Magnetic field strength and spectral distribution of six parsec-scale active galactic nuclei jets

We use observations of six "blazars" with the Very Long Baseline Array (VLBA), at eight frequencies (4.6, 5.1, 7.9, 8.9, 12.9, 15.4, 22.2, 43.1 GHz), to investigate the frequency-dependent position of their VLBI cores ("core-shift") and their overall jet spectral distribution. By cross-correlating the optically thin jet emission, we are able to accurately align the multi-frequency images of three of the jets (1418+546, 2007+777, 2200+420), whose core-shifts and spectra we find consistent with the equipartition regime of the Blandford & Konigl conical jet model, where the position of the radio core from the base of the jet follows $r_{core}\propto\nu^{-1}$. For the jet of 0954+658, we align the higher frequency images using our lower frequency measurements assuming equipartition in the radio core from 4.6-43 GHz. The jet emission of the other two sources in our sample (1156+295, 1749+096) is too sparse for our alignment technique to work. Using our measured core-shifts, we calculate equipartition magnetic field strengths of the order of 10's to 100's of mG in the radio cores of these four AGN from 4.6-43 GHz. Extrapolating our results back to the accretion disk and black hole jet-launching distances, we find magnetic field strengths consistent with those expected from theoretical models of magnetically powered jets.Comment: To match published version in MNRAS. Equation 5 corrected for typesetting error and references update

Air Shower Measurements with LOFAR

Air showers from cosmic rays emit short, intense radio pulses. LOFAR is a new radio telescope, that is being built in the Netherlands and Europe. Designed primarily as a radio interferometer, the core of LOFAR will have a high density of radio antennas, which will be extremely well calibrated. This makes LOFAR a unique tool for the study of the radio properties of single air showers. Triggering on the radio emission from air showers means detecting a short radio pulse and discriminating real events from radio interference. At LOFAR we plan to search for pulses in the digital data stream - either from single antennas or from already beam-formed data - and calculate several parameters characterizing the pulse shape to pick out real events in a second stage. In addition, we will have a small scintillator array to test and confirm the performance of the radio only trigger.Comment: Proceedings of the ARENA 2008 workshop, to be published in NIM

The LOPES experiment - recent results, status and perspectives

The LOPES experiment at the Karlsruhe Institute of Technology has been taking radio data in the frequency range from 40 to 80 MHz in coincidence with the KASCADE-Grande air shower detector since 2003. Various experimental configurations have been employed to study aspects such as the energy scaling, geomagnetic dependence, lateral distribution, and polarization of the radio emission from cosmic rays. The high quality per-event air shower information provided by KASCADE-Grande has been the key to many of these studies and has even allowed us to perform detailed per-event comparisons with simulations of the radio emission. In this article, we give an overview of results obtained by LOPES, and present the status and perspectives of the ever-evolving experiment.Comment: Proceedings of the ARENA2010 conference, Nantes, Franc

Air Shower Measurements with the LOPES Radio Antenna Array

LOPES is set up at the location of the KASCADE-Grande extensive air shower experiment in Karlsruhe, Germany and aims to measure and investigate radio pulses from Extensive Air Showers. Since radio waves suffer very little attenuation, radio measurements allow the detection of very distant or highly inclined showers. These waves can be recorded day and night, and provide a bolometric measure of the leptonic shower component. LOPES is designed as a digital radio interferometer using high bandwidths and fast data processing and profits from the reconstructed air shower observables of KASCADE-Grande. The LOPES antennas are absolutely amplitude calibrated allowing to reconstruct the electric field strength which can be compared with predictions from detailed Monte Carlo simulations. We report about the analysis of correlations present in the radio signals measured by the LOPES 30 antenna array. Additionally, LOPES operates antennas of a different type (LOPES-STAR) which are optimized for an application at the Pierre Auger Observatory. Status, recent results of the data analysis and further perspectives of LOPES and the possible large scale application of this new detection technique are discussed.Comment: 8 pages, 10 figures, Contribution to the Arena 2008 conference, Rome, June 200

Launching of Conical Winds and Axial Jets from the Disk-Magnetosphere Boundary: Axisymmetric and 3D Simulations

We investigate the launching of outflows from the disk-magnetosphere boundary of slowly and rapidly rotating magnetized stars using axisymmetric and exploratory 3D magnetohydrodynamic (MHD) simulations. We find long-lasting outflows in both cases. (1) In the case of slowly rotating stars, a new type of outflow, a conical wind, is found and studied in simulations. The conical winds appear in cases where the magnetic flux of the star is bunched up by the disk into an X-type configuration. The winds have the shape of a thin conical shell with a half-opening angle 30-40 degrees. The conical winds may be responsible for episodic as well as long-lasting outflows in different types of stars. (2) In the case of rapidly rotating stars (the "propeller regime"), a two-component outflow is observed. One component is similar to the conical winds. A significant fraction of the disk matter may be ejected into the winds. A second component is a high-velocity, low-density magnetically dominated axial jet where matter flows along the opened polar field lines of the star. The jet has a mass flux about 10% that of the conical wind, but its energy flux (dominantly magnetic) can be larger than the energy flux of the conical wind. The jet's angular momentum flux (also dominantly magnetic) causes the star to spin-down rapidly. Propeller-driven outflows may be responsible for the jets in protostars and for their rapid spin-down. The jet is collimated by the magnetic force while the conical winds are only weakly collimated in the simulation region.Comment: 29 pages and 29 figures. This version has a major expansion after comments by a referee. The 1-st version is correct but mainly describes the conical wind. This version describes in greater detail both the conical winds and the propeller regime. Accepted to the MNRA

Radio detection of cosmic ray air showers with LOPES

In the last few years, radio detection of cosmic ray air showers has experienced a true renaissance, becoming manifest in a number of new experiments and simulation efforts. In particular, the LOPES project has successfully implemented modern interferometric methods to measure the radio emission from extensive air showers. LOPES has confirmed that the emission is coherent and of geomagnetic origin, as expected by the geosynchrotron mechanism, and has demonstrated that a large scale application of the radio technique has great potential to complement current measurements of ultra-high energy cosmic rays. We describe the current status, most recent results and open questions regarding radio detection of cosmic rays and give an overview of ongoing research and development for an application of the radio technique in the framework of the Pierre Auger Observatory.Comment: 8 pages; Proceedings of the CRIS2006 conference, Catania, Italy; to be published in Nuclear Physics B, Proceedings Supplement

Radio Emission in Atmospheric Air Showers: First Measurements with LOPES-30

When Ultra High Energy Cosmic Rays interact with particles in the Earth's atmosphere, they produce a shower of secondary particles propagating toward the ground. LOPES-30 is an absolutely calibrated array of 30 dipole antennas investigating the radio emission from these showers in detail and clarifying if the technique is useful for largescale applications. LOPES-30 is co-located and measures in coincidence with the air shower experiment KASCADE-Grande. Status of LOPES-30 and first measurements are presented.Comment: Proceedings of ARENA 06, June 2006, University of Northumbria, U

Frequency spectra of cosmic ray air shower radio emission measured with LOPES

AIMS: We wish to study the spectral dependence of the radio emission from cosmic-ray air showers around 100 PeV (1017 eV). METHODS: We observe short radio pulses in a broad frequency band with the dipole-interferometer LOPES (LOFAR Prototype Station), which is triggered by a particle detector array named Karlsruhe Shower Core and Array Detector (KASCADE). LOFAR is the Low Frequency Array. For this analysis, 23 strong air shower events are selected using parameters from KASCADE. RESULTS: The resulting electric field spectra fall off to higher frequencies. An average electric field spectrum is fitted with an exponential, or alternatively, with a power law. The spectral slope obtained is not consistent within uncertainties and it is slightly steeper than the slope obtained from Monte Carlo simulations based on air showers simulated with CORSIKA (Cosmic Ray Simulations for KASCADE). One of the strongest events was measured during thunderstorm activity in the vicinity of LOPES and shows the longest pulse length measured of 110 ns and a spectral slope of -3.6. CONCLUSIONS: We show with two different methods that frequency spectra from air shower radio emission can be reconstructed on event-by-event basis, with only two dozen dipole antennae simultaneously over a broad range of frequencies. According to the obtained spectral slopes, the maximum power is emitted below 40 MHz. Furthermore, the decrease in power to higher frequencies indicates a loss in coherence determined by the shower disc thickness. We conclude that a broader bandwidth, larger collecting area, and longer baselines, as will be provided by LOFAR, are necessary to further investigate the relation of the coherence, pulse length, and spectral slope of cosmic ray air showers.Comment: 13 pages, 21 figures. Nigl, A. et al. (LOPES Collaboration), Frequency spectra of cosmic ray air shower radio emission measured with LOPES, accepted by A&A on 17/06/200

Radio emission of highly inclined cosmic ray air showers measured with LOPES

LOPES-10 (the first phase of LOPES, consisting of 10 antennas) detected a significant number of cosmic ray air showers with a zenith angle larger than 50$^{\circ}$, and many of these have very high radio field strengths. The most inclined event that has been detected with LOPES-10 has a zenith angle of almost 80$^{\circ}$. This is proof that the new technique is also applicable for cosmic ray air showers with high inclinations, which in the case that they are initiated close to the ground, can be a signature of neutrino events.Our results indicate that arrays of simple radio antennas can be used for the detection of highly inclined air showers, which might be triggered by neutrinos. In addition, we found that the radio pulse height (normalized with the muon number) for highly inclined events increases with the geomagnetic angle, which confirms the geomagnetic origin of radio emission in cosmic ray air showers.Comment: A&A accepte

Radio emission of highly inclined cosmic ray air showers measured with LOPES

LOPES (LOFAR Prototype Station) is an array of dipole antennas used for detection of radio emission from air showers. It is co-located and triggered by the KASCADE (Karlsruhe Shower Core and Array Detector) experiment, which also provides informations about air shower properties. Even though neither LOPES nor KASCADE are completely optimized for the detection of highly inclined events, a significant number of showers with zenith angle larger than 50$^o$ have been detected in the radio domain, and many with very high field strengths. Investigation of inclined showers can give deeper insight into the nature of primary particles that initiate showers and also into the possibility that some of detected showers are triggered by neutrinos. In this paper, we show the example of such an event and present some of the characteristics of highly inclined showers detected by LOPES