Astroparticle Physics 2023

| Forewords and news6 Astroparticle physics at DESY Director’s foreword We live in challenging times. Even as the COVID-19 pandemic receded, 2023 saw an increase in conicts that has numerous implications for both the political situation and the scientic landscape in Europe and beyond. As a research centre whose success is founded on international collaboration, DESY rmly upholds the democratic values on which our peaceful coexistence and cooperation in Europe and with our partners around the world is based. As I wrote on the occasion of the second Diversity Day at DESY in May 2023: “DESY is diverse, the people who work here come from many nations, have different origins, bring different identities and perspectives. Each and every one enriches us. This diversity is sometimes exhausting, but it makes us strong. Especially in difcult times, it is important to promote equal opportunities and solidarity and to set an example for diversity.” It is this spirit of open-mindedness, tolerance and mutual respect that makes DESY such a great place to work – and that ensures its success as one of the world’s top centres for research into the structure of matter and the universe. The progress of our research projects testies to the success of this approach, with all the endeavours of the DESY Astroparticle Physics Division reaching decisive milestones in 2023. In particular, preparations are in full swing for the construction of the tele- scopes and infrastructure for the upcoming Cherenkov Telescope Array Observatory (CTAO) – a unique, world-class observatory for gamma-ray astronomy with over 60 telescopes at two sites in Chile and on the Canary Island of La Palma. Construction work on the observatory’s Science Data Management Centre (SDMC) on our Zeuthen campus proceeded as planned, with completion expected in autumn 2024. In addition, DESY is in charge of providing a total of 42 cameras for the Small-Sized Telescopes (SSTs), which are approaching their nal design. In 2023, electronic components for the cameras were manufactured and used to build a development and test model of the camera. Furthermore, DESY is responsible for the design and production of the Medium-Sized Telescopes (MSTs). The DESY design was approved by CTAO at the end of 2023 after a thorough review. As the next step, two prototype MSTs will be manufactured at DESY and prepared for installation at both observatory sites. The development of the camera for the ULTRASAT space tele- scope, a satellite mission led by the Weizmann Institute of Science (WIS) and the Israel Space Agency (ISA), made very good progress. Major steps were taken towards the rst version of the camera, and the production phase was initiated. First tests of the readout chain of the sensors – which will take images in the near-ultraviolet with 90 million pixels every ve minutes to search for time-varying cosmic sources – were successful. In neutrino astronomy, work continued to focus on the IceCube neutrino telescope at the South Pole and on the Radio Neutrino Observatory Greenland (RNO-G). IceCube is currently being upgraded in collaboration with US partners, KIT in Karlsruhe and several university groups. New optical modules with more sensitive light detection, called multi-photomultiplier tube digital optical modules (mDOMs), have been developed at DESY and elsewhere for this purpose. Equipped with 24 photomultipliers each, they have about twice the light-sensitive area of their predecessors in IceCube and allow intrinsic directional determi- nation of the measured light. Their development was successfully completed in 2023 with a production readiness review, and the integration and testing of 225 mDOMs at DESY is now well under way. By the end of 2023, more than 100 mDOMs had been produced and successfully tested. The detection of neutrinos with petaelectronvolt (PeV) energies using radio waves is an important addition to IceCube, which is being studied with the RNO-G pilot array in Greenland. Here, the year 2023 was used to improve the drilling technology and the autonomous energy supply and to calibrate the stations already installed, building on the experience gained with the rst seven stations. IceCube also delivered one of the scientic highlights of the year, which we are pleased to present to you in this report. Using a novel analysis technique developed mainly at DESY and RWTH Aachen, the IceCube researchers performed the most accurate measurement of the cosmic neutrino spectrum to date. For the rst time, evidence was found for a break in the spectral shape, providing important clues about the nature of the environments in which the neutrinos are produced. To further boost analysis capabilities, DESY released the Astrophysical Multi-Messenger Modeling (AM3) software, which can be used to simulate multimessenger emission – including neutrinos – from active galactic nuclei, blazars and gamma-ray bursts. Further scientic highlights included the detection by the H.E.S.S. telescope of very-high-energy gamma-ray emission from the outer jets of a microquasar, insights from observations by the VERITAS telescope that reveal the potential origin of a galactic “PeVatron” candidate – a cosmic-ray factory that accelerates particles to PeV energies – and a novel analysis of the production mechanisms of particle cascades generated by gamma-rays from distant blazars. These successes, which have been published in high-ranking scientic journals, attest to the excellent research being carried out in the DESY Astroparticle Physics Division despite the current tight stafng situation. So do the prizes and awards conferred to members of the division, including two distinctions for outstanding PhD theses – for Annika Rudolph from the German Astronomical Society and for Robert Stein from the German Physical Society – as well as a Starting Grant from the European Research Council (ERC) for Anna Nelles to fund the construction of the RNO-G network of radio antennas on Greenland. The Helmholtz–Weizmann International Research School for Multimessenger Astronomy, in which DESY is strongly involved, continues to deliver excellent results, with the students of the second cohort having nished the School and new students recruited for a doctorate for the sixth time. To further the public understanding of science and inspire young people in particular to take an interest in science, the division continued to actively promote young talent and outreach activities, for example through its School Lab, the rst AstroCamp, featuring the launch of a research balloon, the rst DESY–Ukraine Winter School, the traditional summer student programme and several other events organised on the occasion of the “Science Year 2023 – Our Universe” declared by the German Federal Ministry of Education and Research (BMBF). I would like to thank all our staff and our partners in Germany and around the world for contributing to our joint successes in the past year, and I am looking forward to continuing our cooperation – based on our common values of open-mindedness, tolerance and respect – in the years to come.