Ultimate precision in cosmic-ray radio detection --- the SKA
Tim Huege, Justin D. Bray, Stijn Buitink, David Butler, Richard Dallier, Ron D. Ekers, Torsten Enßlin, Heino Falcke, Andreas Haungs, Clancy W. James, Lilian Martin, Pragati Mitra, Katharine Mulrey, Anna Nelles, Benoît Revenu, Olaf Scholten, Frank G. Schröder, Steven Tingay, Tobias Winchen, Anne Zilles
UUltimate precision in cosmic-ray radio detection — the SKA
Tim
Huege ,(cid:63) , Justin D.
Bray , Stijn
Buitink , David
Butler , Richard
Dallier , , Ron D.
Ekers , Torsten
Enßlin , Heino
Falcke , , Andreas
Haungs , Clancy W.
James , Lilian
Martin , , Pra-gati
Mitra , Katharine
Mulrey , Anna
Nelles , Benoît
Revenu , Olaf
Scholten , , Frank G.
Schröder , Steven
Tingay , , Tobias
Winchen , and Anne
Zilles IKP, Karlsruher Institut für Technologie, Postfach 3640, 76021 Karlsruhe, Germany School of Physics & Astronomy, Univ. of Manchester, Manchester M13 9PL, United Kingdom Astrophysical Institute, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium EKP, Karlsruher Institut für Technologie, Kaiserstr. 12, 76131 Karlsruhe, Germany Subatech, 4 rue Alfred Kastler, 44307 Nantes cedex 3, France Station de radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, Nançay, France CSIRO Astronomy & Space Science, NSW 2122, Australia Max-Planck-Institut für Astrophysik, Karl-Schwarzschildstr. 1, 85748 Garching, Germany Dept. of Astrophysics/IMAPP, Radboud Univ. Nijmegen, 6500 GL Nijmegen, The Netherlands Netherlands Institute for Radio Astronomy (ASTRON), 7990 AA Dwingeloo, The Netherlands ECAP, Univ. of Erlangen-Nuremberg, 91058 Erlangen, Germany Department of Physics & Astronomy, University of California, Irvine, CA 92697, USA Kernfysisch Versneller Instituut, Univ. of Groningen, 9747 AA Groningen, The Netherlands Interuniversity Institute for High-Energy, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium Istituto di Radioastronomy, INAF, via P. Gobetti, Bologna, 4012, Italy International Centre for Radio Astronomy Research, Curtin University, Bentley, WA, 6102, Australia
Abstract.
As of 2023, the low-frequency part of the Square Kilometre Array will goonline in Australia. It will constitute the largest and most powerful low-frequency radio-astronomical observatory to date, and will facilitate a rich science programme in astron-omy and astrophysics. With modest engineering changes, it will also be able to measurecosmic rays via the radio emission from extensive air showers. The extreme antennadensity and the homogeneous coverage provided by more than 60,000 antennas withinan area of one km will push radio detection of cosmic rays in the energy range around10 eV to ultimate precision, with superior capabilities in the reconstruction of arrivaldirection, energy, and an expected depth-of-shower-maximum resolution of <
10 g / cm . Over the past decade, radio detection of cosmic-ray air showers has made tremendous progress [1].Today, radio antenna arrays routinely complement classical cosmic-ray observatories, and have provento contribute valuable information with competitive resolutions achieved in the determination of therelevant air-shower parameters [2]. Arrival direction and energy of the primary particle can be mea-sured accurately even with sparse antenna arrays where individual detector stations are spaced 100 to (cid:63) e-mail: [email protected] a r X i v : . [ a s t r o - ph . I M ] F e b
00 metres apart. Determination of the depth of shower maximum (X max ) is also possible with suchsparse arrays, with resolutions of approximately 40 g / cm achieved with today’s analysis approaches[3, 4]. The true power for X max measurements with radio techniques, however, lies in precision mea-surements of the radio-emission footprints with dense antenna arrays.In this article, we discuss the potential of ultra-precise air-shower measurements with the up-coming Square Kilometre Array (SKA) . We illustrate the expected improvement over the existingLow Frequency Array (LOFAR) telescope, describe the science potential and discuss the necessaryengineering changes to the SKA baseline design. In low-frequency radio astronomy, LOFAR can be seen as a pathfinder towards the SKA. The same istrue for cosmic-ray detection with dense antenna arrays [5]. Only with the impressive results achievedby LOFAR did the potential for air-shower detection with the SKA become apparent. In particular, theresolution in the determination of X max achieved with LOFAR [6] sparked the interest in air-showerdetection with the SKA.LOFAR uses a top-down analysis approach: For each measured air-shower event nearly 100 air-shower simulations with proton- and iron-primaries are generated with the microscopic CoREASMonte Carlo code [7]. The individual simulations are then compared with the measurements to findthe air-shower parameters that yield the best agreement. It turns out that X max is the one quantitythat governs the agreement, and hence it can be read o ff directly from the best-fitting simulations. Theaverage X max resolution achieved with this analysis techique amounts to 17 g / cm [6]. The best eventseven reach an X max determination to within better than 10 g / cm . For reasons explained below, weexpect that the SKA will yield an X max measurement to within 10 g / cm for each and every measuredcosmic-ray shower. This is significantly better than the 20 g / cm achievable with the most precisetechniques available so far, namely fluorescence- and Cherenkov-light detection (see, e.g., [8]). The key factor for the increased measurement precision of the SKA with respect to LOFAR lies inthe layout of the antenna array. While LOFAR constitutes a fairly inhomogeneous array with signif-icant gaps in between radio antennas (see Fig. 1 top-left), the SKA will possess a very dense, veryhomogeneous core of radio antennas (see Fig. 1 top-right). The radio-emission footprint has severalcharacteristics sensitive to the depth-of-shower maximum [1], and for LOFAR only such events wherethese features are sampled favorably with the inhomogeneous antenna array can yield supreme X max resolution. For the SKA, any “contained” event will be sampled very precisely (see Fig. 1 bottom), andwill thus be reconstructable with superior precision. Initial simulation studies confirm this expectationwith typical X max resolutions of ≈ / cm achieved with a LOFAR-style analysis [9]. In combinationwith experimental uncertainties, a practical resolution of ≈
10 g / cm thus seems achievable. Also the increased frequency coverage of 50–350 MHz of the SKA as opposed to 30–80 MHz forLOFAR (in the most-used low-frequency observation mode) will benefit the reconstruction quality.Again, initial simulation studies [9] illustrate this improvement; a pure-simulation reconstruction foran SKA-like array with a frequency coverage from 30–80 MHz achieves an ≈
10 g / cm X max resolu-tion, while the frequency range from 50–350 MHz yields an X max resolution of ≈ / cm . Throughout this article, we use the shorthand SKA to refer to the phase 1 implementation of the low-frequency aperture-array part of the SKA, which will measure in the frequency range from 50–350 MHz.
00 300 200 100 0 100 200 300 400
West-East (m) S o u t h - N o r t h ( m ) i n t e g r a t e d p o w e r ( a . u . ) Figure 1.
CoREAS-based simulation of how the air-shower radio emission would be sampled by LOFAR (top-left) and the SKA core (top-right and zoom-in at bottom). The SKA will measure the radio-emission footprintwith an extremely dense and homogeneous array of antennas, yielding superior reconstruction quality on anevent-to-event basis. The appearance of a clear ring-structure in the SKA simulation is related to the presence ofa Cherenkov ring at higher frequencies. Plots are from the simulation study presented in [9].
While cosmic-ray detection with LOFAR has provided impressive results, it does su ff er from ratherlimited event statistics. The reasons for this are two-fold: So far, only the innermost area of theLOFAR core can be used for air-shower detection, limiting the fiducial area to less than 0.1 km .Furthermore, cosmic-ray detection is not active 100% of the time, mostly because technical and or-anisational hurdles prevent fully commensal operation. This resulted from the fact that cosmic-raydetection capability was not a priority feature from the beginning of the design phase. With the SKA,we aim for 100% commensal operation and a fiducial area of roughly 1 km , increasing the eventstatistics by a factor of more than 100 over LOFAR. This will make the energy range from a few times10 eV up to a few times 10 eV accessible for detailed measurements, with approximately 10,000events measured per year above 10 eV. The science potential for cosmic-ray detection with the SKA lies in precision measurements. Severalscientific goals can be addressed with SKA air-shower measurements.
The energy range accessible with the SKA, from a few times 10 eV up to a few times 10 eV, issuspected to harbour a transition from Galactic to extragalactic cosmic rays [10, 11]. The key met-ric needed to investigate this hypothesis is highly precise composition information. With supremeX max resolution, the SKA will deliver very-high-quality data in this important energy range. Exploit-ing information beyond a per-event determination of X max , we hope that even a separation betweenindividual particle species, possibly even between proton and Helium, could become feasible. The air shower evolution, in particular X max , is sensitive to hadronic interaction physics. It has beenshown before that the proton-air cross section, multiplicity of secondaries in interactions, elasticityand pion charge ratios can be probed with air-shower measurements [12]. With the high-precisionmeasurements provided by the SKA, such studies in the very forward direction and at energies be-yond those accessible by the Large Hadron Collider could be carried out with minimised systematicuncertainties. If the deployed particle detector array allows a measurement of the muonic componentof the air showers, even more detailed studies of the air-shower physics will become possible.
The LOFAR analysis approach employed today only uses the integrated power measured by the an-tennas. In addition, there are measurements of signal timing, pulse shape and polarisation that verylikely carry additional information. Furthermore, there is phase information in the radio signal thatis currently not taken into account at all. We expect that with near-field interferometric techniques,it should be possible to do “tomography” of the electromagnetic cascade in the air shower, and thusprovide information that goes well beyond the pure determination of X max . It has been shown previously that strong electric fields in the atmosphere, in particular those occurringduring thunderstorms, influence the radio emission from air showers [13]. In fact, properties of theatmospheric electric fields can be probed in situ by detailed analysis of the measured radio emissionand comparison with theoretical calculations [14]. The air-shower tomography development maylead as well to imaging techniques for the reconstruction of charge flows during lightnings. With theSKA, it will thus be possible to probe the physics of thunderstorms and also test possible connectionsbetween lightning initiation and cosmic-ray air showers.
Engineering changes to the baseline SKA-design
The SKA baseline design does not foresee the capabilities needed for air-shower detection. Here weshortly describe the required engineering changes that have been proposed to the SKA managementand are currently under consideration. Bu ff ers are foreseen in the current SKA design to temporarily store beam-formed quantities. For air-shower detection these bu ff ers would need to additionally store the raw signals digitised at individualantennas. With 800 MHz sampling, at least 8-bit, preferably 12-bit dynamic range, and a bu ff er depthof 10 ms, 1.4 TB of bu ff ers would be needed for 60,000 dual-polarised antennas. This bu ff ering shouldbe carried out with 100% duty cycle, fully commensal with other observations. On a trigger, 50 µ sworth of data need to be read out, which amounts to 7.2 GB of data per event. We estimate a readout rate of 1 shower per minute at 10 eV. With the above numbers, the averagedata rate for readout would be 120 MB / s. This is very small in comparison with the continuous datastream handled by the SKA. However, cosmic rays arrive with a Poissonian time distribution and willthus cause bursts of data that need to be read out quickly. Assuming a data rate of 2.4 GB / s, leadingto 3 seconds of readout time during which the bu ff ers are frozen, we will accumulate a tolerabledeadtime of 5%. A di ffi cult problem is the generation of an e ffi cient and pure trigger in real time. While it would inprinciple be possible to generate a trigger from the radio data, we propose to install a particle-detectorarray for the purpose of triggering. This will also gather valuable information on the particle contentof the air shower which can be included in the event reconstruction. The particle detector array shouldbecome e ffi cient at 10 eV and thus have an average detector spacing of 50–100 m. The fiducialarea should extend beyond the core of antennas because non-contained events will also be usable. Anattractive option is to deploy 180 scintillating particle detectors with a size of 3.6 m each, availablefrom the dismantled KASCADE array [15], between the SKA antenna stations (see Fig. 2). Thedetectors could be read out via the same kind of RF-over-fibre links that are used for the antennareadout. Assuming 4 channels per particle detector, this would require an additional 720 readoutchannels (on top of the 262,144 needed for the entire antenna array). Special care has to be taken ofthe radio-quietness of the particle detectors. Experience with LOFAR has shown that careful shieldingensures RFI-quietness. The low-level readout capability needed for cosmic-ray measurements will greatly benefit the SKAfor diagnostic purposes. Experience with LOFAR shows that many low-level problems (swappedcables, damaged low-noise amplifiers, broken antennas, time synchronisation problems, ...) could bediagnosed with the per-antenna data taken in the cosmic-ray mode. Such problems are otherwise verydi ffi cult to track down and will generally deteriorate the quality of astronomical observations. igure 2. Possible layout of a particle detector array with 180 detectors (red squares) in between and outside theantenna stations of the SKA (gray dots).
With moderate engineering changes, the SKA can become a cosmic-ray detector that will performextremely precise measurements of individual air showers in the energy range from a few times 10 to a few times 10 eV. This would enable a rich scientific harvest in return on a very limited addi-tional investment. The SKA focus group on high-energy cosmic particles has proposed the necessaryengineering changes to the SKA management, where they are currently under consideration. References [1] T. Huege, Physics Reports , 1 (2016)[2] F.G. Schröder, Prog. Part. Nucl. Phys. (submitted), arXiv:1607.08781 [3] P.A. Bezyazeekov, N.M. Budnev, O.A. Gress, et al., JCAP , 52 (2016)[4] F. Gaté for the Pierre Auger Collaboration, these proceedings (2016)[5] P. Schellart, A. Nelles, S. Buitink, et al., Astronomy & Astrophysics , A98 (2013)[6] S. Buitink, A. Corstanje, H. Falcke, et al., Nature , 70 (2016)[7] T. Huege, M. Ludwig, C.W. James, AIP Conf. Proc. pp. 128–132 (2013)[8] A. Aab, P. Abreu, M. Aglietta, et al., Nucl. Instrum. Meth. A , 172 (2015)[9] A. Zilles, S. Buitink, T. Huege, these proceedings (2016)[10] W.D. Apel, J.C. Arteaga-Velàzquez, K. Bekk, et al., Phys. Rev. D , 081101 (2013)[11] P. Blasi, The Astronomy and Astrophysics Review , 1 (2013)[12] R. Ulrich, R. Engel, M. Unger, Phys. Rev. D , 054026 (2011)[13] S. Buitink, W.D. Apel, T. Asch, et al., Astronomy & Astrophysics , 385 (2007)[14] P. Schellart, T.N.G. Trinh, S. Buitink, et al., Phys. Rev. Lett. , 165001 (2015)[15] T. Antoni, W.D. Apel, F. Badea et al., Nucl. Instrum. Meth. A , 490 (2003) http: // astronomers.skatelescope.org / home / focus-groups / high-energy-cosmic-particleshigh-energy-cosmic-particles