LIGO-detected gravitational-wave events derive from merger of black holes, according to CALET observations

The Calorimetric Electron Telescope (CALET), which was installed on the exposed facility of the Japan experiment module ‘Kibo’ of the International Space Station (ISS) on August 2015, did not observe x-rays and/or gamma rays when the United States’ Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves. This confirmed the existence of not only gravitational waves from astronomical objects but also from a merger of two black holes. CALET’S observation results were published in the American Astronomical Society online journal THE ASTRONOMICAL JOURNAL LETTERS on September 20, 2016. The CALET Project team representative is Professor Shoji Torii from the Faculty of Science and Engineering, Waseda University.

図1 CALET(円で囲った装置)と 「きぼう」船外実験プラットフォーム (出典:NASA/JAXA)

Figure 1. CALET (circled red) and Kibo experiment platform (Source: NASA/JAXA)

Gravitational waves are phenomena in which massive accelerating objects disturb and cause ripples in space-time. These ripples travel at the speed of light and distort space. Albert Einstein predicted the existence of gravitational waves in his Theory of General Relativity in 1916, and attempts have been made to discover gravitational waves ever since. 100 years after Einstein’s prediction, LIGO became the first observatory in the world to detect a gravitational wave event on September 14, 2015.

Before LIGO’s launch, the LIGO team had asked other space observatories to look out for electromagnetic waves (such as radio, optical, near-infrared, X-ray, or high-energy gamma rays) to understand in detail the mechanism of how gravitational waves occur. The CALET Team has offered to collaborate and become a sister facility of LIGO, establishing a Memorandum of Understanding to mutually provide data analysis and results.

CALET consists of two scientific instruments: the Calorimeter (CAL) and CALET Gamma-ray Burst Monitor (CGBM). CAL, the main instrument, is capable of observing high energy electrons. The CGBM is a gamma-ray burst monitor which uses hard x-ray and soft gamma-ray monitors to achieve broad energy coverage.

When the LIGO team first observed gravitational waves in September 2015, CALET was still under initial inspection and started making observations in November 2015. Later, the LIGO team detected the second gravitational-wave event in December 26, 2015. At this time, CALET was making observations in the same direction as where the gravitational waves were coming from, so it immediately looked for electromagnetic waves. However, CALET was unable to detect any signals.

図2 CALETによるガンマ線バースト観測装置(CGBM)による観測結果

Figure 2. Observation results of the CALET Gamma-ray Burst Mointor

According to the LIGO team, the two gravitational-wave events LIGO has detected are believed to be from two black holes merging together. Detecting electromagnetic waves is considered to be difficult in such cases, and CALET’s observation results backed this hypothesis.

In the future, if improved LIGO facilities and highly-sensitive detectors such as Japan’s KAGRA are newly activated, detecting gravitational waves from mergers of black holes and neutron stars or neutron stars with each other becomes possible. Under this circumstance, gamma-ray bursts are expected to happen simultaneously. Therefore, to prepare for future observations on gravitational waves and to better understand the mechanisms of how gravitational waves occur, CALET will continue to make observations.


Journal: Astrophysical Journal Letters 829-L20 2016 September 20

More information on CALET

Contact information

  • On research
    Assistant Professor, Takanori Sakamoto, Department of Physics and Mathematics, Aoyama Gakuin University
    TEL: 042-759-6275 E-mail:
    Professor Shoji Torii, Faculty of Science and Engineering, Waseda University
  • On media coverage and public relations
    Japan Aerospace Exploration Agency
    ISS/Kibo Information and Public Relations Center, Human Space Flight Technology Division
    Inquiry form (Japanese JAXA website)
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