tokens: tens of solar masses (Abbott et al. 2016a ). Based solely on the gravitational-wave signals recorded by two LIGO detectors, the current hypothesis is that GW150914 was the result of a merger of two black holes with initial masses of and at a luminosity distance of Mpc. The Fermi Gamma-ray Burst Monitor ( Fermi -GBM) reported a possible weak gamma-ray transient source above 50 keV at 0.4 s after the GW150914 trigger (Connaughton et al. 2016 ). However, the upper limit provided by the INTEGRAL Advanced Camera for Survey instrument in a gamma-ray energy band similar to the Fermi -GBM energy band is not consistent with a possible gamma-ray counterpart of GW150914 suggested by the Fermi -GBM (Savchenko et al. 2016 ). No electromagnetic counterpart of GW150914 was found in radio, optical, near-infrared, X-ray, or high-energy gamma-rays (Abbott et al. 2016b ). GW151226 (LIGO-Virgo trigger ID: G211117) is the second gravitational-wave candidate identified by both LIGO Hanford Observatory and LIGO Livingston Observatory with a high significance (the false-alarm rate of less than one per 1000 years by the on-line search) at 3:38:53.647 UT on 2015 December 26 (Abbott et al. 2016c ). According to a Bayesian parameter estimation analysis, the event is very likely a binary black hole merger with initial black hole masses of and and a final black hole mass of (Abbott et al. 2016d ). The luminosity distance of the source is estimated as Mpc, which corresponds to a redshift of . As far as the electromagnetic counterpart search of GW151226 in the gamma-ray regime is concerned, Fermi -GBM (Racusin et al. 2016 ), Fermi Large Area Telescope (LAT; Racusin et al. 2016 ), High-Altitude Water Cherenkov Observatory (Wood the HAWC Collaboration 2016 ), and Astrosat -CZTI (Bhalerao et al. 2016 ) reported no detections around the GW trigger time. According to Racusin et al. ( 2016 ), the flux upper limit of Fermi -GBM is from 4.5 × 10 −7 to 9 × 10 −7 erg cm −2 s −1 in the 10–1000 keV band. The Fermi -LAT flux upper limit using the first orbit data after the LIGO trigger is from 2.6 × 10 −10 to 7.8 × 10 −9 erg cm −2 s −1 in the 0.1–1 GeV band. The CALorimetric Electron Telescope ( CALET ; Asaoka the CALET Collaboration 2015 ; Torii et al. 2015 ) mission, which was successfully launched and emplaced on the Japanese Experiment Module—Exposed Facility of the International Space Station (ISS) in 2015 August, was fully operational at the time of GW151226. CALET consists of two scientific instruments. The Calorimeter (CAL) is the main instrument, which is capable of observing high-energy electrons from ∼1 GeV to ∼20 TeV, protons, helium, and heavy nuclei from ∼10 GeV to 1000 TeV and gamma-rays from ∼1 GeV to ∼10 TeV. The field of view (FOV) of CAL is ∼45° from the zenith direction. Another instrument, CALET Gamma-ray Burst Monitor (CGBM; Yamaoka et al. 2013 ), is a gamma-ray burst (GRB) monitor using two different kind of scintillators (LaBr 3 (Ce) and BGO) to achieve a broad
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