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| 2. |
- Buitink, S., et al.
(författare)
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A large light-mass component of cosmic rays at 1017–1017.5 electronvolts from radio observations
- 2016
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 531:7592, s. 70-73
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Tidskriftsartikel (refereegranskat)abstract
- Cosmic rays are the highest-energy particles found in nature. Measurements of the mass composition of cosmic rays with energies of 1017–1018 electronvolts are essential to understanding whether they have galactic or extragalactic sources. It has also been proposed that the astrophysical neutrino signal1 comes from accelerators capable of producing cosmic rays of these energies2. Cosmic rays initiate air showers—cascades of secondary particles in the atmosphere—and their masses can be inferred from measurements of the atmospheric depth of the shower maximum3 (Xmax; the depth of the air shower when it contains the most particles) or of the composition of shower particles reaching the ground4. Current measurements5 have either high uncertainty, or a low duty cycle and a high energy threshold. Radio detection of cosmic rays6, 7, 8 is a rapidly developing technique9 for determining Xmax (refs 10, 11) with a duty cycle of, in principle, nearly 100 per cent. The radiation is generated by the separation of relativistic electrons and positrons in the geomagnetic field and a negative charge excess in the shower front6, 12. Here we report radio measurements of Xmax with a mean uncertainty of 16 grams per square centimetre for air showers initiated by cosmic rays with energies of 1017–1017.5 electronvolts. This high resolution in Xmax enables us to determine the mass spectrum of the cosmic rays: we find a mixed composition, with a light-mass fraction (protons and helium nuclei) of about 80 per cent. Unless, contrary to current expectations, the extragalactic component of cosmic rays contributes substantially to the total flux below 1017.5 electronvolts, our measurements indicate the existence of an additional galactic component, to account for the light composition that we measured in the 1017–1017.5 electronvolt range.
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| 3. |
- Corstanje, A., et al.
(författare)
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Timing calibration and spectral cleaning of LOFAR time series data
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 590
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Tidskriftsartikel (refereegranskat)abstract
- We describe a method for spectral cleaning and timing calibration of short voltage time series data from individual radio interferometer receivers. It makes use of the phase differences in Fast Fourier Transform (FFT) spectra across antenna pairs. For strong, localized terrestrial sources these are stable over time, while being approximately uniform-random for a sum over many sources or for noise. Using only milliseconds-long datasets, the method finds the strongest interfering transmitters, a first-order solution for relative timing calibrations, and faulty data channels. No knowledge of gain response or quiescent noise levels of the receivers is required. With relatively small data volumes, this approach is suitable for use in an online system monitoring setup for interferometric arrays. We have applied the method to our cosmic-ray data collection, a collection of measurements of short pulses from extensive air showers, recorded by the LOFAR radio telescope. Per air shower, we have collected 2 ms of raw time series data for each receiver. The spectral cleaning has a calculated optimal sensitivity corresponding to a power signal-to-noise ratio of 0.08 (or -11 dB) in a spectral window of 25 kHz, for 2 ms of data in 48 antennas. This is well sufficient for our application. Timing calibration across individual antenna pairs has been performed at 0.4 ns precision; for calibration of signal clocks across stations of 48 antennas the precision is 0.1 ns. Monitoring differences in timing calibration per antenna pair over the course of the period 2011 to 2015 shows a precision of 0.08 ns, which is useful for monitoring and correcting drifts in signal path synchronizations. A cross-check method for timing calibration is presented, using a pulse transmitter carried by a drone flying over the array. Timing precision is similar, 0.3 ns.
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| 4. |
- Rossetto, L., et al.
(författare)
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Measurement of cosmic rays with LOFAR
- 2016
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Ingår i: XIV INTERNATIONAL CONFERENCE ON TOPICS IN ASTROPARTICLE AND UNDERGROUND PHYSICS (TAUP 2015), PTS 1-7. - : Institute of Physics Publishing (IOPP).
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Konferensbidrag (refereegranskat)abstract
- The LOw Frequency ARay (LOFAR) is a multipurpose radio -antenna array aimed to detect radio signals in the 10 - 240 MHz frequency range, covering a large surface in Northern Europe with a higher density in the Northern Netherlands. Radio emission in the atmosphere is produced by cosmic -ray induced air showers through the interaction of charged particles with the Earth magnetic field. The detection of radio signals allows to reconstruct several properties of the observed cascade. We review here all important results achieved in the last years. We proved that the radio -signal distribution at ground level is described by a two-dimensional pattern, which is well fitted by a double Gaussian function. The radio -signal arrival time and polarization have been measured, thus providing additional information on the extensive air shower geometry, and on the radio emission processes. We also showed that the radio signal reaches ground in a thin, curved wavefront which is best parametrized by a hyperboloid shape centred around the shower axis. Radio emission has also been studied under thunderstorm conditions and compared to fair weather conditions. Moreover, by using a hybrid reconstruction technique, we performed mass composition measurements in the energy range 10(17) - 10(18) eV.
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| 5. |
- Scholten, O., et al.
(författare)
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Measurement of the circular polarization in radio emission from extensive air showers confirms emission mechanisms
- 2016
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Ingår i: Physical Review D. - 2470-0010 .- 2470-0029. ; 94:10
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Tidskriftsartikel (refereegranskat)abstract
- We report here on a novel analysis of the complete set of four Stokes parameters that uniquely determine the linear and/or circular polarization of the radio signal for an extensive air shower. The observed dependency of the circular polarization on azimuth angle and distance to the shower axis is a clear signature of the interfering contributions from two different radiation mechanisms, a main contribution due to a geomagnetically-induced transverse current and a secondary component due to the build-up of excess charge at the shower front. The data, as measured at LOFAR, agree very well with a calculation from first principles. This opens the possibility to use circular polarization as an investigative tool in the analysis of air shower structure, such as for the determination of atmospheric electric fields.
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| 6. |
- Thoudam, Satyendra, et al.
(författare)
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Measurement of the cosmic-ray energy spectrum above 1016 eV with the LOFAR Radboud Air Shower Array
- 2016
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Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 73, s. 34-43
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Tidskriftsartikel (refereegranskat)abstract
- The energy reconstruction of extensive air showers measured with the LOFAR Radboud Air Shower Array (LORA) is presented in detail. LORA is a particle detector array located in the center of the LOFAR radio telescope in the Netherlands. The aim of this work is to provide an accurate and independent energy measurement for the air showers measured through their radio signal with the LOFAR antennas. The energy reconstruction is performed using a parameterized relation between the measured shower size and the cosmic-ray energy obtained from air shower simulations. In order to illustrate the capabilities of LORA, the all-particle cosmic-ray energy spectrum has been reconstructed, assuming that cosmic rays are composed only of protons or iron nuclei in the energy range between ∼2 × 1016 and 2 × 1018 eV. The results are compatible with literature values and a changing mass composition in the transition region from a Galactic to an extragalactic origin of cosmic rays.
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| 7. |
- Trinh, T. N. G., et al.
(författare)
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Influence of atmospheric electric fields on the radio emission from extensive air showers
- 2016
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Ingår i: Physical Review D. - 2470-0010 .- 2470-0029. ; 93:2
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Tidskriftsartikel (refereegranskat)abstract
- The atmospheric electric fields in thunderclouds have been shown to significantly modify the intensity and polarization patterns of the radio footprint of cosmic-ray-induced extensive air showers. Simulations indicated a very nonlinear dependence of the signal strength in the frequency window of 30–80 MHz on the magnitude of the atmospheric electric field. In this work we present an explanation of this dependence based on Monte Carlo simulations, supported by arguments based on electron dynamics in air showers and expressed in terms of a simplified model. We show that by extending the frequency window to lower frequencies, additional sensitivity to the atmospheric electric field is obtained.
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| 8. |
- Thoudam, Satyendra, et al.
(författare)
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Cosmic-ray energy spectrum and composition up to the ankle : the case for a second Galactic component
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 595
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Tidskriftsartikel (refereegranskat)abstract
- Motivated by the recent high-precision measurements of cosmic rays by several new-generation experiments, we have carried out a detailed study to understand the observed energy spectrum and composition of cosmic rays with energies up to about 10(18) eV. Our study shows that a single Galactic component with subsequent energy cut-offs in the individual spectra of different elements, optimised to explain the observed elemental spectra below similar to 10(14) eV and the "knee" in the all-particle spectrum, cannot explain the observed all-particle spectrum above similar to 2 x 10(16) eV. We discuss two approaches for a second component of Galactic cosmic rays re-acceleration at a Galactic wind termination shock, and supernova explosions of Wolf-Rayet stars, and show that the latter scenario can explain almost all observed features in the all-particle spectrum and the composition up to similar to 10(18) eV, when combined with a canonical extra-galactic spectrum expected from strong radio galaxies or a source population with similar cosmological evolution. In this two-component Galactic model, the knee at similar to 3 x 10(15) eV and the "second knee" at similar to 10(17) eV in the all-particle spectrum are due to the cut-offs in the first and second components, respectively. We also discuss several variations of the extra-galactic component, from a minimal contribution to scenarios with a significant component below the "ankle" (at similar to 4 x 10(18) eV), and find that extragalactic contributions in excess of regular source evolution are neither indicated nor in conflict with the existing data. We also provide arguments that an extra-galactic contribution is unlikely to dominate at or below the second knee. Our main result is that the second Galactic component predicts a composition of Galactic cosmic rays at and above the second knee that largely consists of helium or a mixture of helium and CNO nuclei, with a weak or essentially vanishing iron fraction, in contrast to most common assumptions. This prediction is in agreement with new measurements from LOFAR and the Pierre Auger Observatory which indicate a strong light component and a rather low iron fraction between similar to 10(17) and 10(18) eV.
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