INTEGRAL search for GW counterparts and the GRB170817A/GW170817 detection

Abstract

The INTernational Gamma-ray Astrophysics Laboratory (INTEGRAL) has detected the Short Gamma-Ray Burst (SGRB), GRB170817A with a signal-to-noise ratio of 4.6 and demonstrated its association with the binary neutron star merging event GW170817 detected by the LIGO and Virgo gravitational wave observatories. The association was immediately evident due to the timing and positional coincidence of this event with the initial error box, derived from gravitational wave (GW) measurements and the contemporaneous detection of a similar gamma-ray signal by Fermi/GBM. This SGRB was detected by the INTEGRAL SPI ACS about 1.7 s after the end of the GW emission, with a fluence of ($1.4 \pm 0.4$) $\times 10^{-7} \text { erg cm}^{-2}$ in the 75–2000 keV energy range. After the serendipitous detection of the short prompt GRB, INTEGRAL continued the planned observation for about 20 h, and then performed a targeted follow-up ToO observation lasting several days. This ToO observation provided a stringent upper limit on any electromagnetic signal in a very broad energy range, from 3 keV to 8 MeV, in particular constraining the soft gamma-ray afterglow flux to $< 7.1 \times ^{-11} \text { erg cm}^{-2} \text { s}^{-1}$ in the range 80–300 keV. Exploiting the unique capabilities of INTEGRAL, we constrained the gamma-ray line emission intensity from radioactive decays expected to be the principal source of the energy behind a kilonova event following a NS–NS coalescence. Finally, we put a stringent upper limit on any delayed bursting activity, for example, from a newly formed magnetar. The INTEGRAL prompt detection and the subsequent continuous observations at all wavelengths have provided important constraints on the high energy emission of the resulting kilonova and the post inspiral object: NS, BH, or a new exotic object.