Item

Abstract

On 2017 August 17 a binary neutron star coalescence candidate (later
designated GW170817) with merger time 12:41:04 UTC was observed through
gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The
Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB
170817A) with a time delay of $\sim$1.7 s with respect to the merger time. From
the gravitational-wave signal, the source was initially localized to a sky
region of 31 deg$^2$ at a luminosity distance of $40^{+8}_{-8}$ Mpc and with
component masses consistent with neutron stars. The component masses were later
measured to be in the range 0.86 to 2.26 Msun. An extensive observing campaign
was launched across the electromagnetic spectrum leading to the discovery of a
bright optical transient (SSS17a, now with the IAU identification of AT
2017gfo) in NGC 4993 (at $\sim$40 Mpc) less than 11 hours after the merger by
the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The
optical transient was independently detected by multiple teams within an hour.
Subsequent observations targeted the 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 $\sim$10
days. Following early non-detections, X-ray and radio emission were discovered
at the transient's position $\sim$9 and $\sim$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. (Abridged)