Astro2020 APC White Paper The Southern Wide-Field Gamma-Ray Observatory (SWGO): A Next-Generation Ground-Based Survey Instrument for VHE Gamma-Ray Astronomy

We describe plans for the development of the Southern Wide-field Gamma-ray Observatory (SWGO), a next-generation instrument with sensitivity to the very-high-energy (VHE) band to be constructed in the Southern Hemisphere. SWGO will provide wide-field coverage of a large portion of the southern sky, effectively complementing current and future instruments in the global multi-messenger effort to understand extreme astrophysical phenomena throughout the universe. A detailed description of science topics addressed by SWGO is available in the science case white paper [1]. The development of SWGO will draw on extensive experience within the community in designing, constructing, and successfully operating wide-field instruments using observations of extensive air showers. The detector will consist of a compact inner array of particle detection units surrounded by a sparser outer array. A key advantage of the design of SWGO is that it can be constructed using current, already proven technology. We estimate a construction cost of 54M USD and a cost of 7.5M USD for 5 years of operation, with an anticipated US contribution of 20M USD ensuring that the US will be a driving force for the SWGO effort. The recently formed SWGO collaboration will conduct site selection and detector optimization studies prior to construction, with full operations foreseen to begin in 2026. Throughout this document, references to science white papers submitted to the Astro2020 Decadal Survey with particular relevance to the key science goals of SWGO, which include unveiling Galactic particle accelerators [2–10], exploring the dynamic universe [11–21], and probing physics beyond the Standard Model [22–25], are highlighted in red boldface. Corresponding/Lead Author: Petra Huentemeyer (Michigan Technological University); Contact: petra@mtu.edu; +1 (906) 487-1229 Co-authors/Proposing Team: P. Abreu1 2, A. Albert3, R. Alfaro4, C. Alvarez5, R. Arceo5, P. Assis1 2, F. Barao1 6, J. Bazo7, J. F. Beacom8, J. Bellido9, S. BenZvi10, T. Bretz11, C. Brisbois12, A. M. Brown13, F. Brun14, M. Buscemi15, K. S. Caballero-Mora16, P. Camarri17, A. Carramiñana18, S. Casanova19, A. Chiavassa20, R. Conceição1 2, G. Cotter21, P. Cristofari22, S. Dasso23 24, A. De Angelis25 26 27, M. De Maria28, P. Desiati29 30, G. Di Sciascio31, J. C. Dı́az Vélez32, C. Dib33, B. Dingus3, D. Dorner34, M. Doro25 27, C. Duffy35, M. DuVernois29, R. Engel36, M. Fernandez Alonso37, H. Fleischhack12, P. Fonte1 38, N. Fraija39, S. Funk40, J. A. Garcı́a-González4, M. M. González39, J. A. Goodman41, T. Greenshaw42, J. P. Harding3, A. Haungs36, B. Hona12, A. Insolia43, A. Jardin-Blicq44, V. Joshi40, K. Kawata45, S. Kunwar44, G. La Mura1, J. Lapington35, J.-P. Lenain46, R. López-Coto27, K. Malone3, J. Martinez-Castro47, H. Mart́ınez-Huerta48, L. Mendes1, E. Moreno49, M. Mostafá37, K. C. Y. Ng50, M. U. Nisa51, F. Peron28, A. Pichel23, M. Pimenta1 2, E. Prandini27 52, S. Rainò53, A. Reisenegger54, J. Rodriguez14, M. Roth36, A. Rovero23, E. Ruiz-Velasco44, T. Sako45, A. Sandoval4, M. Santander55, K. Satalecka56, M. Schneider41, H. Schoorlemmer44, F. Schüssler14, R. C. Shellard57, A. Smith41, S. Spencer21, W. Springer58, P. Surajbali44, K. Tollefson51, B. Tomé1, I. Torres18, A. Viana48, T. Weisgarber29 30, R. Wischnewski56, A. Zepeda59, B. Zhou8, H. Zhou3 Introduction This white paper presents our vision and plans for the Southern Wide-field Gamma-ray Observatory (SWGO, www.swgo.org). SWGO is a next-generation, ground-based survey instrument that will provide a unique view on gamma-ray and cosmic-ray emission from tens of GeV to hundreds of TeV. The facility will improve upon the success of the HAWC Gamma-ray Observatory in Mexico that is surveying the Northern gamma-ray sky with nearly 100% duty cycle and an instantaneous field of view of ∼ 2 sr. Since 2015, HAWC has discovered new TeV sources and source classes, set new world-leading limits on dark matter decay and annihilation, and played a crucial role in multi-messenger observations [26–42]. The success of the water Cherenkov technology implemented by HAWC has inspired an ambitious Chinese-lead effort, LHAASO, in the Northern Hemisphere, which uses a similar design [43]. Recent years have seen a wealth of paradigm-shifting discoveries including a kilonova associated with merging neutron stars [34], a gamma-ray burst (GRB) with photons detected above 300 GeV [44], and a detection of a sub-PeV neutrino from a flaring active galactic nucleus (AGN) [39]. The global multi-messenger astrophysics community recognizes the importance of facilities in both hemispheres that continuously survey the gamma-ray sky in the space and time domain. Wide-field-of-view observatories can not only provide prompt alerts of transient events to the astrophysics community, but they also retain archival information about gamma-ray emission covering large regions in the sky. Our international collaboration is committed to the ideals of open science. All gamma-ray data will be made publicly available after a brief proprietary period. We envision an approach similar to those practiced by existing NASA missions such as Fermi, Swift, and NuSTAR, or the planned next-generation imaging atmospheric Cherenkov Telescope Array, CTA [45], which provide a data archive and science tools for the astrophysics and astronomy community. In addition, we will explore ways in which communities beyond astrophysics, e.g. cosmology and particle physics, may use data at all levels from the observatory. We propose, for the first time, to provide a formal guest investigator program for a TeV all-sky instrument. While the team members have experience in and are working on a number of methods to make data from their respective observatories public, this represents an even more ambitious and sweeping plan for data dissemination that is not restricted only to high-level data products. The goal is for observatory data to be prepared in a way that will make it straightforward for a diverse science community to access and combine them – a necessity for any astrophysics experiment in the multi-messenger era. Key Science Goals and Objectives The key science objectives of SWGO appear in Table 1, along with the design requirements necessary to achieve them. The anticipated sensitivity of SWGO is shown in the left panel of Figure 1, in comparison with the existing High Energy Stereoscopic System (HESS) instrument, and CTA-South, which is currently under construction. SWGO will have a total sky coverage of ∼8 sr, as shown in the right panel of Figure 1. In contrast to the imaging atmospheric Cherenkov Telescope (IACT) sensitivities, which apply to the observation of a single source, the SWGO sensitivity applies to a large number of sources throughout its sky coverage simultaneously. Figure 1 also applies strictly to point sources. Sources with moderate angular extents will reduce the sensitivity for IACTs more severely than for SWGO.