Multi-messenger Observations of a Binary Neutron Star Merger

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)


Introduction
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. Neutron stars entered the realm of observational astronomy in the 1960s by providing a physical interpretation of X-ray emission from ScorpiusX-1 (Giacconi et al. 1962;Shklovsky 1967) and of radio pulsars (Gold 1968;Hewish et al. 1968;Gold 1969).
The discovery of a radio pulsar in a double neutron star system by Hulse & Taylor (1975) led to a renewed interest in binary stars and compact-object astrophysics, including the development of a scenario for the formation of double neutron stars and the first population studies (Flannery & van den Heuvel 1975;Massevitch et al. 1976;Clark 1979;Clark et al. 1979;Dewey & Cordes 1987;Lipunov et al. 1987; for reviews see Kalogera et al. 2007;Postnov & Yungelson 2014). The Hulse-Taylor pulsar provided the first firm evidence (Taylor & Weisberg 1982) of the existence of gravitational waves (Einstein 1916(Einstein , 1918 and sparked a renaissance of observational tests of general relativity (Damour & Taylor 1991Taylor et al. 1992;Wex 2014). Merging binary neutron stars (BNSs) were quickly recognized to be promising sources of detectable gravitational waves, making them a primary target for ground-based interferometric detectors (see Abadie et al. 2010 for an overview). This motivated the development of accurate models for the two-body, general-relativistic dynamics  (Blanchet et al. 1995;Buonanno & Damour 1999;Pretorius 2005;Baker et al. 2006;Campanelli et al. 2006;Blanchet 2014) that are critical for detecting and interpreting gravitational waves (Abbott et al. 2016c(Abbott et al. , 2016d(Abbott et al. , 2016e, 2017a(Abbott et al. , 2017c(Abbott et al. , 2017d. In the mid-1960s, gamma-ray bursts (GRBs) were discovered by the Vela satellites, and their cosmic origin was first established by Klebesadel et al. (1973). GRBs are classified as long or short, based on their duration and spectral hardness (Dezalay et al. 1992;Kouveliotou et al. 1993). Uncovering the progenitors of GRBs has been one of the key challenges in high-energy astrophysics ever since (Lee & Ramirez-Ruiz 2007). It has long been suggested that short GRBs might be related to neutron star mergers (Goodman 1986;Paczynski 1986;Eichler et al. 1989;Narayan et al. 1992).
In 2005, the field of short gamma-ray burst (sGRB) studies experienced a breakthrough (for reviews see Nakar 2007;Berger 2014) with the identification of the first host galaxies of sGRBs and multi-wavelength observation (from X-ray to optical and radio) of their afterglows (Berger et al. 2005;Fox et al. 2005;Gehrels et al. 2005;Hjorth et al. 2005b;Villasenor et al. 2005). These observations provided strong hints that sGRBs might be associated with mergers of neutron stars with other neutron stars or with black holes. 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;), 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. 2005cHjorth et al. , 2005aSoderberg 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 electro-magnetically (Metzger & Berger 2012;Nissanke et al. 2013).
Evidence of beaming in some sGRBs was initially found by Soderberg et al. (2006) and Burrows et al. (2006) and confirmed by subsequent sGRB discoveries (see the compilation and analysis by Fong et al. 2015 and also Troja et al. 2016). Neutron star binary mergers are also expected, however, to produce isotropic electromagnetic signals, which include (i) early optical and infrared emission, a so-called kilonova/macronova (hereafter kilonova; Li & Paczyński 1998;Kulkarni 2005;Rosswog 2005;Metzger et al. 2010;Roberts et al. 2011;Kasen et al. 2013;Tanaka & Hotokezaka 2013;Grossman et al. 2014; Barnes et al. 2016;Tanaka 2016;Metzger 2017) due to radioactive decay of rapid neutron-capture process (r-process) nuclei (Lattimer & Schramm 1974 synthesized in dynamical and accretion-disk-wind ejecta during the merger; and (ii) delayed radio emission from the interaction of the merger ejecta with the ambient medium (Nakar & Piran 2011;Piran et al. 2013;Hotokezaka & Piran 2015;Hotokezaka et al. 2016). The late-time infrared excess associated with GRB 130603B was interpreted as the signature of r-process nucleosynthesis (Berger et al. 2013b;Tanvir et al. 2013), and more candidates were identified later (for a compilation see Jin et al. 2016).
Here, we report on the global effort 958 that led to the first joint detection of gravitational and electromagnetic radiation from a single source. An ∼ 100 s long gravitational-wave signal (GW170817) was followed by an sGRB (GRB 170817A) and an optical transient (SSS17a/AT 2017gfo) found in the host galaxy NGC 4993. The source was detected across the electromagnetic spectrum-in the X-ray, ultraviolet, optical, infrared, and radio bands-over hours, days, and weeks. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993, followed by an sGRB and a kilonova powered by the radioactive decay of r-process nuclei synthesized in the ejecta. Figure 1. Localization of the gravitational-wave, gamma-ray, and optical signals. The left panel shows an orthographic projection of the 90% credible regions from LIGO (190 deg 2 ; light green), the initial LIGO-Virgo localization (31 deg 2 ; dark green), IPN triangulation from the time delay between Fermi and INTEGRAL (light blue), and Fermi-GBM (dark blue). The inset shows the location of the apparent host galaxy NGC 4993 in the Swope optical discovery image at 10.9 hr after the merger (top right) and the DLT40 pre-discovery image from 20.5 days prior to merger (bottom right). The reticle marks the position of the transient in both images. 958 A follow-up program established during initial LIGO-Virgo observations (Abadie et al. 2012) was greatly expanded in preparation for Advanced LIGO-Virgo observations. 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.

A Multi-messenger Transient
On 2017 August 17 12:41:06 UTC the Fermi Gamma-ray Burst Monitor (GBM; Meegan et al. 2009) onboard flight software triggered on, classified, and localized a GRB. A Gamma-ray Coordinates Network (GCN) Notice(Fermi-GBM 2017) was issued at 12:41:20 UTC announcing the detection of the GRB, which was later designated GRB 170817A (von Kienlin et al. 2017). Approximately 6 minutes later, a gravitational-wave candidate (later designated GW170817) was registered in low latency (Cannon et al. 2012;Messick et al. 2017) based on a single-detector analysis of the Laser Interferometer Gravitationalwave Observatory (LIGO) Hanford data. The signal was consistent with a BNS coalescence with merger time, t c , 12:41:04 UTC, less than 2 s before GRB 170817A. A GCN Notice was issued at 13:08:16 UTC. Single-detector gravitational-wave triggers had never been disseminated before in low latency. Given the temporal coincidence with the Fermi-GBM GRB, however, a 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 BNS coalescence was associated with the time of the GRB 959 . An extensive observing campaign was launched across the electromagnetic spectrum in response to the Fermi-GBM and LIGO-Virgo detections, and especially the subsequent well-constrained, three-dimensional LIGO-Virgo localization. A bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) was discovered in NGC 4993 (at 40 Mpc ) by the 1M2H team(August 18 01:05 UTC; Coulter et al. 2017a) less than 11 hr after the merger.

Gravitational-wave Observation
GW170817 was first detected online (Cannon et al. 2012;Messick et al. 2017) as a single-detector trigger and disseminated through a GCN Notice at 13:08:16 UTC and a GCN Circular at 13:21:42 UTC (LIGO Scientific Collaboration & Virgo Collaboration et al. 2017a). A rapid re-analysis (Nitz et al. 2017a(Nitz et al. , 2017b of data from LIGO-Hanford, LIGO-Livingston, and Virgo confirmed a highly significant, coincident signal. These data were then combined to produce the first three-instrument skymap (Singer & Price 2016;  ) . The offline gravitational-wave analysis of the LIGO-Hanford and LIGO-Livingston data identified GW170817 with a falsealarm rate of less than one per 8.0×10 4 (Abbott et al. 2017c). This analysis uses post-Newtonian waveform models (Blanchet et al. 1995(Blanchet et al. , 2004(Blanchet et al. , 2006Bohé et al. 2013) to construct a matchedfilter search (Sathyaprakash & Dhurandhar 1991;Cutler et al. 1993;Allen et al. 2012) for gravitational waves from the coalescence of compact-object binary systems in the (detector frame) total mass range M 2 500  -. GW170817 lasted for ∼100 s in the detector sensitivity band. The signal reached Virgo first, then LIGO-Livingston 22 ms later, and after 3 ms more, it arrived at LIGO-Hanford. GW170817 was detected with a combined signal-to-noise ratio across the three-instrument network of 32.4. For comparison, GW150914 was observed with a signal-to-noise ratio of 24 (Abbott et al. 2016c).
The properties of the source that generated GW170817 (see Abbott et al. 2017c for full details; here, we report parameter ranges that span the 90% credible interval) were derived by employing a coherent Bayesian analysis (Veitch et al. 2015;Abbott et al. 2016b) of the three-instrument data, including marginalization over calibration uncertainties and assuming that the signal is described by waveform models of a binary system of compact objects in quasi-circular orbits (see Abbott et al. 2017c and references therein). The waveform models include the effects introduced by the objects' intrinsic rotation (spin) and tides. The source is located in a region of 28 deg 2 at a distance of 40 14 8 -+ Mpc, see Figure 1, consistent with the early estimates disseminated through GCN Circulars (LIGO Scientific Collaboration & Virgo Collaboration et al. 2017b, 2017c. The misalignment between the total angular momentum axis and the line of sight is 56 °. The . The chirp mass, 961 , is the mass parameter that, at the leading order, drives the frequency evolution of gravitational radiation in the inspiral phase. This dominates the portion of GW170817 in the instruments' sensitivity band. As a consequence, it is the best measured mass parameter, is bound to the range 0.4-1.0. These results are consistent with a binary whose components are neutron stars. White dwarfs are ruled out since the gravitational-wave signal sweeps through 200 Hz in the instruments' sensitivity band, implying an orbit of size ∼100km, which is smaller than the typical radius of a white dwarf by an order of magnitude (Shapiro & Teukolsky 1983). However, for this event gravitational-wave data alone cannot rule out objects more compact than neutron stars such as quark stars or black holes (Abbott et al. 2017c).

Prompt Gamma-Ray Burst Detection
The first announcement of GRB 170817A came from the GCN Notice (Fermi-GBM 2017) et al. 2017a). The difference between the binary merger and the 959 The trigger was recorded with LIGO-Virgo ID G298048, by which it is referred throughout the GCN Circulars. 960 Any mass parameter m det ( ) derived from the observed signal is measured in the detector frame. It is related to the mass parameter, m, in the source frame by m zm , where z is the source's redshift. Here, we always report source-frame mass parameters, assuming standard cosmology (Ade et al. 2016) and correcting for the motion of the solar Ssystem barycenter with respect to the cosmic microwave background (Fixsen 2009). From the gravitational-wave luminosity distance measurement, the redshift is determined to be z 0.008 0.003 0.002 = -+ . For full details see Abbott et al. (2016bAbbott et al. ( , 2017c. 961 The binary's chirp mass is defined as m m m m Figure 2. Timeline of the discovery of GW170817, GRB 170817A, SSS17a/AT 2017gfo, and the follow-up observations are shown by messenger and wavelength relative to the time t c of the gravitational-wave event. Two types of information are shown for each band/messenger. First, the shaded dashes represent the times when information was reported in a GCN Circular. The names of the relevant instruments, facilities, or observing teams are collected at the beginning of the row. Second, representative observations (see Table 1) in each band are shown as solid circles with their areas approximately scaled by brightness; the solid lines indicate when the source was detectable by at least one telescope. Magnification insets give a picture of the first detections in the gravitational-wave, gamma-ray, optical, X-ray, and radio bands. They are respectively illustrated by the combined spectrogram of the signals received by LIGO-Hanford and LIGO-Livingston (see Section 2.1), the Fermi-GBM and INTEGRAL/SPI-ACS lightcurves matched in time resolution and phase (see Section 2.2), 1 5×1 5 postage stamps extracted from the initial six observations of SSS17a/AT 2017gfo and four early spectra taken with the SALT (at t c +1.  (Abbott et al. 2017g). Exploiting the difference in the arrival time of the gamma-ray signals at Fermi-GBM and INTEGRAL SPI-ACS (Svinkin et al. 2017c) provides additional significant constraints on the gamma-ray localization area (see Figure 1). The IPN localization capability will be especially important in the case of future gravitational-wave events that might be less well-localized by LIGO-Virgo.
Standard follow-up analyses (Goldstein et al. 2012;Paciesas et al. 2012;Gruber et al. 2014) of the Fermi-GBM trigger determined the burst duration to be T 2.0 0.5 90 =  s, where T 90 is defined as the interval over which 90% of the burst fluence is accumulated in the energy range of 50-300keV. From the Fermi-GBM T 90 measurement, GRB 170817A was classified as an sGRB with 3:1 odds over being a long GRB. The classification of GRB 170817A as an sGRB is further supported by incorporating the hardness ratio of the burst and comparing it to the Fermi-GBM catalog (Goldstein et al. 2017a). The SPI-ACS duration for GRB 170817A of 100 ms is consistent with an sGRB classification within the instrument's historic sample (Savchenko et al. 2012).
Detailed analysis of the Fermi-GBM data for GRB 170817A revealed two components to the burst: a main pulse encompassing the GRB trigger time from T0 0.320 s to T0 0.256 s + followed by a weak tail starting at T0 0.832 s + and extending to T0 1.984 s + . The spectrum of the main pulse of GRB 170817A is best fit with a Comptonized function (a power law with an exponential cutoff) with a power-law photon index of −0.62±0.40, peak energy E 185 62 peak =  keV, and time-averaged flux of 3.1 0.7 10 7 ´-( ) erg cm −2 s −1 . The weak tail that follows the main pulse, when analyzed independently, has a localization consistent with both the main pulse and the gravitationalwave position. The weak tail, at 34% the fluence of the main pulse, extends the T 90 beyond the main pulse and has a softer, blackbody spectrum with kT 10.3 1.5 =  keV (Goldstein et al. 2017a).
Using the Fermi-GBM spectral parameters of the main peak and T 90 interval, the integrated fluence measured by INTEGRAL SPI-ACS is 1.4 0.4 10 7 ´-( ) erg cm −2 (75-2000 keV), compatible with the Fermi-GBM spectrum. Because SPI-ACS is most sensitive above 100keV, it detects only the highest-energy part of the main peak near the start of the longer Fermi-GBM signal (Abbott et al. 2017f).

Discovery of the Optical Counterpart and Host Galaxy
The announcements of the Fermi-GBM and LIGO-Virgo detections, and especially the well-constrained, three-dimensional LIGO-Virgo localization, triggered a broadband observing campaign in search of electromagnetic counterparts. A large number of teams across the world were mobilized using ground-and space-based telescopes that could observe the region identified by the gravitational-wave detection. GW170817 was localized to the southern sky, setting in the early evening for the northern hemisphere telescopes, thus making it inaccessible to the majority of them. The LIGO-Virgo localization region (LIGO Scientific Collaboration & Virgo Collaboration et al. 2017b, 2017c became observable to telescopes in Chile about 10 hr after the merger with an altitude above the horizon of about 45°.
The One-Meter, Two-Hemisphere (1M2H) team was the first to discover and announce(August 18 01:05 UTC; Coulter et al. 2017a) a bright optical transient in an i-band image acquired on August 17 at 23:33 UTC (t c +10.87 hr) with the 1 m Swope telescope at Las Campanas Observatory in Chile. The team used an observing strategy (Gehrels et al. 2016) that targeted known galaxies (from White et al. 2011b) in the three-dimensional LIGO-Virgo localization taking into account the galaxy stellar mass and star formation rate   (Yang et al. 2017a) in an image taken on August 17 23:50 UTC while carrying out high-priority observations of 51 galaxies (20 within the LIGO-Virgo localization and 31 within the wider Fermi-GBM localization region; Valenti et al. 2017, accepted). A confirmation image was taken on August 18 00:41 UTC after the observing program had cycled through all of the high-priority targets and found no other transients. The updated magnitudes for these two epochs are r=17.18±0.03 and 17.28±0.04 mag, respectively. SSS17a/AT 2017gfo was also observed by the VISTA in the second of two 1.5 deg 2 fields targeted. The fields were chosen to be within the high-likelihood localization region of GW170817 and to contain a high density of potential host galaxies (32 of the 54 entries in the list of Cook et al. 2017a (Lipunov et al. 2010), covering the sky location of GW170817, recorded an image that included NGC 4993. The autodetection software identified MASTER OT J130948.10-232253.3, the bright optical transient with the unfiltered magnitude W 17.5 0.2 =  mag, as part of an automated search performed by the MASTER Global Robotic Net (Lipunov et al. 2017a(Lipunov et al. , 2017d Las Cumbres Observatory (LCO; Brown et al. 2013) surveys started their observations of individual galaxies with their global network of 1 and 2 m telescopes upon receipt of the initial Fermi-GBM localization. Approximately five hours later, when the LIGO-Virgo localization map was issued, the observations were switched to a prioritized list of galaxies (from Dalya et al. 2016) ranked by distance and luminosity (Arcavi et al. 2017, in preparation). In a 300 s w-band exposure beginning on August 18 00:15 UTC, a new transient, corresponding to AT 2017gfo/SSS17a/DLT17ck, was detected near NGC 4993 (Arcavi et al. 2017a These early photometric measurements, from the optical to near-infrared, gave the first broadband spectral energy distribution of AT 2017gfo/SSS17a/DL17ck. They do not distinguish the transient from a young supernova, but they serve as reference values for subsequent observations that reveal the nature of the optical counterpart as described in Section 3.1. Images from the six earliest observations are shown in the inset of Figure 2.

Broadband Follow-up
While some of the first observations aimed to tile the error region of the GW170817 and GRB 170817A localization areas, including the use of galaxy targeting (White et al. 2011a;Dalya et al. 2016;D. Cook & M. Kasliwal 2017, in preparation;S. R. Kulkarni et al. 2017, in preparation), most groups focused their effort on the optical transient reported by Coulter et al. (2017) to define its nature and to rule out that it was a chance coincidence of an unrelated transient. The multiwavelength evolution within the first 12-24hr, and the subsequent discoveries of the X-ray and radio counterparts, proved key to scientific interpretation. This section summarizes the plethora of key observations that occurred in different wavebands, as well as searches for neutrino counterparts.

Ultraviolet, Optical, and Infrared
The quick discovery in the first few hours of Chilean darkness, and the possibility of fast evolution, prompted the need for the ultraviolet-optical-infrared follow-up community to have access to both space-based and longitudinally separated ground-based facilities. Over the next two weeks, a network of ground-based telescopes, from 40 cm to 10 m, and space-based observatories spanning the ultraviolet (UV), optical (O), and near-infrared (IR) wavelengths followed up GW170817. These observations revealed an exceptional electromagnetic counterpart through careful monitoring of its spectral energy distribution. Here, we first consider photometric and then spectroscopic observations of the source.
Regarding photometric observations, at t c +11.6 hr, the Magellan-Clay and Magellan-Baade telescopes (Drout et al. 2017a;Simon et al. 2017) initiated follow-up observations of the transient discovered by the Swope Supernova Survey from the optical (g band) to NIR (Ks band). At t c +12.7 hr and t c +12.8 hr, the Rapid Eye Mount (REM)/ROS2 (Melandri et al. 2017b) detected the optical transient and the Gemini-South FLAMINGO2 instrument first detected near-infrared Ksband emission constraining the early optical to infrared color Singer et al. 2017a), respectively. At t c +15.3 hr, the Swift satellite (Gehrels 2004) detected bright, ultraviolet emission, further constraining the effective temperature (Evans et al. 2017a(Evans et al. , 2017b. The ultraviolet evolution continued to be monitored with the Swift satellite (Evans et al. 2017b) and the Hubble Space Telescope (HST; Adams et al. 2017;Cowperthwaite et al. 2017b;Kasliwal et al. 2017).
Over the course of the next two days, an extensive photometric campaign showed a rapid dimming of this initial UV-blue emission and an unusual brightening of the nearinfrared emission. After roughly a week, the redder optical and near-infrared bands began to fade as well. Ground-and spacebased facilities participating in this photometric monitoring effort include (in alphabetic order): CTIO1.3 m, DECam Nicholl et al. 2017aNicholl et al. , 2017d, IRSF, the Gemini-South FLAMINGO2 Chornock et al. 2017b;Troja et al. 2017bTroja et al. , 2017d One of the key properties of the transient that alerted the worldwide community to its unusual nature was the rapid luminosity decline. In bluer optical bands (i.e., in the g band), the transient showed a fast decay between daily photometric measurements Melandri et al. 2017c). Pan-STARRS (Chambers et al. 2017c) reported photometric measurements in the optical/infrared izy bands with the same cadence, showing fading by 0.6 mag per day, with reliable photometry from difference imaging using already existing sky images (Chambers et al. 2016;Cowperthwaite et al. 2017b). Observations taken every 8 hr by LCO showed an initial rise in the w band, followed by rapid fading in all optical bands (more than 1 mag per day in the blue) and reddening with time (Arcavi et al. 2017e). Accurate measurements from Subaru (Tominaga et al. 2017), LSGT/SNUCAM-II and KMTNet (Im et al. 2017c), ESO-VLT/FORS2 (D'Avanzo et al. 2017), and DECam Nicholl et al. 2017b) indicated a similar rate of fading. On the contrary, the near-infrared monitoring reports by GROND and Gemini-South showed that the source faded more slowly in the infrared (Chornock et al. 2017b;Wiseman et al. 2017) and even showed a late-time plateau in the Ks band ). This evolution was recognized by the community as quite unprecedented for transients in the nearby (within 100 Mpc) universe (e.g., Siebert et al. 2017). Table 1 reports a summary of the imaging observations, which include coverage of the entire gravitational-wave sky localization and follow-up of SSS17a/AT 2017gfo. Figure 2 shows these observations in graphical form.
Concerning spectroscopic observations, immediately after discovery of SSS17a/AT 2017gfo on the Swope 1 m telescope, the same team obtained the first spectroscopic observations of the optical transient with the LDSS-3 spectrograph on the 6.5 m Magellan-Clay telescope and the MagE spectrograph on the 6.5 m Magellan-Baade telescope at Las Campanas Observatory. The spectra, just 30 minutes after the first image, showed a blue and featureless continuum between 4000 and 10000 Å, consistent with a power law (Drout et al. 2017a;Shappee et al. 2017). The lack of features and blue continuum during the first few hours implied an unusual, but not unprecedented transient since such characteristics are common in cataclysmic-variable stars and young core-collapse supernovae (see, e.g., Li et al. 2011aLi et al. , 2011b. The next 24 hr of observation were critical in decreasing the likelihood of a chance coincidence between SSS17a/ AT 2017gfo, GW170817, and GRB 170817A. The SALT-RSS spectrograph in South Africa (Buckley et al. 2017;McCully et al. 2017b;Shara et al. 2017), ePESSTO with the EFOSC2 instrument in spectroscopic mode at the ESO New Technology Telescope (NTT, in La Silla, Chile; Lyman et al. 2017), the X-shooter spectrograph on the ESO Very Large Telescope (Pian et al. 2017b) in Paranal, and the Goodman Spectrograph on the 4 m SOAR telescope  obtained additional spectra. These groups reported a rapid fall off in the blue spectrum without any individual features identifiable with line absorption common in supernova-like transients (see, e.g., Lyman et al. 2017). This ruled out a young supernova of any type in NGC 4993, showing an exceptionally fast spectral evolution Nicholl et al. 2017d). Figure 2 shows some representative early spectra (SALT spectrum is from Buckley et al. 2017;McCully et al. 2017b; ESO spectra from Smartt et al. 2017; SOAR spectrum from Nicholl et al. 2017d). These show rapid cooling, and the lack of commonly observed ions from elements abundant in supernova ejecta, indicating this object was unprecedented in its optical and near-infrared emission. Combined with the rapid fading, this was broadly indicative of a possible kilonova (e.g.,  Kasen et al. 2017;Kasliwal et al. 2017;Nicholl et al. 2017d;Smartt et al. 2017). This was confirmed by spectra taken at later times, such as with the Gemini Multi-Object Spectrograph (GMOS; Kasliwal et al. 2017;McCully et al. 2017b;Troja et al. 2017aTroja et al. , 2017b, the LDSS-3 spectrograph on the 6.5 m Magellan-Clay telescope at Las Campanas Observatory Shappee et al. 2017), the LCO FLOYDS spectrograph at Faulkes Telescope South , and the AAOmega spectrograph on the 3.9 m Anglo-Australian Telescope ), which did not show any significant emission or absorption lines over the red featureless continuum. The optical and near-infrared spectra over these few days provided convincing arguments that this transient was unlike any other discovered in extensive optical wide-field surveys over the past decade (see, e.g., Siebert et al. 2017).
The evolution of the spectral energy distribution, rapid fading, and emergence of broad spectral features indicated that the source had physical properties similar to models of kilonovae (e.g., Metzger et al. 2010;Kasen et al. 2013;Tanaka & Hotokezaka 2013;Grossman et al. 2014;Metzger & Fernández 2014;Barnes et al. 2016;Tanaka 2016;Kasen et al. 2017;Metzger 2017). These show a very rapid shift of the spectral energy distribution from the optical to the nearinfrared. The FLAMINGOS2 near-infrared spectrograph at Gemini-South (Chornock et al. 2017c;Kasliwal et al. 2017) shows the emergence of very broad features in qualitative agreement with kilonova models. The ESO-VLT/X-shooter spectra, which simultaneously cover the wavelength range 3200-24800 Å, were taken over 2 weeks with a close to daily sampling Smartt et al. 2017) and revealed signatures of the radioactive decay of r-process nucleosynthesis elements . Three epochs of infrared grism spectroscopy with the HST ( The optical follow-up campaign also includes linear polarimetry measurements of SSS17a/AT 2017gfo by ESO-VLT/FORS2, showing no evidence of an asymmetric geometry of the emitting region and lanthanide-rich late kilonova emission . In addition, the study of the galaxy with the MUSE Integral Field Spectrograph on the ESO-VLT ) provides simultaneous spectra of the counterpart and the host galaxy, which show broad absorption features in the transient spectrum, combined with emission lines from the spiral arms of the host galaxy . Table 2 reports the spectroscopic observations that have led to the conclusion that the source broadly matches kilonovae theoretical predictions.

Gamma-Rays
The fleet of ground-and space-based gamma-ray observatories provided broad temporal and spectral coverage of the source location. Observations spanned 10 orders of magnitude in energy and covered the position of SSS17a/ AT 2017gfo from a few hundred seconds before the GRB 170817A trigger time (T0) to days afterward. Table 3 lists, in chronological order, the results reporting observation 963 HST Program GO 14804 Levan, GO 14771 Tanvir, and GO 14850 Troja.          Note. This is a subset of all the observations made in order to give a sense of the substantial coverage of this event.     Earth Occultation technique (Wilson-Hodge et al. 2012), Fermi-GBM placed limits on persistent emission for the 48 hr period centered at the Fermi-GBM trigger time over the 90% credible region of the GW170817 localization. Using the offline targeted search for transient signals (Blackburn et al. 2015), Fermi-GBM also set constraining upper limits on precursor and extended emission associated with GRB 170817A (Goldstein et al. 2017b). INTEGRAL (Winkler et al. 2003) continued uninterrupted observations after GRB 170817A for 10 hr. Using the PiCSIT (Labanti et al. 2003) and SPI-ACS detectors, the presence of a steady source 10 times weaker than the prompt emission was excluded (Savchenko et al. 2017). The High Energy telescope on board Insight-HXMT monitored the entire GW170817 skymap from T0 650 s to T0 450 s + but, due to the weak and soft nature of GRB 170817A, did not detect any significant excess at T0 (Liao et al. 2017). Upper limits from 0.2-5 MeV for GRB 170817A and other emission episodes are reported in .
The Calorimetric Electron Telescope (CALET) Gamma-ray Burst Monitor (CGBM) found no significant excess around T0. Upper limits may be affected due to the location of SSS17a/ AT 2017gfo being covered by the large structure of the International Space Station at the time of GRB 170817A (Nakahira et al. 2017). AstroSat CZTI (Singh et al. 2014;Bhalerao et al. 2017) reported upper limits for the 100 s interval centered on T0 (Balasubramanian et al. 2017); the position of SSS17a/AT 2017gfo was occulted by the Earth, however, at the time of the trigger.
For the AstroRivelatore Gamma a Immagini Leggero (AGILE) satellite (Tavani et al. 2009) the first exposure of the GW170817 localization region by the Gamma Ray Imaging Detector (GRID), which was occulted by the Earth at the time of GRB 170817A, started at T0 935 s + . The GRID observed the field before and after T0, typically with 150 s exposures. No gamma-ray source was detected above 3s in the energy range 30 MeV-30 GeV(V. Verrecchia et al. 2017, in preparation).
At the time of the trigger, Fermi was entering the South Atlantic Anomaly (SAA) and the Large Area Telescope (LAT) was not collecting science data (Fermi-GBM uses different SAA boundaries and was still observing). Fermi-LAT resumed data taking at roughly T0 1153 s + , when 100% of the lowlatency GW170817 skymap ( INTEGRAL (3 keV-8 MeV) carried out follow-up observations of the LIGO-Virgo localization region, centered on the optical counterpart, starting 24 hr after the event and spanning 4.7 days. Hard X-ray emission is mostly constrained by IBIS (Ubertini et al. 2003), while above 500 keV SPI (Vedrenne et al. 2003) is more sensitive. Besides the steady flux limits reported in Table 3, these observations exclude delayed bursting activity at the level of giant magnetar flares. No gamma-ray lines from a kilonova or e +pair plasma annihilation were detected (see Savchenko et al. 2017).

Discovery of the X-Ray Counterpart
While the UV, optical, and IR observations mapped the emission from the sub-relativistic ejecta, X-ray observations probed a different physical regime. X-ray observations of GRB afterglows are important to constrain the geometry of the outflow, its energy output, and the orientation of the system with respect to the observers' line of sight.
The earliest limits at X-ray wavelengths were provided by the Gas Slit Camera (GSC) of the Monitor of All-Sky X-ray Image (MAXI; Matsuoka et al. 2009). Due to an unfavorable sky position, the location of GW170817 was not observed by MAXI until August 17 17:21 UTC (T0 0.19 + days). No X-ray emission was detected at this time to a limiting flux of 8.6 10 9 erg cm −2 s −1 (2-10 keV; Sugita et al. 2017;S. Sugita 2017, in preparation). MAXI obtained three more scans over the location with no detections before the more sensitive pointed observations began.
The first pointed X-ray observations of GW170817 were obtained by the X-Ray Telescope    Evans et al. 2017aEvans et al. , 2017b. Swift continued to monitor the field, and after stacking several epochs of observations, a weak X-ray source was detected near the location of GW170817 at a flux of 2.6 10 14 erg cm −2 s −1 (Evans et al. 2017c). INTEGRAL (see Section 3.2) performed pointed follow-up observations from one to about six days after the trigger. The X-ray monitor JEM-X (Lund et al. 2003) constrained the average X-ray luminosity at the location of the optical transient to be 2 10 11 <´erg cm −2 s −1 (3-10.0 keV) and 7 10 12 <´erg cm −2 s −1 (10-25 keV; Savchenko et al. 2017).
Chandra obtained a series of observations of GW170817 beginning at August 19 17:10 UTC (T0 2.2 + days) and continuing until the emission from NGC 4993 became unobservable because of SSS17a/AT 2017gfo's proximity to the Sun (Fong et al. 2017;Haggard et al. 2017b;Margutti et al. 2017a;Troja et al. 2017c. Two days post-trigger, Margutti et al. (2017a) reported an X-ray non-detection for SSS17a/AT 2017gfo in a ;25 ks Chandra exposure, 964 along with the detection of an extended X-ray source whose position was consistent with the host NGC 4993 (Margutti et al. 2017b). Refined astrometry from subsequent Swift observations confirmed that the previously reported candidate was indeed associated with the host nucleus (Evans et al. 2017a(Evans et al. , 2017b. Nine days post-trigger, Troja et al. (2017c) reported the discovery of the X-ray counterpart with Chandra. In a 50 ks exposure observation, they detected significant X-ray emission at the same position of the optical/IR counterpart ; top right panel in Figure 2) (Haggard et al. , 2017b. 10 Neither Swift nor Chandra can currently observe GW170817 because it is too close to the Sun ( 47 <  for Swift, 46 <  for Chandra). Hence, until early 2017 December, NuSTAR is the only sensitive X-ray observatory that can continue to observe the location of GW170817.
All X-ray observations of GW170817 are summarized in Table 4.

Discovery of the Radio Counterpart
Radio emission traces fast-moving ejecta from a neutron star coalescence, providing information on the energetics of the explosion, the geometry of the ejecta, as well as the environment of the merger. The spectral and temporal evolution of such emission, coupled with X-ray observations, are likely to constrain several proposed models (see, e.g., Nakar & Piran 2011;Piran et al. 2013;Hotokezaka & Piran 2015;Hotokezaka et al. 2016;Gottlieb et al. 2017).
Prior to detection of SSS17a/AT 2017gfo, a blind radio survey of cataloged galaxies in the gravitational-wave localization volume commenced with the Australia Telescope Compact Array (ATCA; Wilson et al. 2011), and observed the merger events' location on 2017 August 18 at 01:46 UTC ). In addition, the Long Wavelength Array 1 (LWA1; Ellingson et al. 2013) followed up the gravitationalwave localization with observations at t c + 6.5 hr, then on 2017 August 23 and 30 (Callister et al. 2017a;Callister et al. 2017b) using four beams (one centered on NGC 4993, one off-center, and two off NGC 4993). These observations set 3σ upper limits for the appearance of a radio source in the beam centered on NGC 4993, about 8 hours after the GW event, as ∼200 Jy at 25 MHz and ∼100 Jy at 45 MHz.
The first reported radio observations of the optical transient SSS17a/AT 2017gfo's location occurred on August 18 at 02:09:00 UTC (T0+13.5 hr) with the Karl G.Jansky Very Large Array (VLA) by Alexander et al. (2017d). 968 Initially attributed to the optical transient, this radio source was later established to be an AGN in the nucleus of the host galaxy, NGC 4993 (Alexander et al. 2017e, 2017c. Subsequent observations with several radio facilities spanning a wide range of radio and millimeter frequencies continued to detect the AGN, but did not reveal radio emission at the position of the transient (Alexander et al. 2017f;Bannister et al. 2017b;Corsi et al. 2017aCorsi et al. , 2017bCorsi et al. , 2017cDe et al. 2017aDe et al. , 2017bKaplan et al. 2017a;Lynch et al. 2017aLynch et al. , 2017bLynch et al. , 2017cMooley et al. 2017a;Resmi et al. 2017).
The first radio counterpart detection consistent with the HST position (refined by Gaia astrometry) of SSS17a/AT 2017gfo

Neutrinos
The detection of GW170817 was rapidly followed up by the IceCube (Aartsen et al. 2017) and ANTARES (Ageron et al. 2011) neutrino observatories and the Pierre Auger Observatory (Aab et al. 2015a) to search for coincident, high-energy (GeV-EeV) neutrinos emitted in the relativistic outflow produced by the BNS merger. The results from these observations, described briefly below, can be used to constrain the properties of relativistic outflows driven by the merger (A. Albert et al. 2017, in preparation).
In a search for muon-neutrino track candidates (Aartsen et al. 2016), and contained neutrino events of any flavor (Aartsen et al. 2015), IceCube identified no neutrinos that were directionally coincident with the final localization of GW170817 at 90% credible level, within ±500 s of the merger (Bartos et al. 2017a(Bartos et al. , 2017b. Additionally, no MeV supernova neutrino burst signal was detected coincident with the merger. Following the identification via electromagnetic observations of the host galaxy of the event, IceCube also carried out an extended search in the direction of NGC 4993 for neutrinos within the 14 day period following the merger, but found no significant neutrino emission (A. Albert et al. 2017, in preparation).
A neutrino search for upgoing high-energy muon neutrinos was carried out using the online ANTARES data stream (Ageron et al. 2017a). No upgoing neutrino candidates were found over a t 500 s c  time window. The final localization of GW170817 (LIGO Scientific Collaboration & Virgo Collaboration et al. 2017c) was above the ANTARES horizon at the time of the GW event. A search for downgoing muon neutrinos was thus performed, and no neutrinos were found over t c 500 s (Ageron et al. 2017b). A search for neutrinos originating from below the ANTARES horizon, over an extended period of 14 days after the merger, was also performed, without yielding significant detection (A. Albert et al. 2017, in preparation).
The Pierre Auger Observatory carried out a search for ultrahigh-energy (UHE) neutrinos above 10 17 eV using its Surface Detector (Aab et al. 2015a). UHE neutrino-induced extensive air showers produced either by interactions of downward-going neutrinos in the atmosphere or by decays of tau leptons originating from tau neutrino interactions in the Earth's crust can be efficiently identified above the background of the more numerous ultra-high-energy cosmic rays (Aab et al. 2015b). Remarkably, the position of the transient in NGC 4993 was just between 0°. 3 and 3°.2 below the horizon during t 500 s c  . This region corresponds to the most efficient geometry for Earthskimming tau neutrino detection at 10 18 eV energies. No neutrino candidates were found in t

Conclusion
For the first time, gravitational and electromagnetic waves from a single source have been observed. The gravitationalwave observation of a binary neutron star merger is the first of its kind. The electromagnetic observations further support the interpretation of the nature of the binary, and comprise three components at different wavelengths: (i) a prompt sGRB that demonstrates that BNS mergers are the progenitor of at least a fraction of such bursts; (ii) an ultraviolet, optical, and infrared transient (kilonova), which allows for the identification of the host galaxy and is associated with the aftermath of the BNS merger; and (iii) delayed X-ray and radio counterparts that provide information on the environment of the binary. These observations, described in detail in the companion articles cited above, offer a comprehensive, sequential description of the physical processes related to the merger of a binary neutron star. Table 6 collects all of the Gamma-ray Coordinates Network (GCN) notices and circulars related to GW170817 through 2017 October 1 UTC. The results of this campaign demonstrate the importance of collaborative gravitationalwave, electromagnetic, and neutrino observations and mark a new era in multi-messenger, time-domain astronomy.
(1M2H) We thank J.McIver for alerting us to the LVC circular. We thank J.Mulchaey (Carnegie Observatories director), L.Infante (Las Campanas Observatory director), and the entire Las Campanas staff for their extreme dedication, professionalism, and excitement, all of which were critical in the discovery of the first gravitational-wave optical counterpart and its host galaxy as well as the observations used in this study. We thank I.Thompson and the Carnegie Observatory Time Allocation Committee for approving the Swope Supernova Survey and scheduling our program. We thank the University of Copenhagen, DARK Cosmology Centre, and the Niels Bohr International Academy for hosting D. (AGILE) The AGILE Team thanks the ASI management, the technical staff at the ASI Malindi ground station, the technical support team at the ASI Space Science Data Center, and the Fucino AGILE Mission Operation Center. AGILE is an ASI space mission developed with programmatic support by INAF and INFN. We acknowledge partial support through the ASI grant No. I/028/12/2. We also thank INAF, Italian Institute of Astrophysics, and ASI, Italian Space Agency. (CZTI/AstroSat) CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.
(EuroVLBI) The European VLBI Network is a joint facility of independent European, African, Asian, and North American radio astronomy institutes. Scientific results from data presented in this publication are derived from the following EVN project code: RP029. e-MERLIN is a National Facility operated by the University of Manchester at Jodrell Bank Observatory on behalf of STFC. The collaboration between LIGO/Virgo and EVN/e-MERLIN is part of a project that has received funding from the European Unions Horizon 2020 research and innovation programme under grant agreement No. 653477.
(  A.A.M. is funded by the Large Synoptic Survey Telescope Corporation in support of the Data Science Fellowship Program. P.C.Y., C.C.N., and W.H.I. thank the support from grants MOST104-2923-M-008-004-MY5 and MOST106-2112-M-008-007. A.C. acknowledges support from the National Science Foundation CAREER award 1455090, "CAREER: Radio and gravitational-wave emission from the largest explosions since the Big Bang." T.P. acknowledges the support of Advanced ERC grant TReX. B.E.C. thanks SMARTS 1.3 m Queue Manager Bryndis Cruz for prompt scheduling of the SMARTS observations. Basic research in radio astronomy at the Naval Research Laboratory (NRL) is funded by 6.1 Base funding. Construction and installation of VLITE was supported by NRL Sustainment Restoration and Maintenance funding. K.P.M.ʼs research is supported by the Oxford Centre for Astrophysical Surveys, which is funded through the Hintze Family Charitable Foundation. J.S. and A. G. are grateful for support from the Knut and Alice Wallenberg Foundation. GREAT is funded by the Swedish Research Interdisciplinary Programme of the CNRS, the U.K. Science and Technology Facilities Council (STFC), the IPNP of the Charles University, the Czech Science Foundation, the Polish National Science Centre, the South African Department of Science and Technology and National Research Foundation, the University of Namibia, the National Commission on Research, Science and Technology of Namibia (NCRST), the Innsbruck University, the Austrian Science Fund (FWF), and the Austrian Federal Ministry for Science, Research and Economy, the University of Adelaide and the Australian Research Council, the Japan Society for the Promotion of Science and by the University of Amsterdam. We appreciate the excellent work of the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment. This work benefited from services provided by the H.E.S.S.Virtual Organisation, supported by the national resource providers of the EGI Federation.   (LIGO and Virgo) The authors gratefully acknowledge the support of the United States National Science Foundation (NSF) for the construction and operation of the LIGO Laboratory and Advanced LIGO as well as the Science and Technology Facilities Council (STFC) of the United Kingdom, the Max-Planck-Society (MPS), and the State of Niedersachsen/Germany for support of the construction of Advanced LIGO and construction and operation of the GEO600 detector. Additional support for advanced LIGO was provided by the Australian Research Council. The authors gratefully acknowledge the Italian Istituto Nazionale di Fisica Nucleare (INFN), the French Centre National de la Recherche Scientifique (CNRS) and the Foundation for Fundamental Research on Matter supported by the Netherlands Organisation for Scientific Research, for the construction and operation of the Virgo detector and the creation and support of the EGO consortium. The authors also gratefully acknowledge research support from these agencies as well as by the Council of Scientific  Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The European VLBI Network is a joint facility of independent European, African, Asian, and North American radio astronomy institutes. Scientific results from data presented in this publication are derived from the following EVN project code: RP029. e-MERLIN is a National Facility operated by the University of Manchester at Jodrell Bank Observatory on behalf of STFC. The collaboration between LIGO/Virgo and EVN/e-MERLIN is part of a project that has received funding from the European Unionʼs Horizon 2020 research and innovation programme under grant agreement No. 653477. We thank Britt Griswold (NASA/GSFC) for graphic arts. P.G.J. acknowledges ERC-Consolidator grant No.647208. We thank the GMRT staff for prompt scheduling of these observations. The GMRT is run by the National Center for Radio Astrophysics of the Tata  (LOFAR) LOFAR, the Low-Frequency Array designed and constructed by ASTRON, has facilities in several countries that are owned by various parties (each with their own funding sources) and that are collectively operated by the International LOFAR Telescope (ILT) foundation under a joint scientific policy. P.G.J. acknowledges support from ERC grant number 647208. R.F. was partially funded by ERC Advanced Investigator Grant 267607 "4 PI SKY." (Nordic Optical Telescope) J.P.U.F. acknowledges the Carlsberg foundation for funding for the NTE project. D.X. acknowledges the support by the One-Hundred-Talent Program of the Chinese Academy of Sciences (CAS) and by the Strategic Priority Research Program "Multi-wavelength Gravitational Wave Universe" of the CAS (No. XDB23000000). Based on observations made with the Nordic Optical Telescope (program 55-013), operated by the Nordic Optical Telescope Scientific Association.
(OzGrav) Part of this research was funded by the Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav), CE170100004 and the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAAS-TRO), CE110001020. J.C. acknowledges the Australian Research Council Future Fellowship grant FT130101219. Research support to I.A. is provided by the Australian Astronomical Observatory (AAO). A.T.D. acknowledges the support of an Australian Research Council Future Fellowship (FT150100415). Based in part on data acquired through the Australian Astronomical Observatory. We acknowledge the traditional owners of the land on which the AAT stands, the Gamilaraay people, and pay our respects to elders past and present. The Etelman/VIRT team acknowledge NASA grant NNX13AD28A.
(Pan-STARRS) The Pan-STARRS1 observations were supported in part by NASA grant No. NNX14AM74G issued through the SSO Near Earth Object Observations Program and the Queenʼs University Belfast. The Pan-STARRS1 Surveys were made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queenʼs University Belfast, the Harvard-Smithsonian Center for Astrophysics, the LCO Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, and the National Aeronautics and Space Administration under grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), and the Los Alamos National Laboratory. The Pan-STARRS1 Surveys are archived at the Space Telescope Science Institute (STScI) and can be accessed through MAST, the Mikulski Archive for Space Telescopes. Additional support for the Pan-STARRS1 public science archive is provided by the Gordon and Betty Moore Foundation. (SKA) R.F. was partially funded by ERC Advanced Investigator Grant 267607 "4 PI SKY." (Swift) Funding for the Swift mission in the UK is provided by the UK Space Agency. The Swift team at the MOC at Penn State acknowledges support from NASA contract NAS5-00136. The Italian Swift team acknowledge support from ASI-INAF grant I/004/11/3.
(TOROS) We thank support from the USA Air Force Office of International Scientific Research (AFOSR/IO), the Dirección de Investigación de la Universidad de La Serena, the Consejo Nacional de Investigaciones Científicas y Técnicas of Argentina, the FAPESP, and the Observatorio Nacional-MCT of Brasil.
(TTU Group) A.C. and N.T.P. acknowledge support from the NSF CAREER Award 1455090: "CAREER: Radio and gravitational-wave emission from the largest explosions since the Big Bang." The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. (Zadko) The Zadko Telescope was made possible by a philanthropic donation by James Zadko to the University of Western Australia (UWA). Zadko Telescope operations are supported by UWA and the Australian Research Council Centre of Excellence OzGrav CE170100004. The TAROT network of telescopes is supported by the French Centre National de la Recherche Scientifique (CNRS), the Observatoire de la Côte d'Azur (OCA), and we thank the expertise and support of the Observatoire des Sciences de l'Univers, Institut Pythéas, Aix-Marseille University. The FIGARONet network is supported under the Agence Nationale de la Recherche (ANR) grant 14-CE33. The paper-writing team would like to thank Britt Griswold (NASA/GSFC) and Aaron Geller (Northwestern/NUIT/CIERA) for assistance with graphics. 24 INFN, Sezione di Pisa, I-56127 Pisa, Italy