The LOFAR radio telescope is able to measure the radio emission from cosmic
ray induced air showers with hundreds of individual antennas. This allows for
precision testing of the emission mechanisms for the radio signal as well as
determination of the depth of shower maximum $X_{\max}$, the shower observable
most sensitive to the mass of the primary cosmic ray, to better than 20
g/cm$^2$. With a densely instrumented circular area of roughly 320 m$^2$, LOFAR
is targeting for cosmic ray astrophysics in the energy range $10^{16}$ -
$10^{18}$ eV. In this contribution we give an overview of the status, recent
results, and future plans of cosmic ray detection with the LOFAR radio
telescope.Comment: Proceedings of the 26th Extended European Cosmic Ray Symposium
  (ECRS), Barnaul/Belokurikha, 201

Bonardi, A

Buitink, S

Corstanje, A

Falcke, H

Hare, B M

Hörandel, J R

Mitra, P

Mulrey, K

Nelles, A

Rachen, J P

Rossetto, L

Schellart, P

Scholten, O

ter Veen, S

Thoudam, S

Trinh, T N G

Winchen, T

Journal of Physics Conference Series

English

arXiv

Winchen, T.

Bonardi, A.

Buitink, S.

Corstanje, A.

Falcke, H.

Hare, B.M.

Hörandel, J.R.

Mitra, P.

Mulrey, K.

Nelles, A.

Rachen, J.P.

Rossetto, L.

Schellart, P.

Scholten, O.

Veen, S. ter

Thoudam, S.

Trinh, T.N.G.

NARCIS 

Cosmic Ray Physics with the LOFAR Radio Telescope

The LOFAR radio telescope is able to measure the radio emission from cosmic ray induced air showers with hundreds of individual antennas. This allows for precision testing of the emission mechanisms for the radio signal as well as determination of the depth of shower maximum X max , the shower observable most sensitive to the mass of the primary cosmic ray, to better than 20 g cm -2 . With a densely instrumented circular area of roughly 320 m 2 , LOFAR is targeting for cosmic ray astrophysics in the energy range 10 16 -10 18 eV. In this contribution we give an overview of the status, recent results, and future plans of cosmic ray detection with the LOFAR radio telescope. © Published under licence by IOP Publishing Ltd.</p

Winchen, T.,

Bonardi, A.,

Buitink, S.,

Corstanje, A.,

Falcke, H.,

Hare, B. M.,

Hörandel, J. R.,

Mitra, P.,

Mulrey, K.,

Nelles, A.,

Rachen, J. P.,

Rossetto, L.,

Schellart, P.,

Scholten, O.,

Ter Veen, S.,

Thoudam, Satyendra,

Trinh, T. N. G.,

Swepub

Cosmic ray physics with the LOFAR radio telescope

The LOFAR radio telescope is able to measure the radio emission from cosmic ray induced air showers with hundreds of individual antennas. This allows for precision testing of the emission mechanisms for the radio signal as well as determination of the depth of shower maximum Xmax, the shower observable most sensitive to the mass of the primary cosmic ray, to better than 20 g cm−2. With a densely instrumented circular area of roughly 320 m2, LOFAR is targeting for cosmic ray astrophysics in the energy range 1016–1018 eV. In this contribution we give an overview of the status, recent results, and future plans of cosmic ray detection with the LOFAR radio telescope.Peer Reviewe

Nelles, Anna

edoc Server  Dokumenten-Publikationsserver der Humboldt-Universität zu Berlin

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd26th Extended European Cosmic Ray SymposiumIOP Conf. Series: Journal of Physics: Conf. Series 1181 (2019) 012020IOP Publishingdoi:10.1088/1742-6596/1181/1/0120201Cosmic Ray Physics with the LOFAR Radio TelescopeT Winchen 1 and A Bonardi 2, S Buitink 1, A Corstanje 2, H Falcke2,3,4, B M Hare 5, J R Hörandel 2,3, P Mitra 1, K Mulrey 1, A Nelles 6,J P Rachen 2, L Rossetto 2, P Schellart 2,7, O Scholten 5,8, S ter Veen2,4, S Thoudam 2,9, T N G Trinh 51 Astrophysical Institute, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium,2 Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, 6500 GLNijmegen, The Netherlands,3 NIKHEF, Science Park Amsterdam, 1098 XG Amsterdam, The Netherlands,4 Netherlands Institute of Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, TheNetherlands,5 KVI-CART, University Groningen, P.O. Box 72, 9700 AB Groningen,6 Institut für Physik, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin,Germany,7 Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA,8 Interuniversity Institute for High-Energy, Vrije Universiteit Brussel, Pleinlaan 2, 1050Brussels, Belgium,9 Department of Physics and Electrical Engineering, Linnéuniversitetet, 35195 Växjö, SwedenE-mail: tobias.winchen@rwth-aachen.deAbstract. The LOFAR radio telescope is able to measure the radio emission from cosmic rayinduced air showers with hundreds of individual antennas. This allows for precision testing ofthe emission mechanisms for the radio signal as well as determination of the depth of showermaximum Xmax, the shower observable most sensitive to the mass of the primary cosmic ray, tobetter than 20 g cm−2. With a densely instrumented circular area of roughly 320m2, LOFARis targeting for cosmic ray astrophysics in the energy range 10161018 eV. In this contributionwe give an overview of the status, recent results, and future plans of cosmic ray detection withthe LOFAR radio telescope.1. IntroductionThe detection of cosmic rays via the radio technique has matured from application in singleengineering projects only to usage on large scales [1]. All major experiments in the eld nowadaysuse this technique at least complementary with more traditional methods as surface detectorarrays or uorescence telescopes. The radio technique allows for precision measurements ofthe depth of shower maximum of air showers Xmax and the energy deposit in the atmosphere.Observation of cosmic rays with radio telescopes such as the Low Frequency Array (LOFAR) [2]and potentially also the future Square Kilometer Array (SKA) [3] provide observations ofindividual cosmic ray air showers with a high number of antennas, enabling testing details of theradio emission mechanism and shower development. High precision measurements of the cosmicray mass composition in the energy range below the ankle may be important to understand theexpected transition between galactic and extra-galactic cosmic rays [4].26th Extended European Cosmic Ray SymposiumIOP Conf. Series: Journal of Physics: Conf. Series 1181 (2019) 012020IOP Publishingdoi:10.1088/1742-6596/1181/1/0120202Figure 1. Event reconstruction with LOFAR. (left) Amplitude of the radio signal in the showerplane. Color in circles denote the observed power in the LOFAR LBA, color in the backgrounddenotes the signal from simulation of a shower with corresponding input parameters. (right)Reduced chi-square from the comparison of simulated iron (magenta squares) and proton (bluedots) showers with varying Xmax with the data.LOFAR is the rst digital radio telescope with more than 50 stations distributed throughoutEurope. Twenty-four of these stations are located in a dense core in the Netherlands. Five ofthe core stations are located within a circle of 320m radius, the so called `superterp'. Each corestations consists of 96 `low-band antennas' (LBA) operating from 1090MHz and 768 high-bandantennas (HBA) operating from 110240MHz. Both antennas are omnidirectional V-shapeddipoles, but HBA are analog beamformed for astronomical observations which diminish theirusability for cosmic ray physics. In the following we will thus discuss LBA only.2. Cosmic Ray Detection With LOFARCosmic ray detection with LOFAR currently relies on detecting the particle cascade with an arrayof twenty scintillators, the LOfar Radboud air shower Array (LORA) [5]. These scintillators arearranged in groups of four around the stations on the superterp. If thirteen out of the twentystations simultaneously detect particles, indicating a high energy shower, 2ms out of 5 s ofbuered antenna data are read out for further oine analysis.The properties of the primary cosmic-rays are reconstructed by the comparison of the datawith dedicated simulations using the CORSIKA [6] and CoREAS [7] software packages. Thisyields a higher resolution on Xmax then using parametrizations of the radio prole in groundplane [8] and simplies the evaluation of the detection eciency for the given shower parameters.An example of a detected event and the reconstruction is shown in Figure 1. The left plotshows an example of a recorded event together with the best tting simulated shower. The ploton the right the reduced chi-square as measure of the goodness of t for all simulated showers.The Xmax of the individual shower is obtained from a t of a parabola to the simulated values.With this procedure, LOFAR measures the cosmic ray mass composition in the energyrange 10171017.5 eV. The reconstruction achieves an energy resolution of 32% and a statisticaluncertainty of 16 g cm−2 on Xmax. The total systematic uncertainty on the energy calibration26th Extended European Cosmic Ray SymposiumIOP Conf. Series: Journal of Physics: Conf. Series 1181 (2019) 012020IOP Publishingdoi:10.1088/1742-6596/1181/1/0120203Figure 2. Variation of the refractive index along the shower prole for 100 dierent cosmic rayevents observe with LOFAR.is 27% and on the depth of shower maximum +14−10 g cm−2. Improvements of the accuracy ofthe mass-composition data in the energy range covered by LOFAR may be vital to discriminatebetween models for the origin of galactic and extragalactic cosmic rays.3. Improvement of Systematic Uncertainties3.1. Atmospheric CorrectionsThe observed intensity distribution of the radio signal depends on the dierence in propagationtime of particles and radio waves, and therefore on the refractivity prole of the atmosphere at thetime of the event. The refractivity of the atmosphere depends on the density and humidity andis thus expected to vary also on short time scales [9]. In Figure 2 the dierence of the refractiveindex at the time of 100 individual cosmic ray events to the US standard atmosphere [10] isshown. At an altitude of 58 km where the shower maximum is located, and thus the bulk ofradiation is emitted, the variation is of 3-5%. This corresponds to an uncertainty in the inferredXmax of 3.511 g cm−2 [9].So far in LOFAR and, to our knowledge, also in all other cosmic ray experiments, standardizedaverage atmospheric models have been used for the interpretation of all showers. To accountfor the variation of the index of refraction, we started to use refractivity proles obtained fromatmospheric data of the Global Data Assimilation Project (GDAS) [11] in our simulations. Forthis, we contributed a modication to the CORSIKA simulation software to consistently usearbitrary atmospheric proles in the simulation of the particle cascade and radio signals. Themodication is included in CORSIKA version 7.6300 and following. For this purpose we tthe ve layer atmospheric model to the available data for the location of the observatory andcalculate the resulting index of refraction. The t parameters describing the atmosphere layers,respectively the tabualted index of refraction as a function of height, is then fed into the particleand radio simulation. We developed a corresponding software gdastool that manages datadownload, t, and steering le generation. This tool is also distributed alongside the CORSIKAsource code [12].26th Extended European Cosmic Ray SymposiumIOP Conf. Series: Journal of Physics: Conf. Series 1181 (2019) 012020IOP Publishingdoi:10.1088/1742-6596/1181/1/0120204Figure 3. Antenna calibration factor as function of the frequency for galaxy calibration andreference source. (left) Without detailed model of the LOFAR signal chain and (right) withdetailed model of the LOFAR signal chain.3.2. Antenna CalibrationTo reconstruct amplitude and spectral shape of radio signals an accurate calibration of thereceiving system is imperative. However, at meter wavelength achieving the accuracy incalibration necessary for cosmic ray physics with antenna systems is challenging. No anechoicchambers are easily accessible that can contain the antennas to be calibrated as well as thereference sources in the far eld region. Therefore even acquiring reference sources for in-situ calibrations that are well dened in the desired frequency range is dicult. Alternativelyto articial reference sources, the ubiquitous galactic background can be used as reference.Compared to articial sources the signal emitted by the galaxy is well understood in amplitudeand shape. However, the galaxy is not a point source so that calibrations using the galaxy haveto be based on a model for the directivity of the antenna.So far we used a calibration obtained from an in-situ measurement with a reference sourceoptimized for frequencies higher than our observation band. The calibration with a referencesource is consistent with a calibration using the galaxy [13]. However, by modelling of thenoise contributions of the individual components of the signal processing chain we reduced theuncertainty of the galaxy calibration to a value lower than the uncertainty in calibration usingthe reference sources. The calibration constants obtained with and without detailed modellingof the full signal chain methods are displayed in Figure 3. In particular in the frequencyrange between 6080MHz the uncertainty on the galaxy calibration is now smaller than thesystematic uncertainty arising from two dierent calibrations of the reference source providedby the manufacturer. This reduces a frequency dependent bias of the measurement of the slopeof the spectral shape of the signal as shown in 4. This also enables detailed analysis of thefrequency spectrum of the observed events that may be used as an additional observable sensitiveto Xmax [14].4. Future ExtensionsSeveral extensions are currently planned, respectively already under construction, that willincrease the amount of data and extend the energy range for astrophysics analysis with LOFAR.4.1. LORA ExpansionIn April 2018 we installed infrastructure for twenty more particle detectors at ve LOFAR stationsclose to the superterp. Installation of the detectors and electronics is expected to be nished26th Extended European Cosmic Ray SymposiumIOP Conf. Series: Journal of Physics: Conf. Series 1181 (2019) 012020IOP Publishingdoi:10.1088/1742-6596/1181/1/0120205Figure 4. Dierence of the slope of the frequency spectrum at high and low frequencies forLOFAR data using dierent calibrations and simulations.within 2018. These additional detectors will increase the eective area for cosmic ray detectionby approximately 40% and enable the development of new trigger modes for specic classes ofevents. Of particular interest are showers whose Cherenkov cone is located outside the superterp.While the radio signal is greatly diminished in this geometry, it may be possible to retrieve moreinformation on the longitudinal shower development, thus improving composition measurementsand eventually constraining hadronic interaction models.4.2. Advanced TriggerThe trigger rate for cosmic ray detection is limited to approximately one trigger per hour to notdisturb astronomical observations of the LOFAR telescope. This is achieved by requiring a signalover threshold in 13 out of 20 LORA particle detectors. This yields virtually 100% detectioneciency for showers above an energy of 0.1EeV, thereby dening the lower energy threshold forthe data useable for composition studies. However, as a consequence of these trigger conditionsmost of the recorded showers are of lower energies and approximately 80% do not even containan observable radio pulse.Reduction of the energy threshold for bias-free composition measurements while keeping thetrigger rate constant requires to focus on showers with a detectable radio signal only. This ispossible with a hybrid trigger that requires a strong signal in the antennas in coincidence witha signal in the particle array. Such an hybrid trigger will be implemented using a monitoringchannel implemented in the LOFAR system that records the voltage levels in the antennas andcan be used to send information to the LORA system.Still, the current setup in LOFAR and all other radio experiments outside Antarctica requiresparticle detectors to trigger on cosmic ray air showers. Triggering on radio data only wouldinstead allow for a very cost-eective increase of the instrumented area. Therefore we aredeveloping a self-trigger aiming for a high reduction of RFI [15].4.3. LOFAR 2.0In the current conguration, LOFAR observations cannot overlap and data of HBA and LBAcannot be recorded at the same time. As currently roughly 50% of the observations use HBA,and also data in-between observations cannot be processed, the available time for cosmic rayobservations is limited. LOFAR 2.0 is a planned extension of the technical capabilities of LOFAR26th Extended European Cosmic Ray SymposiumIOP Conf. Series: Journal of Physics: Conf. Series 1181 (2019) 012020IOP Publishingdoi:10.1088/1742-6596/1181/1/0120206allowing in particular for simultaneous observations with high-band and low-band antennasand also to perform multiple observations independently in parallel. This extension can thussignicantly increase the available cosmic-ray data.5. ConclusionPrecise measurements of the mass composition of cosmic rays are important to understand theirorigin and propagation. LOFAR continues to measure the cosmic ray mass composition withhigh precision in the expected transition region between cosmic rays of galactic and extragalacticorigin. Several improvements of the analysis chain are being implemented that will allow for thereduction of the systematic uncertainties on composition and energy measurement. Additionalupgrades are currently planned, respectively are already under construction, that will increasethe duty cycle of LOFAR for cosmic ray measurements and reduce the energy threshold for masscomposition analysis.AcknowledgementsThe LOFAR cosmic ray key science project acknowledges funding from an Advanced Grant of theEuropean Research Council (FP/2007-2013) / ERC Grant Agreement n. 227610. The project hasalso received funding from the European Research Council (ERC) under the European Union'sHorizon 2020 research and innovation programme (grant agreement No 640130). We furthermoreacknowledge nancial support from FOM, (FOM-project 12PR3041-3) and NWO (Top Grant614- 001-454, and Spinoza Prize SPI 78-409). TW is supported by DFG grant WI 4946/1-1. LOFAR, the Low Frequency Array designed and constructed by ASTRON, has facilities inseveral countries, that are owned by various parties (each with their own funding sources), andthat are collectively operated by the International LOFAR Telescope foundation under a jointscientic policy.References[1] Huege T 2016 Phys. Rept. 620 152 (Preprint 1601.07426)[2] van Haarlem M P et al. 2013 Astronomy and Astrophysics 556 A2 (Preprint 1305.3550)[3] Huege T et al. 2015 Proceedings, Advancing Astrophysics with the Square Kilometre Array (AASKA14):Giardini Naxos, Italy, June 9-13, 2014 vol AASKA14 p 148 (Preprint 1408.5288)[4] Thoudam S, Rachen J P, van Vliet A, Achterberg A, Buitink S, Falcke H and Hörandel J R 2016 Astron.Astrophys. 595 A33 (Preprint LORA)[5] Thoudam S et al. 2014 Nucl. Instrum. Meth. A767 339346 (Preprint 1408.4469)[6] Heck D et al. 1998 CORSIKA: a Monte Carlo code to simulate extensive air showers. Tech. Rep. FZKA 6019Forschungszentrum Karlsruhe[7] Huege T, Ludwig M and James C W 2013 AIP Conf. Proc. 1535 128 (Preprint 1301.2132)[8] Nelles A et al. 2015 JCAP 1505 018 (Preprint 1411.7868)[9] Corstanje A et al. 2017 Astropart. Phys. 89 2329 (Preprint 1701.07338)[10] National Aeronautics and Space Administration (NASA) 1976 U.S. standard atmosphere Tech. Rep. NASA-TM-X-74335 NASA[11] NOAA Air Resources Laboratory (ARL) 2004 Global data assimilation system (gdas1) Tech. rep. NOAA-AirURL https://ready.arl.noaa.gov/gdas1.php[12] Mitra P et al. 2018 Proceedings of the ARENA Conference (2018), Catania, Italy[13] Nelles A et al. 2015 JINST 10 P11005 (Preprint 1507.08932)[14] Rossetto L et al. 2017 Proceedings of the 35th International Cosmic Ray Conference (ICRC), Busan, Republicof Korea (Preprint 1711.07749)[15] Bonardi A et al. 2017 Proceedings of the 35th International Cosmic Ray Conference (ICRC), Busan, Republicof Korea (Preprint 1711.07749)

Cosmic Ray Physics with the LOFAR Radio Telescope

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