CALET Search for electromagnetic counterparts of gravitational waves during the LIGO/Virgo O3 run

TL;DR

This paper reports on CALET's search for gamma-ray counterparts to gravitational wave events during LIGO/Virgo O3, detailing the instrument's performance, analysis methods, and upper flux limits, with no counterparts detected.

Contribution

The study provides the first systematic search for electromagnetic counterparts to gravitational waves using CALET during LIGO/Virgo O3, including detailed instrument performance and analysis procedures.

Findings

No gamma-ray counterparts detected during O3 events.

Established upper limits on gamma-ray fluxes for GW events.

Detailed description of CALET's detection capabilities and algorithms.

Abstract

The CALorimetric Electron Telescope (CALET) on the International Space Station (ISS) consists of a high-energy cosmic ray CALorimeter (CAL) and a lower-energy CALET Gamma ray Burst Monitor (CGBM). CAL is sensitive to electrons up to 20 TeV, cosmic ray nuclei from Z = 1 through Z $\sim$ 40, and gamma rays over the range 1 GeV - 10 TeV. CGBM observes gamma rays from 7 keV to 20 MeV. The combined CAL-CGBM instrument has conducted a search for gamma ray bursts (GRBs) since Oct. 2015. We report here on the results of a search for X-ray/gamma ray counterparts to gravitational wave events reported during the LIGO/Virgo observing run O3. No events have been detected that pass all acceptance criteria. We describe the components, performance, and triggering algorithms of the CGBM - the two Hard X-ray Monitors (HXM) consisting of LaBr$_{3}$(Ce) scintillators sensitive to 7 keV to 1 MeV gamma rays and a Soft Gamma ray Monitor (SGM) BGO scintillator sensitive to 40 keV to 20 MeV - and the high-energy CAL consisting of a CHarge-Detection module (CHD), IMaging Calorimeter (IMC), and fully active Total Absorption Calorimeter (TASC). The analysis procedure is described and upper limits to the time-averaged fluxes are presented.