Multi-messenger Observations of a Binary Neutron Star Merger

Abstract

On 2017 August 17 a binary neutron star coalescence candidate ( later designated GW170817 ) with merger time 12:41:04 UTC was observed through gra vitational waves by the Advanced LIG O and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst ( GRB 170817A ) with a time delay of 1.7 s ~ with respect to the merger time. From the gravitational- wave signal, the source was initially localized to a sky region of 31 deg 2 at a luminosity distance of 4 0 8 8 - + Mpc and with component masses cons istent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M

. An extensive observing campaign was launched across the electromagnetic spectrum leadi ng to the discovery of a bright optical transient ( SSS17a, now with the IAU identi fi cation of AT 2017gfo ) in NGC 4993 ( at 40 Mpc ~ ) less than 11 hours after the merger by the One- Meter, Two Hemisphere ( 1M2H ) team using the 1 m Swope Telescope. The o ptical transient was independently detected by multiple teams within an hour . Subsequent observations targeted th e object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼ 10 days. Following early non-detections, X-ray and radio emission were discovered at the transient ’ s position 9 ~ and 16 ~ days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV / optical / near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993followedbyashortgamma-rayburst ( GRB 170817A ) and a kilonova / macronova powered by the radioactive decay of r -process nuclei synthesized in the ejecta.