Abstract
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 ∼ 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 deg2 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 M ⊙ . 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 ∼ 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 ∼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 NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.
Highlights
Over 80 years ago Baade & Zwicky (1934) proposed the idea of neutron stars, and soon after, Oppenheimer & Volkoff (1939) carried out the first calculations of neutron star models
Given the temporal coincidence with the Fermi-Gamma-ray Burst Monitor (GBM) gamma-ray bursts (GRBs), a Gamma-ray Coordinates Network (GCN) Circular was issued at 13:21:42 UTC (LIGO Scientific Collaboration & Virgo Collaboration et al 2017a) reporting that a highly significant candidate event consistent with a binary neutron stars (BNSs) coalescence was associated with the time of the GRB959
Five other teams took images of the transient within an hour of the 1M2H image using different observational strategies to search the Laser Interferometer Gravitationalwave Observatory (LIGO)-Virgo sky localization region. They reported their discovery of the same optical transient in a sequence of GCNs: the Dark Energy Camera (01:15 UTC; Allam et al 2017), the Distance Less Than 40 Mpc survey (01:41 UTC; Yang et al 2017a), Las Cumbres Observatory (LCO; 04:07 UTC; Arcavi et al 2017a), the Visible and Infrared Survey Telescope for Astronomy (VISTA; 05:04 UTC; Tanvir et al 2017a), and MASTER (05:38 UTC; Lipunov et al 2017d)
Summary
Over 80 years ago Baade & Zwicky (1934) proposed the idea of neutron stars, and soon after, Oppenheimer & Volkoff (1939) carried out the first calculations of neutron star models. These hints included: (i) their association with both elliptical and star-forming galaxies (Barthelmy et al 2005; Prochaska et al 2006; Berger et al 2007; Ofek et al 2007; Troja et al 2008; D’Avanzo et al 2009; Fong et al 2013), due to a very wide range of delay times, as predicted theoretically (Bagot et al 1998; Fryer et al 1999; Belczynski et al 2002); (ii) a broad distribution of spatial offsets from host-galaxy centers (Berger 2010; Fong & Berger 2013; Tunnicliffe et al 2014), which was predicted to arise from supernova kicks (Narayan et al 1992; Bloom et al 1999); and (iii) the absence of associated supernovae (Fox et al 2005; Hjorth et al 2005c, 2005a; Soderberg et al 2006; Kocevski et al 2010; Berger et al 2013a) Despite these strong hints, proof that sGRBs were powered by neutron star mergers remained elusive, and interest intensified in following up gravitational-wave detections electromagnetically (Metzger & Berger 2012; Nissanke et al 2013). Partners have followed up binary black hole detections, starting with GW150914 (Abbott et al 2016a), but have discovered no firm electromagnetic counterparts to those events
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