Subaru Telescope 2.0

Science Goal #3: To contribute to multi-messenger astronomy

What we receive from the Universe through celestial phenomena is not limited to electromagnetic waves, such as visible, infrared, and radio wavelengths, but includes gravitational waves and particles like neutrinos. Currently, the spotlight is on multi-messenger astronomy which comprehensively studies astronomical phenomena based on the interpretation of multiple pieces of information.

The first detection of gravitational waves was made in 2015. When GW170817, the gravitational wave produced by a merger of two neutron stars, was detected, the electromagnetic waves emitted from the gravitational wave source were observed by the Subaru Telescope and other observatories all over the world. This observation marked a major milestone in multi-messenger astronomy for the coordinated observation of the gravitational and electromagnetic waves. The Subaru Telescope’s achievement was beyond detecting the optical and infrared emission from GW170817. It found evidence for the production of heavy elements by the neutron star merger through its successful follow-up observations of the time evolution of its brightness.

The Subaru Telescope in the 2020s will swiftly identify gravitational wave source objects in visible and infrared light over the wide sky coverage to conduct follow-up observations for a long period. Along with ULTIMATE-Subaru’s search and tracking, we will reveal the synthesis process of heavy elements like gold, platinum, and uranium in the Universe by studying the elements yielded by neutron star mergers. We also investigate the origin of ultra-high-energy cosmic rays in collaboration with neutrino observatories.

(Left) The gravitational wave source GW170817 imaged by J-GEM, Japan’s team for follow-up observations of the gravitational wave. Three-color composite image (blue: z band, green: H band, and red: Ks band) from the optical observation by the Subaru Telescope (z band: wavelength 0.9 micrometers), and the near-infrared observation by the IRSF telescope in South Africa (H band: wavelength 1.6 micrometers, and Ks band: 2.2 micrometers). The observation on August 24 and 25, 2017, indicated that as the object attenuated the light, it became redder (brighter in the near-infrared band). (Credit: NAOJ / Nagoya University)
(Right) Artist’s impression of neutron star merger and kilonova, an explosive phenomenon caused by emissions during the merger process (Credit: NAOJ)

Observation Targets

Gravitational wave sources and neutrino sources

Subaru Telescope’s Objectives

(1) Conduct search and follow-up observations of optical and infrared counterparts of gravitational wave sources with HSC. While gravitational wave telescopes’ accuracy in determining direction is over 10 square degrees, HSC’s wide-field capability plays a significant role in identifying counterparts of gravitational wave sources.

(2) Conduct search and follow-up observation for neutron star mergers with ULTIMATE-Subaru. By ULTIMATE-Subaru’s infrared observation, we investigate ratios of heavy elements produced by the “rapid neutron-capture process,” which is known as the origin of gold, platinum, and uranium during the neutron star mergers. The data will lead to the discovery of the origin of heavy elements in the Universe.

(3) Identify from which directions ultra-high energy neutrinos are emitted and determine their sources with HSC.

Certain explosive phenomena in the Universe produce particles such as neutrinos and gravitational waves, as well as electromagnetic waves, including visible and infrared light. Simultaneous observation, which aims to advance understanding of those events in the Universe, is called multi-messenger astronomy. Multi-messenger astronomy offers new gates to explore phenomena that cannot be accomplished by electromagnetic observations alone, such as the characteristics of black hole horizons, nuclear physics of neutron stars and other extreme objects, and the origin of heavy elements. Leveraging the Subaru Telescope’s wide-field capability, Subaru Telescope 2.0 will significantly advance multi-messenger astronomy in partnership with KAGRA, Hyper-Kamokande, and other Japan’s cutting-edge research facilities.

The first direct observation of gravitational waves was made in September 2015 by the United States gravitational wave telescope called the Laser Interferometer Gravitational-Wave Observatory (LIGO), which led to the birth of gravitational-wave astronomy. In August 2017, another gravitational wave event produced by a neutron star merger was first observed by LIGO and Europe’s gravitational wave telescope, the Virgo interferometer. The electromagnetic waves emitted by this phenomenon (designated GW170817) were detected by observatories all around the world. This detection marked a significant milestone in multi-messenger astronomy for the coordinated observation of both gravitational waves and electromagnetic waves. The Subaru Telescope succeeded in the detection and follow-up observation of the optical and infrared emission from GW170817 and found evidence for the production of heavy elements by the neutron star merge (the Subaru Telescope’s observation result released on October 16, 2017 ).

No other coordinated observations of gravitational and electromagnetic waves have been successful as of February 2022, which, in other words, represents multi-messenger astronomy’s potential for significant growth in the future. In the mid-2020s, gravitational wave telescopes in the world (LIGO, Virgo, and Japan’s KAGRA) are expected to capture gravitational waves from neutron star mergers that occurred about 600 million years ago. Such distant mergers of neutron stars would be fainter than 22 magnitudes even at the peak in the optical and infrared wavelengths and decrease their brightness to 24 magnitudes over 1 to 2 days. What challenges observation by gravitational wave telescopes is their precision for pinpointing such phenomena as wide as dozens of square degrees. Making full use of HSC, Subaru Telescope 2.0 will identify a gravitational wave source object in the optical and infrared wavelength range over such a vast space with dispatch for follow-up observations over a long time. ULTIMATE-Subaru will be capitalized on for the search and follow-up observation when near-infrared light is strongly emitted from a merger of neutron stars. This allows us to analyze optical and infrared emissions from numerous mergers of neutron stars and their time evolution to investigate which and what amount of heavy elements are produced during neutron star mergers among those emitted as a result of explosive nucleosynthesis processes. It will make headway in understanding the origin of heavy elements in the Universe.

Additionally, a collaboration of detection of neutrinos and observation of electromagnetic waves is part of multi-messenger astronomy. While the detector of high-energy neutrinos called the IceCube Neutrino Observatory and other observatories detect neutrinos coming from outer space, Subaru Telescope 2.0 pinpoints their sources by observing their light. Those neutrino detectors can only determine the direction over one square degree, whereas HSC can bring its real power for identifying source objects. Given that high-energy neutrinos are considered to originate from the same source as ultra-high-energy cosmic rays, Subaru Telescope 2.0’s identification of sources of high-energy neutrinos is expected to contribute significantly to answering astrophysics’ major questions about the origin of ultra-high-energy cosmic rays.