Precision measurements of cosmic rays up to the highest energies with a large radio array at the Pierre Auger Observatory

The Pierre Auger Observatory is presently being upgraded to enlarge its detection capabilities for ultra-high-energy cosmic rays. Part of this upgrade is a radio detector array, aimed to cover a surface area of 3000 km2 to measure the properties of the highest-energy cosmic rays. The plans for this radio upgrade are outlined


Introduction
The Pierre Auger Observatory, located in western Argentina, is the world's largest cosmic-ray Observatory [1]. The objectives of the Observatory are to probe the origin and characteristics of cosmic rays above 10 17 eV and to study the interactions of these, the most energetic particles observed in Nature. The Auger design features an array of 1600 water-Cherenkov detector stations on a 1500 m grid, spread over 3000 km 2 , and overlooked by 24 air fluorescence telescopes. The layout is illustrated in Fig. 1. In addition, three high-elevation fluorescence telescopes overlook a 23.5 km 2 , 61-detector infilled array with 750 m spacing. Radio emission from extensive air showers is measured with the Auger Engineering Radio Array (AERA), comprising more than 150 radio detector stations, covering an area of about 17 km 2 , co-located with the infill array [2,3].
At present, the Pierre Auger Collaboration is working on an upgrade of the Observatory, Auger Prime [4]. The physics case of the upgrade is outlined in [5]. The key science questions to be addressed are: What are the sources and acceleration mechanisms of ultra-high-energy cosmic rays (UHECRs)? Do we understand particle acceleration and physics at energies well beyond the LHC (Large Hadron Collider) scale? What is the fraction of protons, photons, and neutrinos in cosmic rays at the highest energies?
Objectives of the upgrade are to: • Elucidate the mass composition and the origin of the flux suppression at the highest energies, i.e. the differentiation between the energy loss effects due to propagation, and the maximum energy of particles injected by astrophysical sources. * e-mail: horandel@astro.ru.nl -http://particle.astro.ru.nl * * full authorlist available at http://www.auger.org/archive/authors_2018_10.html • Search for a flux contribution of protons up to the highest energies. We aim to reach a sensitivity to a contribution as small as 10% in the flux suppression region. The measurement of the fraction of protons is the decisive ingredient for estimating the physics potential of existing and future cosmic-ray, neutrino, and gammaray detectors; thus prospects for proton astronomy with future detectors will be clarified. Moreover, the flux of secondary gamma-rays and neutrinos due to proton energy loss processes will be predicted.  uncertainties, it is difficult to obtain a satisfactory description of the flux and composition data of the PAO mum-rigidity scenario." A somewhat better description of the PAO data, in particular the X max fluctuations at high energy, can be obtained if an additional light component is assumed to appear in a limited energy range.  Figure 2. Examples of fluxes of different mass groups (as indicated) for describing the Auger spectrum and composition data [5]. Two scenarios are shown: particles being accelerated up to a maximum rigidity (left) and particles undergoing photo-disintegration during propagation (right).
• Study extensive air showers and hadronic multiparticle production. This will include the exploration of fundamental particle physics at energies beyond those accessible at man-made accelerators, and the derivation of constraints on new physics phenomena, such as Lorentz invariance violation or extra dimensions.
A key to understanding the origin of the highestenergy cosmic rays is to precisely measure the elemental composition up to the highest energies (10 20 eV and above). Two baseline scenarios are illustrated in Fig. 2, showing the expected energy spectra for elemental groups. The left-hand side illustrates expectations for a scenario in which cosmic rays are accelerated up to a maximum rigidity. The elemental groups exhibit fall-offs proportional to their elemental charge Z. The right-hand side shows an expectation for particles undergoing photo-disintegration during propagation. In this scenario a significant fraction of light particles (protons) are expected even at the highest energies, this is the main difference to the maximum rigidity scenario, in which heavier particles (large Z) are expected at the highest energies.
To achieve these objectives, a layer of scintillators is being installed above the water-Cherenkov detectors (the Surface Scintillator Detector), the observation time of the fluorescence detectors is being increased, and underground muon detectors are being installed in a part of the Surface Detector array. In addition, radio antennas will be added to each Surface Detector station as described below.

Radio detection of (horizontal) air showers
The radio emission from extensive air showers [6][7][8] is investigated at the Auger Observatory with AERA [9]. Radio emission is detected in the frequency range from 30 to 80 MHz [10]. The antenna response is calibrated with a reference source in-situ in the field [11]. Timing synchronization is performed with a reference transmitter and us-ing radio pulses emitted from commercial airplanes [12]. A resolution of better than 2 ns has been achieved.
Measurements of the polarization of the radio signals allow us to have a closer look at the emission processes of the radiation in extensive air showers [13]. The majority of the emission in the atmosphere is found to be due to the interaction with the geomagnetic field (transverse separation of charges, geomagnetic effect) [14]. Of the order of 15% of the radiation is due to the longitudinal separation of charges (Askaryan effect) [15].
The renaissance of radio detection of cosmic rays began in the early 2000s [6][7][8]. In parallel with the experimental activities, strong efforts were devoted to the modeling of the radio emission physics. Two lines of research contributed most to the detailed understanding we have today. On the one hand, macroscopic, semianalytical calculations [16][17][18][19][20][21] revisited and refined the original concepts for geomagnetic [14] and charge-excess emission [15] in an atmosphere with a realistic refractive index gradient, thereby providing important insights into the relevant mechanisms. On the other hand, microscopic Monte Carlo simulations in the atmosphere [22][23][24][25][26][27], building on experience in calculations for showers in dense media [28], allowed detailed and parameter-free calculations of the radio signals. The measurement of the radio footprint of an air shower yields a precise determination of the shower energy [29,30]. A cosmic ray with an energy of 1 EeV delivers about 15.8 MeV of energy to the ground in the frequency range from 30 to 80 MHz. The radio emission measured on the ground E 30−80 MHz can be used to establish an absolute calibration of the energy scale E CR , using the universal formula knowing the magnetic field B Earth at the location of the detector and the angle α between the direction of the Earth magnetic field and the direction of the air shower. Dark-grey circles indicate isolated particle detections rejected in the reconstruction. The radio signal extends over a significantly larger area than the particle distribution.
simulations. There is thus still room for improvement when employing a detailed reconstruction of the radio signals of inclined air showers, which is currently under investigation.

Presence of Cherenkov signature
The example event shown in Fig. 3 exhibits a clear maximum in the lateral signal distribution at an axis distance of approximately 1000 m. Assuming that the emission arises predominantly from the shower maximum, we can estimate the o -axis angle under which this feature is seen. At the event energy of 2◊10 19 eV, the average depth of shower maximum measured with the Auger fluorescence detector [30,31] amounts to ≥780 g/cm 2 . Using an average density profile for the atmosphere above the observatory [32], we relate this depth of shower maximum X max to a geometrical source distance d by solving the equation Here, X 0 denotes the atmospheric depth of the observatory level, and fl(l) denotes the atmospheric density at the distance l measured along the shower axis from the impact point to the shower maximum. For inclined air showers, the atmospheric curvature needs to be taken into account, therefore the above equation can in general not be solved analytically and d is determined numerically. For a depth of shower maximum of 780 g/cm 2 and the event zenith angle of 82.8 ¶ , the geometric source distance d amounts to 116 km. From this geometrical distance and the axis distances of the antennas, an o -axis angle for each antenna is then calculated using trigonometric relations, the result of which is shown in Fig. 8. The maximum in the lateral signal distribution corresponds to an o -axis angle -8 - Figure 3. A measured horizontal air shower. The blue to green circles indicate the measurement with the Surface Dector, their sizes indicating energy deposit and the color encoding arrival times. Dark-grey circles indicate isolated particle detections rejected in the reconstruction. The radio signal extends over a significantly larger area than the particle distribution [31].
AERA has also been used to measure radio emission from air showers with large zenith angles between 60 • and 84 • ("horizontal air showers") [31,32]. An interesting air shower is shown in Fig. 3. The air shower has been detected with four antennas at the edge of AERA. Its readout was triggered because an isolated Surface Detector station with significant energy deposit (dark-grey circles in Fig. 3) was closer than the 5 km maximum readout distance, presently set in the AERA data acquisition. The locations of the antennas with a signal (marked in red) are in alignment with the ground projection of the air-shower axis reconstructed from the Surface Detector data. The azimuth angles reconstructed from the radio signals and particle-detector measurements agree to within better than 0.5 • . The zenith angle reconstructed with the particle detectors amounts to 83 • , while the zenith angle determined from the arrival times of the radio signals corresponds to 87 • . The low number of radio antennas with signal and their approximate alignment along a line perpendicular to the air-shower axis limit the zenith-angle resolution of the radio measurement in this particular case. The maximum axial distance at which a signal has been measured amounts to 2150 m and the exceptionally large ground distance of the measured radio signals more than 15 km away from the shower axis arises from projection effects. Nevertheless, this example illustrates that the ground area illuminated by radio signals can be significantly larger than the "particle footprint" on the ground.
Large footprints for horizontal air showers are also expected from simulations [7]. Simulated footprints of the radio emission of extensive air showers with various zenith angles in the 30 − 80 MHz frequency band are shown in Fig. 4 for an air shower with an energy of 5 · 10 18 eV. The detection threshold governed by Galactic noise typically Source: Adapted from [30].
A particularly important effect is the dependence of the radio emission on the air shower zenith angle. As the zenit increases, the traversed atmospheric depth grows as 5 cos 1 (✓). The air shower reaches its maximum at a given atmos depth, thus for more inclined showers this maximum will be at significantly larger geometrical distances from the ob than for vertical air showers. As a consequence, the forward-beamed radio emission illuminates a much larger a is illustrated impressively in Fig. 11. The average electric field amplitude is lower (the radiated power is distribute a larger area), but also the LDF is less steep. This makes inclined air showers more favorable for detection with a antenna grid [29]. For a fixed zenith angle, another important factor influencing the geometrical source distance is the depth maximum of an individual air shower, X max . This quantity undergoes statistical fluctuations, but is one of the most imp observables to determine the mass of the primary particle. Changes in X max are also reflected in the geometrical d between radio source and observer, and thus can be exploited to determine X max from radio measurements. Th obvious way to access this information is the LDF, as discussed above and illustrated in the comparison between Fi an iron-induced air shower (small X max ) and Fig. 12 for a proton-induced air shower (large X max ) (note the different s For the same geometrical reasons, the geometrical source distance also influences the shape of the radio wavefront, can be determined by precise timing measurements, and the pulse shape (or spectral index of the frequency spe measured at a given lateral distance. We will discuss the sensitivity of the radio signal to the mass of the primary par more depth in the chapter on experimental results.

Modern models and simulations of air shower radio emission
In parallel with the modern experimental efforts, modeling efforts for the radio emission from extensive air sh were started. We give an overview here of approaches that have been tried out, but will focus on those that are stil maintained at the time of writing this review.

Flawed modern approaches
A number of efforts that were started in the early 2000s later turned out to be flawed. The ''geosynchrotron'' schem followed initially with a semi-analytic calculation in the frequency domain [31]. The explicit assumption of synch radiation of particles on long orbits dominating the emission, however, later turned out to be untrue. corresponds to 1 − 2 mV/m/MHz. The footprint is small for air showers with zenith angles up to 60 • , but becomes very large for inclined showers with zenith angles of 70 • or higher. The white rectangle denotes the size of the 50 • inset. The strong increase of the area illuminated by inclined air showers is due to the large geometrical distance of their emission region from the ground, as sketched in the inset in Fig. 5.
The size of the radio footprint of an air shower has been measured. The distance from the shower axis to the radio detector furthest away (in the shower plane, perpendicular to the shower axis) is depicted in Fig. 5 as a function of the zenith angle of the air shower. In the figure also the expected number of radio stations (on the standard Auger 1500 m grid) with a signal above threshold are indicated together with an estimate of the size of the footprint on the ground. As can be seen, for horizontal air showers (i.e. large zenith angles) the footprints reach sizes exceeding tens of km 2 and dozens of stations will have a signal above the threshold. This is a very important result since it confirms experimentally that the radio emission from horizontal air showers can be measured with radio antennas on the standard Auger 1500 m grid.

The Auger radio upgrade
We are going to install a radio antenna on each of the 1661 stations of the Surface Detector array of the Observatory (see Fig. 1), forming a 3000 km 2 radio array, the largest radio array for cosmic-ray detection in the world. An artist's impression of the planned set-up is given in Fig. 6. It shows (from bottom to top) the water-Cherenkov detector with a layer of scintillators on top (Surface Scintillator Detector) and a radio antenna (Radio Detector on standard 1500 m grid Figure 5. Furthest axis distance at which a radio signal above noise background has been detected with AERA as a function of the air-shower zenith angle [31]. concept of radio antennas on top of the Surface Detector stations has been studied earlier at the Auger Observatory on smaller scales [33,34]. With the combination of water-Cherenkov detector and Surface Scintillator Detector the electron-to-muon ratio (e/µ) is measured for vertical showers. In a similar way the combination of water-Cherenkov detector and Radio Detector will be used to measure the ratio of the electromagnetic energy and the number of muons for horizontal air showers. In turn, the e/µ ratio will be used to derive the particle type of the incoming cosmic ray up to the highest energies. This is the main goal of the Auger upgrade, to measure the particle type of each incoming cosmic ray. Thus, the radio upgrade will increase the aperture of the Observatory for mass-sensitive investigations, allowing the e/µ separation for showers with a broad zenith angle range, from zenith with the Surface Scintillator Detector to the horizon with the Radio Detector.
There is an important difference related to the mass measurement of cosmic rays with AERA (for vertical showers) and the radio upgrade (for horizontal air showers). For vertical showers we use a geometrical method, correlating the size of the footprint on the ground to the distance from the observer to the shower maximum [2,3,35]. From this quantity, the depth of the shower maximum X max is derived, which in turn is dependent on the logarithm of the number of nucleons (nuclear mass), A, of the incoming cosmic ray X max ∝ ln A [2]. For horizontal showers, we aim to apply a different method: We will use a combination of radio antennas and the water-Cherenkov detectors to measure the e/µ ratio in air showers to determine ln A.
Two effects are important for the detection of horizontal air showers with the radio upgrade. First, the enlarged footprint of the radio emission, as explained above, and, second, the absorption behavior of air showers in the atmosphere, as sketched in Fig. 7. For horizontal air showers the electromagnetic as well as the hadronic components are absorbed in the atmosphere and only the muons reach the detector. The atmosphere is transparent to radio emission in the frequency band 30 − 80 MHz, thus, the radiation (most of it being produced near the shower Increasing the mass sensitivity of the Observatory is crucial to understand the origin of cosmic rays, e.g. a good mass sensitivity is needed to distinguish between the baseline scenarios illustrated in Fig. 2. Precise mass (and charge Z) information will also allow to constructing more meaningful sky maps using intervals of rigidity E/Z in place of energy E (as done so far). Isolating particles with a small (or no) charge such as protons, neutrinos, and gamma rays is important to conduct astronomy with such particles at energies exceeding EeV energies. This will give direct hints to the cosmic-ray sources. Finally, the Radio Detector will provide a clean measurement of the electromagnetic (e/m) component of the air showers. This is of interest to study shower physics in the atmosphere. In particular, in the densely populated part of the Surface Detector array [5], we will have water-Cherenkov detectors and Surface Scintillator Detectors with a 750 m spacing, underground muon detectors (AMIGA), and also radio antennas. We will have a clean measurement of the e/m component from the Radio Detector and a clean measurement of the muons with AMIGA. This will allow us to cross check the unfolding of the e/µ ratio with the Surface Scintillator Detector-water-Cherenkov detector combination, based on measured showers. Last but not least, the Radio Detector will provide an independent mass and energy scale to cross check the Surface Scintillator Detectorwater-Cherenkov detector results [29,30].

Performance
In a first analysis the physics potential of the radio upgrade has been studied using CORSIKA [36]/CoREAS [22] simulations. A total of 192 showers has been simulated, half of them protons, half iron nuclei, with energies from 4 to 40 EeV and zenith angles between 60 • and 80 • . A full water-Cherenkov detector simulation and reconstruction has been included in the analysis, but only a simplified treatment of the radio signals was available. The corrected radiation energy has been calculated as described in [37]. The radiation energy has been "smeared out" in order to mimic reconstruction uncertainties. Preliminary system of the SD station will be used to control the hardware parameters of the new RDs and to transmit the We aim for <5 W power required for the additional electronics. If the capacity of the existing SD systems is not sufficient, we will replace the existing solar panel of the SD station with a (now available) more efficient version, providing sufficient electrical energy. Based on the AERA experience and the much simpler design, we expect to deploy ~10-20 RDs per day. The deployment will be conducted in close collaboration with the staff at the PAO and with colleagues of the Auger Collaboration who will support us for this huge effort. As it has been successfully done for AERA, the components will be shipped in sea containers to the PAO and the RDs will be pre-assembled and tested at the main campus of the PAO, followed by the actual deployment of RDs in the field. First, 61 RDs in the region of the 750 m grid will be deployed. This first array will be used to prove the concept of the proposed design. After successful first measurements and verification of the concept, the next 739 stations will be deployed in the field. In parallel, we aim to raise the funds for the additional 861 stations (to fully equip all 1600 SD stations on the 1500-m grid) within the Netherlands and in collaboration with our international partners in the PAO. Including shipment and deployment ~935 k€ will be needed. Since the physics potential of the new radio array is very large and fully in-line with the physics objectives of the PAO upgrade, we are optimistic to find further financial support.
*Sub project #4: Optimization of radio reconstruction for horizontal air showers -PhD student 1, PD 1.
HASs have been already measured with AERA, 47,48 as discussed above. The measurements reveal a large radio footprint of HAS, signals up to 5 km from the shower axis have been detected with up to 80 RDs exhibiting a measurable signal above the noise level. The measurements indicate that the footprint of the EASs covers several (tens) km 2 . These measurements also reveal that HAS are well measured with RDs on a 1500 m grid. These measurements are supported by theoretical considerations and simulations, which also predict a large radio footprint for HAS. 55,56 The method to derive the shower energy from the radio measurements as described above has been adapted for HAS 48 . The resulting CR energy derived from the radio measurements is depicted in Fig. 14, left as a function of the energy, reconstructed with the SD. A good correlation of the two parameters is found. The relative difference between the two parameters in this first analysis exhibits an almost Gaussian distribution with a σ of ~48%. Recent AERA measurements for vertical EASs 42,43 indicate that the e/m component is measured with an accuracy of 17% for EASs with 5 and more RDs with a measurable signal. In the future we will determine  results are illustrated in Fig. 8. The left-hand panel depicts the detected radiation energy as a function of the shower energy. A clear correlation can be seen and only a small dependence on the mass of the incoming cosmic rays (represented in the figure through the extrema protons and iron nuclei). The mass-sensitivity is illustrated on the righthand side. The size of the e/m component (as measured with the Radio Detector) is plotted as a function of the muon number (as obtained with the water-Cherenkov detector). A clear separation between showers induced by protons and iron nuclei is visible. Please note: the flattening at the top-right is due to an artifact, only showers up to 40 EeV have been simulated. This is work in progress. We are working on more precise simulation studies in order to quantify various properties of the radio upgrade such as mass resolution, energy resolution, effective aperture as a function of energy, etc.

Technical implementation
At present we are working on the details of the technical implementation of the radio upgrade. The Radio Detector will be fully integrated in the Surface Detector stations, they will form one unit, being comprised of water-Cherenkov detector, Surface Scintillator Detector, and Radio Detector. The different detectors will share the infrastructure such as solar power, battery, communications system, GPS timing, and an integrated data acquisition system. The envisaged system is schematically shown in Fig. 9.
We aim to use a short aperiodic loaded loop antenna (SALLA) to detect the radio emission from air showers in the frequency range 30 to 80 MHz. The SALLA realizes a Beverage antenna as a dipole loop of 1.2 m diameter [10]. The SALLA has been developed to provide a minimal design that fulfills the need for both, ultra-wideband sensitivity, and low costs for production and maintenance of the antenna in a large-scale radio detector. The compact structure of the SALLA makes the antenna robust and easy to manufacture. A picture of a SALLA, showing its two polarization planes, is displayed in Fig. 12 below. Beverage antennas include a resistor load within the antenna structure to give a specific shape to the directivity [10]. In the case of the SALLA a resistance of 500 Ω connects the ends of the dipole arms at the bottom of the antenna. The antenna is read out at the top which is also the position of the Low-Noise Amplifier (LNA). While signals coming from above will induce a current directly at the input of the amplifier, the reception from directions below the antenna is strongly suppressed as the captured power is primarily consumed within the ohmic resistor rather than amplified by the LNA. The resulting strong suppression of sensitivity towards the ground reduces the dependence of the antenna on environmental conditions which might vary as a function of time and are thus a source of systematic uncertainty. With the inclusion of an ohmic resistor the SALLA especially challenges its amplifier as only ∼ 10%  Figure 11. Block diagram of the front-end board for the radio detectors (draft -work in progress). of the captured signal intensity is available at the input of the LNA. Proper matching between the antenna structure and the LNA is realized with a 3:1 transmission line transformer. The structure of the SALLA creates a sensitivity which is flat as a function of frequency.

A 98
The antenna response has been characterized in field measurements [10] and the vector effective length has been determined as a function of the zenith angle over our frequency band from 30 to 80 MHz. The results are shown in Fig. 10 for the SALLA antenna together with the antennas installed at AERA. The SALLA antenna exhibits a good sensitivity for large zenith angles. This is an important aspect for the application in the radio upgrade. Also longterm experience is available, about 60 SALLA antennas are already used at the Tunka site to measure air showers on a regular basis [38]. The water-Cherenkov detector will issue a trigger signal when energy deposition has been detected. The data from the radio antenna will be passed to the readout electronics of the Surface Scintillator Detector/water-Cherenkov detector system (UUB) and will be transmitted together with all data from the station to the central data acquisition of the Auger Observatory.
A schematic view of the radio front-end board is depicted in Fig. 11. We foresee two polarization directions of the antenna, oriented orthogonal to each other (see also Fig. 12). The signals of the two analogue channels will be pre-amplified in a LNA at the antenna. The signals are transmitted through shielded coaxial cables to the filter amplifier on the front-end board. They will be digitized with a sampling frequency of 200 Msps. A FPGA controls the data flow and the communication with the existing electronics of each Surface Detector station (UUB).
To test the performance of the SALLA antennas we have installed three prototype stations inside the AERA field at the Auger Observatory. Such a prototype station is shown in Fig. 12; An AERA station with SALLA antenna is situated next to a water-Cherenkov detector with a Surface Scintillator Detector module on top. First air showers have been measured in coincidence with the prototype stations. As an example, the amplitude of the radio signal for a measured air shower is presented in Fig. 13 as a function of the distance to the shower axis. The signals from the SALLA antennas are compared to the "regular" AERA stations. The signals measured with the SALLA antennas are slightly bigger, due to a preliminary calibration used for the SALLA antennas.

Schedule
Our plan for the installation of the radio upgrade at the Auger Observatory is as follows. We are presently working on the details of the technical implementation. We aim to conclude this throughout the year 2019. We envisage starting the mass production of the detector components in the second half of 2019 and are looking forward to deploying the detectors in the field in 2020.
The development of the radio detection technique is illustrated schematically in Fig. 14. For various experimental set-ups the approximate energy range and the approximate time of operation are indicated. The modern era of the radio detection of air showers started around the year 2000. Within two decades the experiments grew in size from about 0.1 km 2 to now 3000 km 2 . As a consequence, the energy range of interest shifted from a few 10 16 eV to above 10 20 eV. With the upgraded Auger Observatory and its 3000 km 2 radio array we will be able to investigate the properties of cosmic rays up to the highest energies, exceeding 10 20 eV. This will be a milestone in the development of the radio technique to measure extensive air showers and the investigation of the origin of the highest-energy cosmic rays.