| 1. |
- Bonardi, A., et al.
(författare)
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Central acceptance testing for camera technologies for the cherenkov telescope array
- 2015
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Ingår i: Proceedings of Science.
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Konferensbidrag (refereegranskat)abstract
- The Cherenkov Telescope Array (CTA) is an international initiative to build the next generation ground based very-high energy gamma-ray observatory. It will consist of telescopes of three different sizes, employing several different technologies for the cameras that detect the Cherenkov light from the observed air showers. In order to ensure the compliance of each camera technology with CTA requirements, CTA will perform central acceptance testing of each camera technology. To assist with this, the Camera Test Facilities (CTF) work package is developing a detailed test program covering the most important performance, stability, and durability requirements, including setting up the necessary equipment. Performance testing will include a wide range of tests like signal amplitude, time resolution, dead-time determination, trigger efficiency, performance testing under temperature and humidity variations and several others. These tests can be performed on fully-integrated cameras using a portable setup at the camera construction sites. In addition, two different setups for performance tests on camera sub-units are being built, which can provide early feedback for camera development. Stability and durability tests will include the long-term functionality of movable parts, water tightness of the camera housing, temperature and humidity cycling, resistance to vibrations during transport or due to possible earthquakes, UV-resistance of materials and several others. Some durability tests will need to be contracted out because they will need dedicated equipment not currently available within CTA. The planned test procedures and the current status of the test facilities will be presented.
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| 2. |
- Bonardi, A., et al.
(författare)
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Study of the LOFAR radio self-trigger and single-station acquisition mode
- 2018
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Ingår i: 35th International Cosmic Ray Conference, ICRC 2017, 10-20 July 2017. - : Sissa Medialab Srl.
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Konferensbidrag (refereegranskat)abstract
- The LOw Frequency ARay (LOFAR) observatory is a multipurpose radio antenna array aimed to detect radio signals in the frequency range 10-240 MHz. Radio antennas are clustered into over 50 stations, and are spread along Central and Northern Europe. The LOFAR core, where the density of stations is highest, is instrumented with the LOfar Radboud air shower Array (LORA), covering an area of about 300 m diameter centered at the LOFAR core position. Since 2011 the LOFAR core has been used for detecting radio-signals associated to cosmic-ray air showers in the energy range 1016 - 1018 eV. Data acquisition is triggered by the LORA scintillator array, which provides energy, arrival direction, and core position estimates of the detected air shower too. Thus only the core of the LOFAR array is currently used for cosmic-ray detection. In order to extend the energy range of the detected cosmic rays, it is necessary to expand the effective collecting area to the whole LOFAR array. On this purpose, a detailed study about the LOFAR potentialities of working in self-trigger mode, i.e. with the cosmic-ray data acquisition trigger provided by the radio-antenna only, is presented here. A new method based on the intensity and the frequency spectrum for determining the air shower position to be implemented on LOFAR remote stations is presented too. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License (CC BY-NC-ND 4.0).
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| 3. |
- 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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| 4. |
- Buitink, S., et al.
(författare)
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Cosmic ray mass composition with LOFAR
- 2018
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Ingår i: 35th International Cosmic Ray Conference — ICRC2017. 10–20 July, 2017. Bexco, Busan, Korea. - Trieste, Italy : Sissa Medialab Srl.
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Konferensbidrag (refereegranskat)abstract
- The LOFAR radio telescope measures the radio emission from extensive air showers with unprecedented precision. In the dense core individual air showers are detected by hundreds of dipole antennas. By fitting the complex radiation pattern to Monte Carlo radio simulation codes we obtain measurements of the atmospheric depth of the shower maximum X max with a precision of < 20 g/cm 2 . This quantity is sensitive to the mass composition of cosmic rays. We discuss the first mass composition results of LOFAR and the improvements that are currently being made to enhance the accuracy of future analysis. Firstly, a more realistic treatment of the atmosphere will decrease the systematic uncertainties due to the atmosphere. Secondly, a series of upgrades to the LOFAR system will lead to increased effective area, duty cycle, and the possibility to extend the composition analysis down to the energy of 10 16.5 eV. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License (CC BY-NC-ND 4.0).
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| 5. |
- Buitink, S., et al.
(författare)
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Method for high precision reconstruction of air shower Xmax using two-dimensional radio intensity profiles
- 2014
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Ingår i: Physical Review D. - 1550-7998 .- 1550-2368. ; 90:8, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- The mass composition of cosmic rays contains important clues about their origin. Accurate measurements are needed to resolve longstanding issues such as the transition from Galactic to extra-Galactic origin and the nature of the cutoff observed at the highest energies. Composition can be studied by measuring the atmospheric depth of the shower maximum Xmax of air showers generated by high-energy cosmic rays hitting the Earth’s atmosphere. We present a new method to reconstruct Xmax based on radio measurements. The radio emission mechanism of air showers is a complex process that creates an asymmetric intensity pattern on the ground. The shape of this pattern strongly depends on the longitudinal development of the shower. We reconstruct Xmax by fitting two-dimensional intensity profiles, simulated with CoREAS, to data from the Low Frequency Array (LOFAR) radio telescope. In the dense LOFAR core, air showers are detected by hundreds of antennas simultaneously. The simulations fit the data very well, indicating that the radiation mechanism is now well understood. The typical uncertainty on the reconstruction of Xmax for LOFAR showers is 17 g/cm2.
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| 6. |
- Corstanje, A., et al.
(författare)
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LOFAR : Detecting Cosmic Rays with a Radio Telescope
- 2011
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Ingår i: Proceedings of the 32nd International Cosmic Ray Conference. - : International Union of Pure and Applied Physics (IUPAP). ; , s. 192-195
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- LOFAR (the Low Frequency Array), a distributed digital radio telescope with stations in the Netherlands, Germany, France, Sweden, and the United Kingdom, is designed to enable full-sky monitoring of transient radio sources. These capabilities are ideal for the detection of broadband radio pulses generated in cosmic ray air showers. The core of LOFAR consists of 24 stations within 4 square kilometers, and each station contains 96 low-band antennas and 48 high-band antennas. This dense instrumentation will allow detailed studies of the lateral distribution of the radio signal in a frequency range of 10-250 MHz. Such studies are key to understanding the various radio emission mechanisms within the air shower, as well as for determining the potential of the radio technique for primary particle identification. We present the status of the LOFAR cosmic ray program, including the station design and hardware, the triggering and filtering schemes, and our initial observations of cosmic-ray-induced radio pulses.
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| 7. |
- Corstanje, A., et al.
(författare)
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The effect of the atmospheric refractive index on the radio signal of extensive air showers using Global Data Assimilation System (GDAS)
- 2018
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Ingår i: 35th International Cosmic Ray Conference, ICRC 2017, 10-20 July 2017, Bexco, Busan, Korea. - : Sissa Medialab Srl.
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Konferensbidrag (refereegranskat)abstract
- One of the major systematic uncertainties in the measurement of Xmax from radio emission of EAS arises from variations of the refractive index in the atmosphere. The refractive index n varies with temperature, humidity and pressure, and the variations can be on the order of 10% for (n-1) at a given altitude. The effect of a varying refractive index on Xmax measurements is evaluated using CoREAS: a microscopic simulation of the radio emission from the individual particles in the cascade simulated with CORSIKA. We discuss the resulting offsets in Xmax for different frequency regimes, and compare them to a simple physical model. Under typical circumstances, the offsets in Xmax range from 4-11 g/cm2 for the 30-80 MHz frequency band. Therefore, for precise measurements it is required to include atmospheric data at the time and place of observation of the air shower into the simulations. The aim is to implement this in the next version of CoREAS/CORSIKA using the Global Data Assimilation System (GDAS), a global atmospheric model based on meteorological measurements and numerical weather predictions. This can then be used to re-evaluate the air shower measurements of the LOFAR radio telescope. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License (CC BY-NC-ND 4.0).
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| 8. |
- Corstanje, A., et al.
(författare)
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The shape of the radio wavefront of extensive air showers as measured with LOFAR
- 2015
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Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 61, s. 22-31
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Tidskriftsartikel (refereegranskat)abstract
- Extensive air showers, induced by high energy cosmic rays impinging on the Earth’s atmosphere, produce radio emission that is measured with the LOFAR radio telescope. As the emission comes from a finite distance of a few kilometers, the incident wavefront is non-planar. A spherical, conical or hyperbolic shape of the wavefront has been proposed, but measurements of individual air showers have been inconclusive so far. For a selected high-quality sample of 161 measured extensive air showers, we have reconstructed the wavefront by measuring pulse arrival times to sub-nanosecond precision in 200 to 350 individual antennas. For each measured air shower, we have fitted a conical, spherical, and hyperboloid shape to the arrival times. The fit quality and a likelihood analysis show that a hyperboloid is the best parameterization. Using a non-planar wavefront shape gives an improved angular resolution, when reconstructing the shower arrival direction. Furthermore, a dependence of the wavefront shape on the shower geometry can be seen. This suggests that it will be possible to use a wavefront shape analysis to get an additional handle on the atmospheric depth of the shower maximum, which is sensitive to the mass of the primary particle.
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| 9. |
- 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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| 10. |
- De Gasperin, F., et al.
(författare)
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Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 635
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Tidskriftsartikel (refereegranskat)abstract
- The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (< 100 MHz) can reach several thousands of janskys, and they often contaminate observations of the low-frequency sky by interfering with image processing. Furthermore, these sources are foreground objects for all-sky observations hampering the study of faint signals, such as the cosmological 21 cm line from the epoch of reionisation. Aims. We aim to produce robust models for the surface brightness emission as a function of frequency for the A-team sources at ultra-low frequencies. These models are needed for the calibration and imaging of wide-area surveys of the sky with low-frequency interferometers. This requires obtaining images at an angular resolution better than 15″ with a high dynamic range and good image fidelity. Methods. We observed the A-team with the Low Frequency Array (LOFAR) at frequencies between 30 MHz and 77 MHz using the Low Band Antenna system. We reduced the datasets and obtained an image for each A-team source. Results. The paper presents the best models to date for the sources Cassiopeia A, Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to obtain the aimed resolution and dynamic range in all cases. Owing to its compactness and complexity, observations with the long baselines of the International LOFAR Telescope will be required to improve the source model for Cygnus A further.
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