Periodic Reporting for period 3 - PeV-Radio (Digital Radio Detectors for Galactic PeV Particles)
Berichtszeitraum: 2022-02-01 bis 2023-07-31
The unknown origin of the most energetic Galactic and extragalactic cosmic rays remains one of the puzzles of nature. These particles are atomic nuclei accelerated by - yet to be discovered - natural objects to energies far beyond the reach of human technology. High-energy cosmic rays are measured by special observatories through atmospheric particle cascades, called air showers. IceTop, the surface array of the IceCube Neutrino Observatory at the South Pole, is one such detector for cosmic rays. By adding radio antennas to a planned enhancement of IceTop by an array of scintillation panels, its measurement capabilities for air showers will be significantly enhanced. The improvement applies to air showers initiated by all types of cosmic particles, including cosmic-ray nuclei of different masses as well as PeV photons, which may directly reveal the sources of the cosmic rays.
In addition to IceCube, where a prototype station was installed in 2020 and subsequently upgraded, other cosmic-ray observatories may provide alternative locations for the proposed radio enhancement, such as the Pierre Auger Observatory in Argentina, which is a backup site for this project. Due to the restrictions imposed by the Covid pandemic, we have thus installed also a prototype station at the Auger site while preparing simultaneously for the installation of additional detectors at the South Pole. Moreover, we have used the time to further analyze data of the existing prototype station at the South Pole and prepare new analysis methods for more accurate measurements of the types (mass) of primary cosmic-ray particles.
In addition to IceCube, where a prototype station was installed in 2020 and subsequently upgraded, other cosmic-ray observatories may provide alternative locations for the proposed radio enhancement, such as the Pierre Auger Observatory in Argentina, which is a backup site for this project. Due to the restrictions imposed by the Covid pandemic, we have thus installed also a prototype station at the Auger site while preparing simultaneously for the installation of additional detectors at the South Pole. Moreover, we have used the time to further analyze data of the existing prototype station at the South Pole and prepare new analysis methods for more accurate measurements of the types (mass) of primary cosmic-ray particles.
After adapting the data-acquisition electronics accordingly, a prototype station including three radio antennas was installed at the South Pole in January 2020, and data has regularly been transferred by satellite. The subsequent data analysis showed that indeed we successfully measure cosmic-ray air showers with the prototype station.
However, in March 2020 the Covid pandemic and world-wide travel restrictions disrupted these plans. In particular, access to the South Pole station has been limited such that only existing instrumentation can be maintained, but no new deployments were possible. As travel to the best among the backup sites for this project, the Pierre Auger Observatory in Argentina, became possible as expected during 2022, and we have completed the installation of a prototype station there end of 2022. In parallel, we prepared the hardware for further installations at the South Pole: six stations (18 antennas) are completely ready to ship and all other antennas are on stock and can be prepared within a short time as soon as we get green light to resume deployment activities at the South Pole (or finally decide for the Pierre Auger Observatory as a backup site). Preliminary results of the Auger site show that the radio background is not ideal, but tolerable, and in the next step we want to experimentally check that we achieve a similarly low detection threshold for cosmic-ray air showers at the Auger site as we have demonstrated at the South Pole.
Although the deployment of new instrumentation was not allowed at the South Pole through the 2022/23 season, one PhD student of the project was permitted to upgrade the existing prototype station. After successfully defending her PhD in November 2022, she was at the South Pole changing the antenna mount to the final design that she developed as one part of her PhD work on the project. Meanwhile she has moved on to a postdoc position in a different field and has finalized the publication of her PhD thesis.
Being limited in the deployment of instrumentation, we also used the time to accelerate other aspects of the project, in particular, the preparation of software, simulations studies, the analysis of the data of the prototype station, and preparations for further data analysis. These preparations with simulation studies cover in particular important points: a) the reconstruction of the energy and position of the shower maximum of the cosmic-ray air showers, as the latter is one of the most accurate estimators for the mass of the primary particles, and b) the separation of photons against other cosmic particles using all of IceCube’s instrumentation.
However, in March 2020 the Covid pandemic and world-wide travel restrictions disrupted these plans. In particular, access to the South Pole station has been limited such that only existing instrumentation can be maintained, but no new deployments were possible. As travel to the best among the backup sites for this project, the Pierre Auger Observatory in Argentina, became possible as expected during 2022, and we have completed the installation of a prototype station there end of 2022. In parallel, we prepared the hardware for further installations at the South Pole: six stations (18 antennas) are completely ready to ship and all other antennas are on stock and can be prepared within a short time as soon as we get green light to resume deployment activities at the South Pole (or finally decide for the Pierre Auger Observatory as a backup site). Preliminary results of the Auger site show that the radio background is not ideal, but tolerable, and in the next step we want to experimentally check that we achieve a similarly low detection threshold for cosmic-ray air showers at the Auger site as we have demonstrated at the South Pole.
Although the deployment of new instrumentation was not allowed at the South Pole through the 2022/23 season, one PhD student of the project was permitted to upgrade the existing prototype station. After successfully defending her PhD in November 2022, she was at the South Pole changing the antenna mount to the final design that she developed as one part of her PhD work on the project. Meanwhile she has moved on to a postdoc position in a different field and has finalized the publication of her PhD thesis.
Being limited in the deployment of instrumentation, we also used the time to accelerate other aspects of the project, in particular, the preparation of software, simulations studies, the analysis of the data of the prototype station, and preparations for further data analysis. These preparations with simulation studies cover in particular important points: a) the reconstruction of the energy and position of the shower maximum of the cosmic-ray air showers, as the latter is one of the most accurate estimators for the mass of the primary particles, and b) the separation of photons against other cosmic particles using all of IceCube’s instrumentation.
We use the SKALA v2 antenna type for our project, which was originally developed for the Square Kilometer Array (SKA). This antenna type combines a wide sky coverage with a broad frequency band, which includes our measurement band from below 100 MHz to 350 MHz. With the prototype station we confirmed cosmic-ray measurements in this wide frequency band, and we expect that we can lower the detection threshold down to the intrinsic limit given by the Galactic radio noise. Despite not being able to deploy new instrumentation, the IceCube winterovers helped us and upgraded the data-acquisition electronics in the 2021/22 season, and in the 2022/23 a PhD student from the ERC project was at the South Pole to upgrade the antenna mount to the final design developed as part of her PhD research work.
While continuing data taken with the upgraded electronics and further preparation for the deployment of the full array, we have also intensified the analysis of existing data. In particular, one of the PhD students is working on simulation studies to exploit the IceCube data available to the extent possible for the first science goal of searching for PeV photons, improving analysis methods regarding the angular resolution, threshold, and gamma-hadron separation. Generally, the enhanced mass separation of cosmic rays as well as the detection of photons have the potential to solve the long standing puzzle of the origin of the highest energy cosmic rays from our own Galaxy. Hence, the simulation studies and analysis techniques developed will contribute to this scientific quest despite the Covid imposed delays on the instrumentation.
Regarding the analysis of the prototype station at the South Pole, we can report already an important result. In addition to demonstrating that we indeed achieve a relatively low detection threshold with the SKALA-v2 antennas, we could also demonstrate the measurement of the position of the shower maximum with the prototype station for several cosmic-ray events. This came unexpected, as many scientists in the field believed that at least 5 antennas are required for a reasonable resolution on the shower maximum, and the prototype station has only 3 antennas. Nonetheless, through further improvements of the state-of-the-art template fitting analysis technique, we could demonstrate that for a subset of the measured events already three antennas provide a competitive resolution (shown at the ICRC 2023, the most important conference in the field of astroparticle physics).
Finally, due to the convincing results of the prototype station, complemented with simulation studies, the IceCube-Gen2 collaboration has decided to adopt the station design for the planned IceCube-Gen2 Surface Array. This is described in part II of the Technical Design Report, which has recently been published online. Therefore, this ERC project will have a lasting impact much beyond its duration, as IceCube-Gen2 is expected to operate at least into the 2040’s.
While continuing data taken with the upgraded electronics and further preparation for the deployment of the full array, we have also intensified the analysis of existing data. In particular, one of the PhD students is working on simulation studies to exploit the IceCube data available to the extent possible for the first science goal of searching for PeV photons, improving analysis methods regarding the angular resolution, threshold, and gamma-hadron separation. Generally, the enhanced mass separation of cosmic rays as well as the detection of photons have the potential to solve the long standing puzzle of the origin of the highest energy cosmic rays from our own Galaxy. Hence, the simulation studies and analysis techniques developed will contribute to this scientific quest despite the Covid imposed delays on the instrumentation.
Regarding the analysis of the prototype station at the South Pole, we can report already an important result. In addition to demonstrating that we indeed achieve a relatively low detection threshold with the SKALA-v2 antennas, we could also demonstrate the measurement of the position of the shower maximum with the prototype station for several cosmic-ray events. This came unexpected, as many scientists in the field believed that at least 5 antennas are required for a reasonable resolution on the shower maximum, and the prototype station has only 3 antennas. Nonetheless, through further improvements of the state-of-the-art template fitting analysis technique, we could demonstrate that for a subset of the measured events already three antennas provide a competitive resolution (shown at the ICRC 2023, the most important conference in the field of astroparticle physics).
Finally, due to the convincing results of the prototype station, complemented with simulation studies, the IceCube-Gen2 collaboration has decided to adopt the station design for the planned IceCube-Gen2 Surface Array. This is described in part II of the Technical Design Report, which has recently been published online. Therefore, this ERC project will have a lasting impact much beyond its duration, as IceCube-Gen2 is expected to operate at least into the 2040’s.