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The Laser Interferometer Space Antenna (LISA) will be a transformative experiment for gravitational wave astronomy, and, as such, it will offer unique opportunities to address many key astrophysical questions in a completely novel way. The synergy with ground-based and space-born instruments in the electromagnetic domain, by enabling multi-messenger observations, will add further to the discovery potential of LISA. The next decade is crucial to prepare the astrophysical community for LISA's first observations. This review outlines the extensive landscape of astrophysical theory, numerical simulations, and astronomical observations that are instrumental for modeling and interpreting the upcoming LISA datastream. To this aim, the current knowledge in three main source classes for LISA is reviewed; ultra-compact stellar-mass binaries, massive black hole binaries, and extreme or interme-diate mass ratio inspirals. The relevant astrophysical processes and the established modeling techniques are summarized. Likewise, open issues and gaps in our understanding of these sources are highlighted, along with an indication of how LISA could help making progress in the different areas. New research avenues that LISA itself, or its joint exploitation with upcoming studies in the electromagnetic domain, will enable, are also illustrated. Improvements in modeling and analysis approaches, such as the combination of numerical simulations and modern data science techniques, are discussed. This review is intended to be a starting point for using LISA as a new discovery tool for understanding our Universe.

Full list of authors: Amaro-Seoane, Pau; Andrews, Jeff; Sedda, Manuel Arca; Askar, Abbas.; Baghi, Quentin; Balasov, Razvan; Bartos, Imre; Bavera, Simone S.; Bellovary, Jillian; Berry, Christopher P. L.; Berti, Emanuele; Bianchi, Stefano; Blecha, Laura; Blondin, Stephane; Bogdanovic, Tamara; Boissier, Samuel; Bonetti, Matteo; Bonoli, Silvia; Bortolas, Elisa; Breivik, Katelyn; Capelo, Pedro R.; Caramete, Laurentiu; Cattorini, Federico; Charisi, Maria; Chaty, Sylvain; Chen, Xian; Chruslinska, Martyna; Chua, Alvin J. K.; Church, Ross; Colpi, Monica; D'Orazio, Daniel; Danielski, Camilla; Davies, Melvyn B.; Dayal, Pratika; De Rosa, Alessandra; Derdzinski, Andrea; Destounis, Kyriakos; Dotti, Massimo; Dutan, Ioana; Dvorkin, Irina; Fabj, Gaia; Foglizzo, Thierry; Ford, Saavik; Fouvry, Jean-Baptiste; Franchini, Alessia; Fragos, Tassos; Fryer, Chris; Gaspari, Massimo; Gerosa, Davide; Graziani, Luca; Groot, Paul; Habouzit, Melanie; Haggard, Daryl; Haiman, Zoltan; Han, Wen-Biao; Istrate, Alina; Johansson, Peter H.; Khan, Fazeel Mahmood; Kimpson, Tomas; Kokkotas, Kostas; Kong, Albert; Korol, Valeriya; Kremer, Kyle; Kupfer, Thomas; Lamberts, Astrid; Larson, Shane; Lau, Mike; Liu, Dongliang; Lloyd-Ronning, Nicole; Lodato, Giuseppe; Lupi, Alessandro; Ma, Chung-Pei; Maccarone, Tomas; Mandel, Ilya; Mangiagli, Alberto; Mapelli, Michela; Mathis, Stephane; Mayer, Lucio; McGee, Sean; McKernan, Berry; Miller, M. Coleman; Mota, David F.; Mumpower, Matthew; Nasim, Syeda S.; Nelemans, Gijs; Noble, Scott; Pacucci, Fabio; Panessa, Francesca; Paschalidis, Vasileios; Pfister, Hugo; Porquet, Delphine; Quenby, John; Ricarte, Angelo; Roepke, Friedrich K.; Regan, John; Rosswog, Stephan; Ruiter, Ashley; Ruiz, Milton; Runnoe, Jessie; Schneider, Raffaella; Schnittman, Jeremy; Secunda, Amy; Sesana, Alberto; Seto, Naoki; Shao, Lijing; Shapiro, Stuart; Sopuerta, Carlos; Stone, Nicholas C.; Suvorov, Arthur; Tamanini, Nicola; Tamfal, Tomas; Tauris, Thomas; Temmink, Karel; Tomsick, John; Toonen, Silvia; Torres-Orjuela, Alejandro; Toscani, Martina; Tsokaros, Antonios; Unal, Caner; Vazquez-Aceves, Veronica; Valiante, Rosa; van Putten, Maurice; van Roestel, Jan; Vignali, Christian; Volonteri, Marta; Wu, Kinwah; Younsi, Ziri; Yu, Shenghua; Zane, Silvia; Zwick, Lorenz; Antonini, Fabio; Baibhav, Vishal; Barausse, Enrico; Bonilla Rivera, Alexander; Branchesi, Marica; Branduardi-Raymont, Graziella; Burdge, Kevin; Chakraborty, Srija; Cuadra, Jorge; Dage, Kristen; Davis, Benjamin; de Mink, Selma E.; Decarli, Roberto; Doneva, Daniela; Escoffier, Stephanie; Gandhi, Poshak; Haardt, Francesco; Lousto, Carlos O.; Nissanke, Samaya; Nordhaus, Jason; O'Shaughnessy, Richard; Portegies Zwart, Simon; Pound, Adam; Schussler, Fabian; Sergijenko, Olga; Spallicci, Alessandro; Vernieri, Daniele; Vigna-Gomez, Alejandro.-- This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/., The Laser Interferometer Space Antenna (LISA) will be a transformative experiment for gravitational wave astronomy, and, as such, it will offer unique opportunities to address many key astrophysical questions in a completely novel way. The synergy with ground-based and space-born instruments in the electromagnetic domain, by enabling multi-messenger observations, will add further to the discovery potential of LISA. The next decade is crucial to prepare the astrophysical community for LISA’s first observations. This review outlines the extensive landscape of astrophysical theory, numerical simulations, and astronomical observations that are instrumental for modeling and interpreting the upcoming LISA datastream. To this aim, the current knowledge in three main source classes for LISA is reviewed; ultra-compact stellar-mass binaries, massive black hole binaries, and extreme or interme-diate mass ratio inspirals. The relevant astrophysical processes and the established modeling techniques are summarized. Likewise, open issues and gaps in our understanding of these sources are highlighted, along with an indication of how LISA could help making progress in the different areas. New research avenues that LISA itself, or its joint exploitation with upcoming studies in the electromagnetic domain, will enable, are also illustrated. Improvements in modeling and analysis approaches, such as the combination of numerical simulations and modern data science techniques, are discussed. This review is intended to be a starting point for using LISA as a new discovery tool for understanding our Universe. © The Author(s) 2023, corrected publication 2023., P. Dayal acknowledges support from the European Research council (ERC-717001) and from the Netherlands Research Council NWO (016.VIDI.189.162). P.H. Johansson acknowledges the support from the European Research Council (ERC-818930). S. Toonen acknowledges support from the Netherlands Research Council NWO (VENI 639.041.645 Grants) C. Unal is supported by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project CoGraDS - CZ.02.1.01/0.0/0.0/15_003/0000437). S. Chaty acknowledges the LabEx UnivEarthS for the funding of Interface project I10 “From binary evolution towards merging of compact objects”. A. De Rosa acknowledges financial contribution from the agreement ASI-INAF n.2017-14-H.O E. Berti is supported by NSF Grants No. PHY-1912550 and AST-2006538, NASA ATP Grants No. 17-ATP17-0225 and 19-ATP19-0051, NSF-XSEDE Grant No. PHY-090003, and NSF Grant PHY-20043. D. Gerosa is supported by European Union’s H2020 ERC Starting Grant No. 945155–GWmining, Leverhulme Trust Grant No. RPG-2019-350 and Royal Society Grant No. RGS-R2-202004. T. Bogdanovic acknowledges support by the NASA award No. 80NSSC19K0319 and by the NSF award AST-1908042. D. Porquet acknowledges funding support from CNES. C. Danielski acknowledges financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709) B.L. Davis acknowledges support from Tamkeen under the NYU Abu Dhabi Research Institute Grant CAP3. F. Pacucci acknowledges support from a Clay Fellowship by the SAO and from the Black Hole Initiative, which is funded by grants from the John Templeton Foundation and the Gordon and Betty Moore Foundation. A.J. Ruiter acknowledges support from the Australian Research Council Future Fellowship Grant FT170100243. V. Paschalidis is supported by NSF Grant PHY-1912619 and NASA Grant 80NSSC20K1542 to the University of Arizona, and NSF-XSEDE Grant TG-PHY190020. D. Haggard acknowledges support from the NSERC Discovery Grant and Canada Research Chairs programs, and the Bob Wares Science Innovation Prospectors Fund. M. Toscani acknowledges European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 823823 (RISE DUSTBUSTERS project) and COST Action CA16104 - Gravitational waves, black holes and fundamental physics, supported by COST (European Cooperation in Science and Technology). M. Chruslinska, A. Istrate and G. Nelemans acknowledge support from Netherlands Research Council NWO. T. Fragos and S. Bavera acknowledge support from a Swiss National Science Foundation Professorship Grant (project numbers PP00P2_176868 and PP00P2_211006)., With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2021-001131-S).

We describe the use of Multi Order Coverage (MOC) maps as a practical way to manage complex regions of the sky for the planning of multi-messenger observations. MOC maps are a data structure that provides a multi-resolution representation of irregularly shaped and fragmentary regions over the sky based on the HEALPix (Hierarchical Equal Area isoLatitude Pixelization) tessellation. We present a new application of MOC, in combination with the \texttt{astroplan} observation planning package, to enable the efficient computation of sky regions and the visibility of these regions from a specific location on the Earth at a particular time. Using the example of the low-latency gravitational-wave alerts, and a simulated observational campaign with three observatories, we show that the use of MOC maps allows a high level of interoperability to support observing schedule plans. Gravitational-wave detections have an associated credible region localization on the sky. We demonstrate that these localizations can be encoded as MOC maps, and how they can be used in visualisation tools, and processed (filtered, combined) and also their utility for access to Virtual Observatory services which can be queried 'by MOC' for data within the region of interest. The ease of generating the MOC maps and the fast access to data means that the whole system can be very efficient, so that any updates on the gravitational-wave sky localization can be quickly taken into account and the corresponding adjustments to observing schedule plans can be rapidly implemented. We provide example python code as a practical example of these methods. In addition, a video demonstration of the entire workflow is available., To be published in: Astronomy and Computing

The Super-Kamiokande detector is a 50-kton water tank instrumented with ∼ 13k photomultipliers, running since 1996. It is sensitive to neutrinos with energies ranging from 4.5 MeV to several TeV. Except for supernova bursts, it is difficult to use Super-Kamiokande as an independent astrophysical trigger as its data are dominated by atmospheric neutrinos in this energy range. However, it can be used to search for neutrino events in time and spatial coincidence with various external triggers. We developed a framework for the follow-up of gravitational wave alerts. We searched for neutrinos in a 1000 seconds time window centered on alert time. Results using LIGO/Virgo O3 datasets will be presented, as well as the plans of future realtime public release for O4 and beyond., on behalf of the Super-Kamiokande collaboration, {"references":["IceCube Collaboration • Raamis Hussain (Wisconsin U., Madison) et al. e-Print: 1908.07706 [astro-ph.HE] DOI: 10.22323/1.358.0918","IceCube Collaboration • M.G. Aartsen (Canterbury U.) et al. e-Print: 2004.02910 [astro-ph.HE] DOI: 10.3847/2041-8213/ab9d24","Super-Kamiokande Collaboration • Y. Fukuda (Tokyo U., ICRR) et al. DOI: 10.1016/S0168-9002(03)00425-X","Bruny Baret (APC, Paris), Imre Bartos (Columbia U.), Boutayeb Bouhou (APC, Paris), Alessandra Corsi (LIGO Lab., Caltech), Irene Di Palma (Hannover, Max Planck Inst. Grav.) et al. e-Print: 1101.4669 [astro-ph.HE] DOI: 10.1016/j.astropartphys.2011.04.001"]}

En el presente manuscrito se expone el trabajo realizado en el seno de la Collaboración ANTARES-KM3NeT sobre la astronomía multi-mensanjero con neutrinos, que se ha enfocado en dos ejes: la detección de neutrinos de supernova a bajas energías (MeV) con los telecopios KM3NeT y la búsqueda de neutrinos de alta energía (TeV-PeV) en ANTARES en coincidencia con fuentes transitorias detectadas en ondas gravitationales o rayos gamma de muy alta energía O(TeV). El primer análisis, explora la capacidad de los telescopios de neutrinos KM3NeT para detectar la señal de una explosión de supernova (CCSN), así como el potencial de explotar dicha detección para aprender sobre la física que hay detrás, en particular la resolución del perfil temporal y el espectro en energía de esta señal. Junto con el Sol, las explosiones de supernova son la única fuente astrofísica de neutrinos que ha sido observada. Durante la tesis se ha desarrollado un método de búsqueda para detectar estos neutrinos astrofísicos de baja energía, que se basa en el análisis de los primeros datos de KM3NeT para caracterizar el ruido de fondo y la respuesta del detector. Los resultados de dicho análisis muestran que los detectores KM3NeT son sensibles a un flujo de neutrinos con energías del MeV, y serán capaces de detectar fuentes supernova en nuestra Galaxia con una probabilidad a 5 sigmas, y por tanto contribuirán a la observación de la próxima explosión Galáctica. Este análisis se ha implementado para búsquedas a tiempo real de forma que nos permita mandar alertas en caso en una detección probable, y está en funcionamiento constante desde 2019. De hecho, este análisis se ha aplicado al seguimiento de las alertas de eventos detectados por ondas gravitacionales como fuentes no modelizadas, que son posibles candidatos a ser supernova, lo que ha llevado a los primeros resultados multi-mensajero de KM3NeT. Además, los resultados de este análisis ha permitido que KM3NeT entre a formar parte de la colaboración SNEWS, que reúne a todos los detectores sensibles a la señal de neutrinos de supernova y recibe sus alertas. El segundo análisis, utiliza los datos del detector ANTARES para buscar neutrinos de alta energía (TeV-PeV) en coincidencia espacial y temporal con la detección de fuentes astrofísicas mediante otros mensajeros. En particular, con señales detectados por ondas gravitacionales (GWs) y con las primeras detecciones de “bursts” de rayos gamma (GRBs) a muy alta energía O(TeV) por telescopios de tipo Cherenkov (IATC’s). De hecho, las colisiones de objetos compactos en sistemas binarios (fuentes de ondas gravitacionales) y las fuentes de rayos gamma energéticos, han sido sugeridos durante mucho tiempo como posibles emisores de neutrinos cósmicos. De normal, las búsquedas de fuentes de neutrinos se centran en neutrinos muónicos que atraviesan la tierra antes de ser detectados en el detector (trazas ascendentes), lo que permite de deshacerse del ruido de fondo de los muones atmosféricos. Por primera vez, todos los sabores de neutrinos (incluyendo los llamados eventos de tipo cascada) han sido considerados en esto tipo de análisis. Además, la búsqueda realizada en el este trabajo, es aplicable a fuentes tanto por encima como por debajo del horizonte de ANTARES (eventos ascendentes y descendentes), lo que permite una cobertura de todo el cielo. Esto es posible gracias a las reducidas ventanas espacial y temporal utilizadas. Los análisis multi-mensajero con ANTARES llevados a cabo durante la tesis han dado resultado que no se han encontrado neutrinos en coincidencia con ninguno de los señales GWs ni GRBs considerados en las búsquedas. Sin embargo, los resultados de este análisis muestran una mejora de la sensibilidad de un 15-30% para fuentes en el horizonte de ANTARES, y hasta un 200% cuando la búsqueda utiliza eventos descendentes, y han llevado a los mejores límites de emisión de neutrinos publicados por ANTARES para este tipo de fuentes. Two analyses are detailed in this thesis related to neutrino and multi-messenger astronomy with Cherenkov telescopes in the Mediterranean Sea. The first analysis explores the capabilities of the KM3NeT neutrino telescopes to detect the signal from a Galactic core-collapse supernova (CCSN), as well as the physical constraints that could be extracted from such a detection. Together with the Sun, CCSNe are the only confirmed sources of astrophysical neutrinos. A search method for these astrophysical neutrino sources with KM3NeT has been developed during this thesis, based on the analysis of the first data which has allowed for a good characterisation of the background and the detector performance. The results show that the KM3NeT detector might be sensitive to this MeV neutrino flux, with a coverage at 5 sigma discovery potential of more than 95% of Galactic CCSN progenitors. Therefore, KM3NeT will contribute to the observation of the next Galactic explosion. The CCSN analysis has been implemented in a real-time trigger, that is active since summer 2019. Moreover, it has yielded the first KM3NeT real-time results with the follow-up of the unmodelled candidate gravitational-wave (GW) events. These results have allowed the KM3NeT experiment to join the SNEWS network, to which all detectors sensitive to CCSN neutrino send their alerts. The second analysis exploits the data of the ANTARES neutrino telescope to search for high-energy neutrinos (TeV-PeV) in time and space coincidence with gravitational-wave sources and very-high energy gamma-ray bursts (GRBs). In fact, compact binary mergers and gamma-ray bursts have long been suggested as potential high-energy neutrino emitters. Typically, these searches look for muon neutrinos coming through the Earth (upgoing tracks). For the first time, all-flavors (including the so-called shower events) were included in this kind of searches. Moreover, these analyses have been applied to sources both below the ANTARES horizon (seen as upgoing events), and above the horizon of the ANTARES telescope (downgoing). This has lead to an improvement of 15-30% for upgoing events and up to a 200% for searches above the horizon. The analyses carried out during the thesis yielded no neutrino in coincidence with any of the gravitational-wave sources from the first GW catalog, neither with the first GRBs detected at very high energies.

For the first time in history, advanced detectors are available to observe the stretching and squeezing of space---gravitational waves---from violent astrophysical events. This opens up the prospect of joint detections with instruments of traditional astronomy, creating the new field of multi-messenger astrophysics. Joint detections allow us to form a coherent picture of the unfolding event as told by the various channels of information: mass and energy dynamics from gravitational waves, charged particle environments (along with magnetic field and specific element environments) from electromagnetic radiation, and thermonuclear reactions/relativistic particle outflows from neutrinos. In this work, I motivate low-latency electromagnetic and neutrino follow-up of sources known to emit gravitational radiation in the sensitivity band of ground-based interferometric detectors, Advanced LIGO and Advanced Virgo. To this end, I describe the low-latency infrastructure I developed with colleagues to select and enable successful follow-up of the first few gravitational-wave candidate events in history, including the first binary black hole merger, named GW150914, and binary neutron star coalescence, named GW170817, from the first and second observing runs. As a review, I outline the theory behind gravitational waves and explain how the advanced detectors, low-latency searches, and data quality vetting procedures work. To highlight the newness of the field, I also share results from an offline search for a more speculative source of gravitational waves, intersecting cosmic strings, from the second observing run. Finally, I address how LIGO/Virgo is prepared to adapt to challenges that will arise during the upcoming third observing run, an era that will be marked by near-weekly binary black hole candidate events and near-monthly binary neutron star candidate events. To handle this load, we made several improvements to our low-latency infrastructure, including a new, streamlined candidate event selection process, expansions I helped develop for temporal coincidence searches with electromagnetic/neutrino triggers, and data quality products on source classification and probability of astrophysical origin to provide to our observing partners for potential compact binary coalescences. These measures will further our prospects for multi-messenger astrophysics and increase our science returns.

In recent years we have seen major advances in multi-messenger astronomy. A milestone was achieved by identifying the electromagnetic counterpart of the gravitational wave event GW170817 detected by LIGO and Virgo. Similar efforts led to a set of neutrinos detected by IceCube to be associated with the blazar TXS 0506+056. Both demonstrate the potential of using multiple types of probes to study an astrophysical source. The High-Altitude Water Cherenkov Observatory (HAWC), located in the state of Puebla, Mexico, is a wide field instrument (~2 sr) sensitive to very-high-energy gamma rays (~0.1-100 TeV) which can operate with a large duty cycle (>95%). These characteristics make it well suited to look for transient events correlated with other astronomical messengers. In this work we present a maximum likelihood analysis framework developed to search and analyze signals in HAWC data of arbitrary timescales. We apply this method to search for very-high-energy gamma-ray counterparts of gravitational waves in short timescales (0.3-1000 s). We show that we would be able to either detect or meaningfully constrain the very-high-energy component of a gamma-ray burst within the binary neutron star merger horizon of current gravitational wave detectors if it occurs in our field of view. We did not find evidence for emission for any of the events analyzed. The source location of GW170817 was not observable by HAWC at the time of the merger. We also set flux upper bounds for TXS 0506+056 during the periods when the neutrino flares were identified. For the flare between September 2014 and March 2015 these are the only available limits at very high energy, and are consistent with the low state in high-energy gamma rays reported by the Fermi-LAT Collaboration.

We summarise the parallel session C7 Multi-Messenger Astronomy of GW Sources in the GR20 Amaldi 10 Conference. The talks in this session covered a wide range of topics in multi-messenger astronomy.