Radio detection of Cosmic Rays with LOFAR

Abstract

When high-energy cosmic rays (ionized atomic nuclei) impinge on the atmosphere of the Earth they interact with atomic nuclei and initiate cascades of secondary particles -the extensive air showers. Many of the secondary particles in the air showers are electrons and positrons. They cause radiation in the frequency range of tens of MHz. The LOFAR radio telescope detects this radiation in the frequency range 30 to 240 MHz. LOFAR has a high antenna density and good time resolution. In turn, the properties of the radio emission are measured in detail. The properties of the shower-inducing cosmic rays are derived from the air shower measurements, namely their direction, energy, and particle type (atomic mass). The uncertainties achieved are competative to established techniques. This demonstrates that the radio technique is now a standard tool to measure extensive air showers and to study the properties of the incoming cosmic rays. The mean logarithmic mass of cosmic rays as measured with LOFAR is derived as a function of energy. In an examplary study, these data are used to show that the radio measurements of air showers are now in a state to discriminate astrophysical models of the origin of cosmic rays. PoS(ICRC2015)033Radio Detection of Cosmic Rays with LOFAR J.R. Hörandel thus the amount of observing time in a given frequency band is not controlled by the cosmic ray project. In parallel to any observation the tile-beamformed data from all tiles are filled into ring buffers (Transient Buffer Boards), from which the last 5 s of data can be recorded when triggered. Triggers can be generated by inspecting the data with an on-board FPGA 1 or received through the LOFAR control software. In order to record cosmic-ray pulses, the dense core of LOFAR is equipped with an array of particle detectors [20], as also shown in Fig. 2. In routine observations, coincidences of several particle detectors trigger a read-out of the ring buffers [18].The HBAs can be sampled at two different clock frequencies, which allows for several different observing bands. The one mostly used in the present data-set is an event. During processing, the signals from all tiles in a station are first coherently beamformed in the cosmic-ray arrival direction as reconstructed from the particle data. When a significant signal, with a signal-to-noise ratio exceeding three in amplitude, is detected in the beamformed signal the station is selected for further processing and the event is selected as a cosmic ray candidate. Using a smaller search window, around the peak in the beamformed signal, a pulse search is then performed on the Hilbert envelope of the up-sampled signals from each tile. Up-sampling, by a factor 16, is needed such that the pulse maximum search is not the limiting factor in achieving the required time resolution. From the arrival times of those pulse maxima for which the signal-to-noise ratio in amplitude exceeds three, the direction of the cosmic ray is reconstructed. Additionally the amplitude (in each instrumental polarization) and...

Cosmic rays: air showers from low to high energies

“…The typical working frequencies range from a few to hundreds of MHz. The Low Frequency Array (LOFAR) [84] located in the Netherlands is a small but very dense array of antennas which allows a very precise measurement of the radio footprint at the ground. In LOFAR, scintillator detectors are used for triggering purposes [85].…”

“…In LOFAR X max is determined by performing a fit to the measured LDF against the predictions of the CoREAS simulations [97]. The high density of the array and the accurate calibration of the antennas [98] allow one to get a resolution on X max of about 15 g/cm 2 , comparable to the one of the fluorescence telescopes. A similar method has been developed in CODALEMA [99].…”

“…The calibration of the Tunka-Rex antennas is described in [101]. A first measurement of the mean depth of the shower maximum by LOFAR using a sample of 118 showers is shown in the left panel of Figure 17 [84,102]. Another measurement of the mean X max based on the radio array installed in the Yakutsk site was presented in [103].…”

“…Left panel: first measurement of the mean depth of the shower maximum by LOFAR[84,102]. Right panel: energy spectrum measured by ANITA[104].…”

The mass composition of cosmic rays measured with LOFAR

“…The LOFAR measurements together with the predictions of the CoREAS [13] simulation package result in a complete understanding of the emission mechanisms. With LOFAR the properties of the radio emission have been measured with high accuracy [14][15][16] in the frequency range 30 − 80 MHz, which allows us to establish key features, such as the lateral density distribution of the radio signals [17,18], the shape of the shower front [19], and the polarization of the radio signal [20,21]. These measurements help to understand the emission processes in the atmosphere and to quantify the contributions of the two mechanisms, being responsible for the radio emission of air showers -namely the geomagnetic effect (i.e.…”