INTEGRAL follow-up of the gravitational wave events

We use observations of the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) to search for gamma-ray and hard X-rayemission associated with the gravitational waveevents discovered during the first and the second scientific runs of Advanced LIGOand Advanced Virgo. The highly eccentric orbit of INTEGRAL ensures high duty cycle, long-term stable background, and unobstructed view of nearly the entire sky. This enables us to use a combination of INTEGRAL instruments (SPectrometer onboard INTEGRAL - Anti-Coincidence Shield (SPI-ACS), Imager on Board the INTEGRAL Satellite ( IBIS), and IBIS/ Veto) to search for a hard X-rayelectromagnetic signal in the full high-probability sky region for almost every single LIGOtrigger. INTEGRAL observations of the binary black hole(BBH) mergers GW150914, LVT151012, GW170104, and GW170814 allowes to constrain the fraction of the energy promptly released in gamma-rays in 75 keV - 2 MeV energy range in the direction of the observer down to as little as one millionthof the gravitational waveenergy, in the majority of the localization region. Moreover, in the case of LVT151012 INTEGRAL high-energy imaging instruments, IBIS, SPectrometer onboard INTEGRAL (SPI), and Joint European X-Ray Monitor (JEM-X), provided the unique opportunity to search also for long-lasting electromagnetic counterparts of this event over 3 decades in energy, from 5 keV to 8 MeV. Finally, we discuss the INTEGRAL detection of the short gamma-ray burstGRB 170817A (discovered by Fermi- Gamma-ray BurstMonitor (GBM)) with a signal-to-noise ratio of 4.6, and, for the first time, its association with the gravitational waves(GWs) from binary neutron star (BNS) merging event GW170817 detected by the LIGOand Virgo observatories. The significance of association between the gamma-ray burstobserved by INTEGRAL and GW170817 is 3.2 σ, while the association between the Fermi-GBM and INTEGRAL detections is 4.2 σ. GRB 170817A was detected by the SPI-ACS instrument about 2 s after the end of the gravitational waveevent. We measure a fluence of 1.4±0.4±0.6×10−7 erg cm−2 (75–2000 keV), where, respectively, the statistical error is given at the 1 σ confidence level, and the systematic error corresponds to the uncertainty in the spectral model and instrument response.