Introduction

• Stefan Wagner

Multimessenger studies with gravitational waves

• Samaya Nissanke

Observations of low-frequency gravitational waves

• Michael Kramer

Multi-messenger astrophysics with information field theory

• Torsten Enßlin (MPA)

Combining data from several instruments to recover signals in time, space, and frequency domains has several challenges, which can be addressed via information field theory (IFT). The potential of IFT-based multi-messenger signal reconstruction is highlighted with recent results on QPO search in magnetar X-ray bursts, multi-frequency all-sky gamma-ray imaging, 3D galactic dust tomography, and black hole filming.

MM aspects of CR and UHE astrophysics

• Karl-Heinz Kampert

Radio Detection of Cosmic Particles

• Frank Schröder (KIT - IAP)

Digital radio detection has become an additional standard technique for cosmic rays above 1016 eV. Antenna arrays measure the geomagnetic radio emission of extensive air showers and feature a competitive accuracy for the cosmic-ray energy and position of the shower maximum, a key parameter to estimate the type of primary particle. As enhancement to particle-detector arrays, radio antennas can thus increase the total measurement accuracy for air showers, such as at the Pierre Auger Observatory or the planned IceCube-Gen2 Surface Array. Moreover, radio arrays are a promising technique for ultra-high-energy photons and neutrinos, the latter, by searching for Earth skimming neutrinos through tau-induced air showers or by deploying radio antennas in the ice. Finally, there are synergies by using astronomical radio observatories also for cosmic-ray detection as demonstrated by LOFAR and planned for SKA-low. This talk will provide an overview of the state-of-the-art of the radio technique, including recent examples such as a prototype station operating at IceTop at the South Pole, and provide an outlook of future activities and science goals.

Astroparticle Opportunities with SKA-low

• Anna Nelles (DESY)

The low-frequency array of SKA will consist of more than 60,000 individual antennas on an area of only a few tens of square kilometres. Apart from providing radio astronomical data of unprecedented resolution at frequencies between 50 and 350 MHz, these antennas also present an opportunity for air shower detection. Break-through work at LOFAR has shown that dense arrays of radio antennas provide one of the highest-resolution measurements of air shower properties. This contribution will summarise the work of the SKA Science Working Group High Energy Particles towards Cosmic Ray and potential Gamma Ray Detection with SKA-low.

Radio-neutrino studies of blazars: inspiring progress and exciting opportunities

• Yuri Kovalev (MPIfR)

We present results from a joint analysis of VLBI observations of blazars and neutrino data from IceCube supplemented by ANTARES and Baikal-GVD. We have found a 4sigma-significant observational evidence that high energy neutrinos are generated in radio-bright blazars and arrive preferentially during their flares. VLBI effectively selects active galaxies with jets whose electromagnetic and neutrino emission is relativistically boosted towards us. We summarize ongoing efforts and new opportunities for multi-messenger studies to explore the nature of blazars' nuclei as powerful proton accelerators.

Perspectives of optical/NIR surveys for MM and time-domain astrophysics

• Matthias Steinmetz

AMPEL: An analysis engine for the era of high-throughput multi-messenger observatories

• Jakob Nordin (HU Berlin)

Astronomical facilities will produce an ever increasing real-time avalanche of photon, neutrino and gravitational wave detections during the coming decade. The scientific posibilites are endless, but assume efficient tools for managing information flows: Computations need to be fast and scalable, while upholding FAIR principles and allow scientific creativity. We here present the AMPEL platform as a solution to these challenges: Complex analysis schema are designed hierarchically from analysis modules, tested in a local environment and uploaded to a computer center for large scale processing. The AMPEL system ensures the scalability and reproducibility of this process, while automatically guaranteeing the provenance of each datapoint and analysis result. I will here both introduce the AMPEl system as well as give an overview of current and future applications. An AMPEL instance hosted at DESY Zeuthen is currently processing data from the Zwicky Transient Facility, which is combined with IceCube and Ligo data for applications in multi-messenger science and cosmology. Looking ahead, the first large facility to go online will be the Vera Rubin Observatory (VRO), which will distribute real-time alerts to AMPEL as one of seven world-wide endpoints. Finally, machine learning based techniques will be essential for many science applications, and is naturally incorporated into AMPEL. We recently demonstrated this in the ELAsTiCC VRO data challenge, where the AMPEL classifiers produced the best overall ML scores.

Neutron Stars and Black Holes as Compact Remnants of Stellar Death

• Hans-Thomas Janka

At the end of their lives massive stars collapse to neutron stars or black holes, often associated with the violent ejection of matter in supernova explosions. When born in binary systems, these compact remnants act as sources of the gravitational waves measured by advanced LIGO-VIRGO, the Japanese KAGRA detector, and of their next-generation successors. However, the links between single-star and binary progenitor systems on the one hand and stellar explosions and their remnants on the other hand are still largely unclear. Supernova 1987A in the Large Magellanic Cloud was not only the closest supernova observable by the naked eye within 400 years, but also marked the beginning of multi-messenger astronomy due to the first detection of neutrinos from an extragalactic source concomitant with superb observations over a wide spectrum of electromagnetic channels. A next galactic stellar collapse event will provide far better neutrino statistics and is also a promising source of gravitational waves, in particular for the next-generation interferometers. The combined measurement of these signals together with the firework of multi-waveband radiation will help us deciphering long-standing questions of the supernova explosion mechanism, of the birth properties of neutron stars, and of nuclear, neutrino, and particle physics that play a role in the hot, newly formed neutron star. Meanwhile detailed observations of pulsars and of neutron stars with their associated gas remnants, a growing catalog of gravitational-wave signals from compact object mergers, and an exponentially bursting number of supernovae in conjunction with increasingly more refined and realistic theoretical models will expand our understanding of the population systematics of stellar death events and their compact remnants.

Chasing Shadows: Unveiling the Electromagnetic Secrets of Binary Black Hole Mergers

• Julian Sommer (LMU München)

The detection of electromagnetic counterparts of gravitational wave events from binary neutron star mergers has laid the foundation for multi-messenger astronomy. However, counterparts to neutron star-black hole (NSBH) mergers and binary black hole (BBH) mergers have not been detected with high confidence. The state-of-the-art theoretical description of physical processes involving merged stellar-mass to intermediate-mass black holes within the accretion disks of active galactic nuclei (AGNs) opens the possibility of discovering transient emissions within a feasible timeframe, just days after gravitational wave detection. Our focus is on conducting follow-up observations of gravitational wave detections LIGO/Virgo/KAGRA, utilizing Mount Wendelstein's 2.1 m telescope with the 3KK imager in visible to near-infrared bands. In close collaboration with the "Gravitational Wave MultiMessenger Astronomy Decam Survey" (GW-MMADS) and the DESI time domain team, we aim to identify astrophysical transients directly associated with BNS, NSBH and BBH merger events. For BBH merger, our target selection involves matching AGNs from catalogs within a constrained localization area determined by gravitational wave detections and narrowing the sample down with reasonable constraints. As the theory evolves rapidly, our plan is to improve selection criteria and gain a better understanding of BBH light curve modelling given sensible parameter estimates. We would like to take advantage of the opportunity of this conference to showcase the first successes and results of our follow-up campaigns.

Time-dependent multi-messenger modeling in the open source era

• Xavier Rodrigues (ESO)

The IceCube experiment has now accumulated over a decade of observations, and a suite of next-generation multi-messenger experiments line up on the horizon. Theorists try to keep pace by developing increasingly refined astrophysical neutrino source models. In this talk, I discuss recent advancements in numerical modeling tools. I introduce a cutting-edge numerical cosmic-ray simulation tool developed at DESY and recently published as open-source software. I discuss some of the recent applications and future directions, including modeling of multi-wavelength catalogs and time-domain multi-messenger data.

Fast Radio Bursts and multifrequency searches

• Laura Spitler

Multimessenger Transient Astronomy with ZTF and Ultrasat

• Marek Kowalski

Deutsches Zentrum für Astrophysik DZA

• Günther Hasinger

Strategies in Astrophysics: RDS Denkschrift - Astronet Roadmap

• Jörn Wilms

Strategies in Astroparticlephysics: KAT Strategy - APPEC Roadmap

• Andreas Haungs

DZA Science Strategy

• Stefan Wagner

The DFG Priority Program

• Stefan Krückeberg

Gaining physics insights through the analysis of various messengers requires a careful theoretical basis.

• Martin Pohl

A neutrino association with a blazar PKS 0735+178 will be discussed as an example. Detailed computations require extensive simulations. One multimessenger simulation code was recently published as an open-source tool for the community, and more should follow. Institutional structures in Germany are not ideal for the professional development and maintenance of such codes. I shall outline options how DZA may help to address this issue.

Multi-messanger cosmology

• Dominik Schwarz

Modern cosmology has always made use of different messengers, but basically used each of these probes to test a particular aspect of the Universe. The combination of those recently lead to prominently discussed tensions. I will argue, that a full multi-messanger approach to cosmology, in which different probes probe the same aspect would help us to resolve these tensions. As an example I’ll discuss the quest for the cosmic rest-frame.

Modeling the strong-field dynamics of binary neutron star merger

• Sebastiano Bernuzzi

Binary neutron star mergers (BNSM) are associated to powerful gravitational and electromagnetic astronomical transients. Multimessenger observations of BNSMs promise to deliver unprecedented insights on fundamental physics questions, including constraints on dense matter models and the production of heavy elements. Detailed theoretical predictions of the merger dynamics are a crucial aspect for extracting information from such observations. This talk reviews recent progress on the modeling of BNSMs using simulations in 3+1 numerical general relativity. I will discuss the first predictions for the complete (inspiral-merger-postmerger) gravitational-wave spectrum and their application in gravitational-wave astronomy. Then, I will discuss selected recent results on the mechanisms behind kilonova light and their application to the analyses of astrophysical data.