editorial Pushing the limits of discovery Be it neutrinos, ultra-high-energy photons or gravitational waves, new cosmic messengers have expanded the available discovery space of astronomy by exploring previously inaccessible astrophysical environments.

he neutrino, an elusive elementary other cosmic neutrinos, the poor angular There are interesting parallels particle with a rest mass so small localization of neutrino detectors (at best to be drawn between neutrino and Tthat it can effectively be considered ~0.7° for the highest energies for IceCube) very-high-energy gamma-ray astronomy. zero, is the perfect messenger from afar. makes it extremely difficult to pinpoint the Both probe highly energetic astrophysical Because they only interact with matter direction on the sky from which a specific processes that might otherwise be through the weak interaction and gravity event is coming. In 2017, the coincidence hidden from conventional astronomical (also weak, given their vanishingly small of a neutrino event with a flaring blazar observations. Furthermore, given the mass), neutrinos are able to escape most was a big step forward. It confirmed that challenges linked to the technology required environments unscathed and unchanged, active galactic nuclei are potent particle for their detection, both probes have only hence delivering pristine information on a accelerators and one of the sources of recently entered an era of reliable detections number of (astro)physical processes. That is high-energy cosmic neutrinos (see Elena of multiple events. While high-energy the theory of course, because in practice and Pian’s News & Views). gamma-ray astronomy may have a head exactly due to their elusiveness, detecting In this issue of Nature Astronomy, start — it is after all an extension of these neutrinos is no easy feat. Robert Stein and collaborators report the the electromagnetic spectrum, which While the direct detection of a association of a high-energy neutrino astronomers have been studying for a very (man-made) neutrino through the beta event with a different type of astrophysical long time — both fields are set to grow capture process occurred in 1956, it was source: a tidal disruption event, when exponentially over the coming years. not until 1968 that the first extraterrestrial a star is pulled apart during its infall The next milestone for high-energy neutrinos, from the Sun, were detected towards a supermassive black hole. In a gamma-ray astronomy will be the by the Homestake experiment, marking companion paper, Walter Winter and Cecilia commissioning of the Cherenkov Telescope the start of neutrino astronomy. Two Lunardini present an astrophysical model Array (first operations are expected to more decades passed before a fortuitous explaining the observations. As for the commence next year), which will afford supernova in our nearby Large Magellanic blazar association before it, the additional unprecedented sensitivity and localization Cloud led to the detection of neutrinos from information carried by the IceCube-detected capabilities up to energies of 300 TeV further away in our cosmic neighbourhood. neutrino event provides valuable insights and will tackle a number of fundamental The neutrino detector Kamiokande-II into tidal disruption events (see a discussion physical and astrophysical questions (see a measured eleven neutrinos from the by Kimitake Hayasaki). Meeting Report from the first Cherenkov supernova SN 1987A in February 1987. Neutrinos are not the only tool currently Telescope Array symposium in 2019). This discovery, which together with the used for pushing the limits of astrophysical Concerning neutrinos, in addition to detection of solar neutrinos led to a Nobel discovery. Ultra-high-energy astrophysics is upgrading the capabilities of IceCube, Prize for Masatoshi Koshiba and Raymond also enjoying a productive era, with multiple several next-generation neutrino detectors Davis Jr, further established the field of facilities now online to investigate this as yet are planned (including KM3NeT and the neutrino astronomy. scarcely explored part of the electromagnetic Pacific Ocean Neutrino Experiment). Fast-forward to today and the field spectrum. The detection of the Crab nebula The observational arsenal of astronomers is in full bloom. There are now multiple in the very-high-energy gamma rays by the keeps increasing and every additional way of neutrino experiments around the globe. Whipple telescope in 1989 led to a number looking at the night sky leads to an expanded Whether using ice, water, heavy water or of concurrent and subsequent experiments discovery space, a deeper understanding of even chlorine, the principle of all neutrino (including HESS, MAGIC and VERITAS) long-studied astrophysical phenomena and detectors is the same: put a large enough looking to expand our understanding of the in some cases the discovery of completely volume of a homogeneous medium in very-high-energy gamma-ray sky. new phenomena. Most tantalizingly, for the path of neutrinos and one of them In this issue, we publish two new sets both neutrinos and very-high-energy is bound to interact. The most sensitive of observations pushing the limits of gamma-rays, these observational probes and high-resolution detector currently very-high-energy gamma-ray astronomy straddle the (admittedly fuzzy) division in operation, the IceCube Neutrino in the TeV range. The High-Altitude Water between physics and astrophysics. Observatory in Antarctica, has been Cherenkov Observatory and the Tibet air The potential of cross-pollination and detecting cosmic neutrinos since the shower array report the detection of the serendipitous discoveries should motivate early 2010s. Cygnus cocoon and the supernova remnant both communities to keep pushing those Despite routine detections of cosmic G106.3+2.7, respectively. Both studies limits further. ❐ neutrinos, understanding and studying their constrain the astrophysical processes leading origins has not been straightforward. On top to the acceleration of cosmic-ray particles of the very fundamental issues of separating to PeV energies in two different and Published online: 17 May 2021 signal from noise, and solar neutrinos from unexpected astrophysical environments. https://doi.org/10.1038/s41550-021-01374-4

Nature Astronomy | VOL 5 | May 2021 | 425 | www.nature.com/natureastronomy 425