Air showers and cosmic rays through the eyes of digital radio telescopes Anna Nelles University of California Irvine LOFAR Key Science Project: Cosmic Rays A. Bonardi, S. Buitink, A. Corstanje, H. Falcke, J.R. Hörandel, P. Mitra, K. Mulrey, J.P. Rachen, L. Rossetto, P. Schellart, O.Scholten, S. ter Veen, S. Thoudam, T.N.G. Trinh, T. Winchen
Cosmic rays and air showers Cosmic ray Primary π 0 γ K + π 0 p π + p γ π − e + π 0 γ e − • Cosmic rays of K 0 γ energies > 10 14 eV n p n µ + γ π + are not observed µ − ¯ ν µ n e − n p directly on Earth π − e + γ • Flux decreases with γ γ e − energy, 1 particle per µ + ν µ ν µ γ m 2 per hour to ¯ ν µ ν e 1 particle per km 2 per e + e + e − year at highest ¯ γ ν µ µ − e − energies • Extended detectors Hadronic Muonic component Electromagnetic needed component component 2 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Radio emission of air showers cosmic ray Electromagnetic component of shower responsible for radio emission atmospheric nucleus Emission arises from : deflection of particles in geomagnetic field • e+ and e- are accelerated in B e - e + geomagnetic field (geomagnetic effect) air shower • more e- than e+ in the shower by collecting e- from e - drift atmosphere e + drift (charge excess) e - e - e - e + e - e + e + e + e - e + Emission is affected by : coherent • Superposition of emission radio pulse • Cherenkov effects 3 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Traditional methods & radio detection Air showers can be detected in many ways Cosmic ray • Particle detectors : 100% duty cycle little sensitivity to primary Air shower particle • Cherenkov and Fluorescence light Fluorescence detectors : 10% duty cycle and high quality observing conditions, sensitive to primary • Radio detectors : > 95% duty cycle and sensitive to primary particle Fluorescence telescope Cherenkov Particle Radio antenna detector detectors 4 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Detection at radio telescopes Single antenna data LOFAR 30-80 MHz Particle detectors provide trigger • Signals are short non-repeating broad-band pulses • Need access to raw voltage data • full frequency range: 10 - 300 MHz, about 50 nanoseconds • Arrival times in antennas determined by shower arrival direction, source in atmosphere 5 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Measuring composition 700 g/cm 2 650 g/cm 2 X max ~ 600 g/cm 2 X max X max • Particle type determines interaction height, which determines signal distribution • Prediction can be simulated Proton Iron Projection onto v x B axis 6 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Measuring composition Buitink et al., Phys. Rev D, 2014 • Fit quality of simulated pattern to measured data, determines most probable value for shower height • LOFAR data is extremely precise, often better than 20 g/cm 2 , which is current standard of field • Detailed measurement of single shower only possible with radio • Examples: Proton and Iron simulations Fe proton 7 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Measuring energy Nelles et al. JCAP 2015 • Radio emission also excellent in determining energy • Fitted intensity pattern is directly proportional to energy of the shower • Energy resolution better than particle detectors • Very small systematic uncertainties • With energy and composition we can do Astrophysics 8 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Astrophysical results Buitink et al, Nature 2016 proton Proton fraction X max iron Helium fraction log(Energy) • Already with 100 showers, measurements competitive to other experiments in the field • High precision measurements determine strong light component at transition energies of 10 17 - 10 18 eV 9 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Astrophysical implications • LOFAR results already now put tension on theories: Thoudam et al, A&A 2016 • Strong light component argues 6 TUNKA (QGSJET − II − 04) KASCADE TUNKA (EPOS − LHC) LOFAR (QGSJET − II − 04) against single type of source of Yakutsk (QGSJET − II − 04) LOFAR (EPOS − LHC) 5 Auger (QGSJET − II − 04) Yakutsk (EPOS − LHC) Galactic cosmic rays after the WR − CRs (C/He=0.1) Auger (EPOS − LHC) WR − CRs (C/He=0.4) Kampert & Unger 2012 4 Fe knee, which suppresses GW − CRs Si protons 3 〉 lnA C 〈 2 • Strong light component, but not He 1 purely protons, argues against imprint of pair-productions 0 H 6 8 9 10 7 11 10 10 10 10 10 10 • More likely a second Galactic Energy E (GeV) component, caused by for example Galactic-Wind or Wolf-Rayet stars 10 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Synergies in astrophysics Detailed source observations Improved composition and energy of cosmic rays Magnetic field measurements and models φ (rad m − 2 ) Buitink et al (2016) van Eck et al. (2016) better understanding of sources better understanding of propagation 11 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Synergies in calibration methods Cosmic ray measurement Astronomical observation • Single antenna, raw • Combined antenna voltage data signals, visibilities • no beamforming • beamformed • no time-integration • time-integrated • Very detailed • Detailed understanding of understanding of station-beam needed individual antenna needed • Time-dependent • Time-dependent monitoring of array monitoring of single performance antenna performance • Absolute calibration on • Absolute calibration on astronomical sources and artificial sources sky models 12 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Antenna calibration Flying reference Model of the source electronics and antenna response iterative improvement correction Stationary data reference source • Totally independent from “real units” sky models, agreement provides confidence Nelles et al. JINST 2015 13 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
RFI cleaning • In raw voltage data: A stable phase difference between two-antenna pairs reveals RFI transmitter • Data can be recorded and flagged offline • Better accuracy than baseline fitting and continuous monitoring of RFI environment • Phase difference also reveal timing stability of system Corstanje et al. A&A 2016 14 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Timing calibration hyperboloid • Monitoring of phase differences • Cosmic rays signals arrive as shows that also LOFAR clock hyperboloid with shows small drifts subnanosecond structure • Larger jumps (sample shifts) • Perfect cross-check for system are immediately recognized stability 15 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Instrument health • Any radio telescope can detect air showers, if there is access to raw voltage data • Unexpected failures are easily identified in raw voltage data normal polarization • Timing instability shows in • Swapped cable in raw data polarization reconstruction • Identifiable without • No monitoring run needed analysis 16 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Thunderstorms Schellart et al. PRL 2015, Trinh et al. PRD 2015, Scholten et al PRD 2016 • Cosmic rays during thunderstorm show unique polarization signature • Traces the strength and the height of electric fields • Cosmic rays radio signals are a surprising tool to study thunderclouds 17 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Future plans • LOFAR will continue to do high impact cosmic ray science, better statistics, higher energies, improved systematics • Continued thunderstorm measurements — little statistics in the Netherlands • Long-term effort: SKA - ultimate precision for cosmic rays and particle interactions in shower LOFAR core SKA core • Requires engineering change proposal, currently under discussion 18 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
Conclusions • Exciting astrophysics with LOFAR Proton fraction • LOFAR can resolve shower maximum to better than 20 g/cm 2 • good resolution reconstruction of cosmic ray particle type • will lead to improved understanding of sources and propagation Helium fraction • Cosmic ray data is perfect monitoring tool • continous RFI monitoring • continuous timing-calibration and monitoring • in-depth study of antenna properties • absolute calibration without sky models • Unexpected science such as studying electric fields during thunderstorms data antenna model 19 Anna Nelles, Science at Low Frequencies III, Pasadena, 2016
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