of the setup are measured, the frequency dependence of these delays (dispersion) is corrected in the subsequent data analysis, and variations of the delays with time are monitored. We use a transmitting reference antenna, a beacon, which continuously emits sine waves at known frequencies. Variations of the relative delays between the antennas can be detected and corrected for at each recorded event by measuring the phases at the beacon frequencies.

title: Cherenkov light based measurement of extensive air showers around the knee with the HEGRA experiment
abstract: Data of the wide angle atmospheric Cherenkov light detector array AIROBICC and the scintillator matrix of the HEGRA air shower detector complex are combined to determine the energy spectrum and coarse composition of charged cosmic rays in the energy interval from 300 TeV to 10 PeV.

title: Rejection of the hypothesis that Mkn-501 TeV photons are pure Bose – Einstein condensates
abstract: The TeV energy spectrum of the Blazar type galaxy Mkn-501 as measured by the HEGRA air Cherenkov telescopes extends beyond 16 TeV and constitutes the most energetic photons observed from an extragalactic object. A fraction of the emitted spectrum is possibly absorbed in interactions with low energy photons of the diffuse extragalactic infrared radiation, which in turn offers the unique possibility to measure the diffuse infrared radiation density by exact TeV spectroscopy. The upper limit on the density of the extragalactic infrared radiation derived from the TeV observations imposes constraints on models of galaxy formation and stellar evolution. One of the recently published ideas to reduce the absorption of TeV photons is based upon the assumption Max-Planck-Institut für Kernphysik, Postfach 103980, D-69029 Heidelberg, Germany Max-Planck-Institut für Physik, Föhringer Ring 6, D-80805 München, Germany Universidad Complutense, Facultad de Ciencias F́isicas, Ciudad Universitaria, E-28040 Madrid, Spain Universität Hamburg, II. Institut für Experimentalphysik, Luruper Chaussee 149, D-22761 Hamburg, Germany Universität Kiel, Institut für Experimentelle und Angewandte Physik, Leibnizstraße 15-19, D-24118 Kiel, Germany Universität Wuppertal, Fachbereich Physik, Gaußstr.20, D-42097 Wuppertal, Germany Yerevan Physics Institute, Alikhanian Br. 2, 375036 Yerevan, Armenia On leave from Altai State University, Dimitrov Street 66, 656099 Barnaul, Russia Corresponding author: Dieter Horns Dieter.Horns@desy.de

title: Multiwavelength observations of Markarian 501 during the 1997 high state
abstract: During the observation period 1997, the nearby blazar Mrk 501 showed extremely strong emission and high variability. We examine multiwavelength aspects of this event using radio, optical, soft and hard X-ray, and TeV data. We concentrate on the medium-timescale variability of the broadband spectra, averaged over weekly intervals. We confirm the previously found correlation between soft and hard X-ray emission and the emission at TeV energies, while the source shows only minor variability at radio and optical wavelengths. The nonlinear correlation between hard X-ray and TeV fluxes is consistent with a simple analytic estimate based on an SSC model in which Klein-Nishina effects are important for the highest-energy electrons in the jet, and flux variations are caused by variations of the electron density and/or the spectral index of the electron injection spectrum. The time-averaged spectra are fitted with a synchrotron self-Compton (SSC) dominated leptonic jet model, using the full Klein-Nishina cross section and following the self-consistent evolution of relativistic particles along the jet, accounting for γγ absorption and pair production within the source as well as due to the intergalactic infrared background radiation. The contribution from external inverse-Compton scattering is tightly constrained by the low maximum EGRET flux and found to be negligible at TeV energies. We find that high levels of the X-ray and TeV fluxes can be explained by a hardening of the energy spectra of electrons injected at the base of the jet, in remarkable contrast to the trend found for γ-ray flares of the flat-spectrum radio quasar PKS 0528+134.

title: A Search for gamma-ray emission from the galactic plane in the longitude range between 37 degrees and 43 degrees