CALET Upper Limits on X-Ray and Gamma-Ray Counterparts of GW151226

Abstract

We present upper limits in the hard X-ray and gamma-ray bands at the time of the Laser Interferometer Gravitational-wave Observatory (LIGO) gravitational-wave event GW151226 derived from the CALorimetric Electron Telescope (CALET) observation. The main instrument of CALET, CALorimeter (CAL), observes gamma-rays from ∼1 GeV up to 10 TeV with a field of view of ∼2 sr. The CALET gamma-ray burst monitor (CGBM) views ∼3 sr and ∼2π sr of the sky in the 7 keV–1 MeV and the 40 keV–20 MeV bands, respectively, by using two different scintillator-based instruments. The CGBM covered 32.5% and 49.1% of the GW151226 sky localization probability in the 7 keV–1 MeV and 40 keV–20 MeV bands respectively. We place a 90% upper limit of 2 × 10−⁷ erg cm−² s−¹ in the 1–100 GeV band where CAL reaches 15% of the integrated LIGO probability (∼1.1 sr). The CGBM 7σ upper limits are 1.0 × 10−⁶ erg cm−² s−¹ (7–500 keV) and 1.8 × 10−⁶ erg cm−² s−¹ (50–1000 keV) for a 1 s exposure. Those upper limits correspond to the luminosity of 3–5 × 10⁴⁹ erg s−¹, which is significantly lower than typical short GRBs.