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Observation of the Second LIGO/Virgo Event Connected with a Binary Neutron Star Merger S190425z in the Gamma-Ray Range
Authors:Pozanenko  A S  Minaev  P Yu  Grebenev  S A  Chelovekov  I V
Institution:1.Space Research Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow, 117997, Russia
;2.National Research University “Higher School of Economics”, Myasnitskaya ul. 20, Moscow, 101000, Russia
;
Abstract:

The results of observations of the gravitational-wave (GW) event S190425z recorded by the LIGO/Virgo detectors with the anti-coincidence shield (ACS) of the SPI gamma-ray spectrometer onboard the INTEGRAL observatory are presented. With a high probability (>99%) it was associated with a neutron star (NS) merger in a close binary system. This is only the second event of such a type in the history of gravitational-wave observations (after GW170817). A weak gamma-ray burst, GRB190425, consisting of two pulses ~0.5 and ~5.9 s after the NS merger in the event S190425z with an a priori significance of 3.5 and 4.4σ (taken together 5.5σ) was detected by SPI-ACS. Analysis of the SPI-ACS count rate history recorded on these days (a total of ~125 ks of observations) has shown that the rate of random occurrence of two close spikes with the characteristics of GRB190425 does not exceed 6.4 × 10?5 s?1 (i.e., such events occur by chance, on average, every ~4.3 hours). Note that the time profile of GRB190425 has much in common with the profile of GRB170817A accompanying the event GW170817, that both NS mergers were the nearest (≤150 Mpc) of all the events recorded by the LIGO/Virgo detectors, and that no significant excesses of the gamma-ray flux above the background were detected in any of ~30 black hole merger events recorded to date by these detectors. No bursts of hard radiation were detected in the field of view of the SPI and IBIS/ISGRI gamma-ray telescopes onboard INTEGRAL. This, along with the absence of detection of gamma-ray emission from GRB190425 by the GBM gamma-ray burst monitor of the Fermi observatory suggesting its occultation by the Earth, allows the localization region for the source of this GWevent to be reduced significantly. The parameters Eiso and Ep for GRB190425 are estimated and compared with those for GRB170817A.

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