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.

On 17 August 2017, the Advanced LIGO and Virgo detectors observed the gravitational-wave event GW170817-a strong signal from the merger of a binary neutron-star system. Less than two seconds after the merger, a γ-ray burst (GRB 170817A) was detected within a region of the sky consistent with the LIGO-Virgo-derived location of the gravitational-wave source. This sky region was subsequently observed by optical astronomy facilities, resulting in the identification of an optical transient signal within about ten arcseconds of the galaxy NGC 4993. This detection of GW170817 in both gravitational waves and electromagnetic waves represents the first 'multi-messenger' astronomical observation. Such observations enable GW170817 to be used as a 'standard siren' (meaning that the absolute distance to the source can be determined directly from the gravitational-wave measurements) to measure the Hubble constant. This quantity represents the local expansion rate of the Universe, sets the overall scale of the Universe and is of fundamental importance to cosmology. Here we report a measurement of the Hubble constant that combines the distance to the source inferred purely from the gravitational-wave signal with the recession velocity inferred from measurements of the redshift using the electromagnetic data. In contrast to previous measurements, ours does not require the use of a cosmic 'distance ladder': the gravitational-wave analysis can be used to estimate the luminosity distance out to cosmological scales directly, without the use of intermediate astronomical distance measurements. We determine the Hubble constant to be about 70 kilometres per second per megaparsec. This value is consistent with existing measurements, while being completely independent of them. Additional standard siren measurements from future gravitational-wave sources will enable the Hubble constant to be constrained to high precision.

We present UV, optical, and near-infrared (NIR) photometry of the first electromagnetic counterpart to a gravitational wave source from Advanced Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo, the binary neutron star merger GW170817. Our data set extends from the discovery of the optical counterpart at 0.47-18.5 days post-merger, and includes observations with the Dark Energy Camera (DECam), Gemini-South/ FLAMINGOS-2 (GS/F2), and the Hubble Space Telescope (HST). The spectral energy distribution (SED) inferred from this photometry at 0.6 days is well described by a blackbody model with » T 8300 K, a radius of »Ŕ 4.5 10 14 cm (corresponding to an expansion velocity of » v c 0.3 ), and a bolometric luminosity of »Ĺ 5 10 bol 41 erg s −1 . At 1.5 days we find a multi-component SED across the optical and NIR, and subsequently we observe rapid fading in the UV and blue optical bands and significant reddening of the optical/ NIR colors. Modeling the entire data set, we find that models with heating from radioactive decay of 56 Ni, or those with only a single component of opacity from r-process elements, fail to capture the rapid optical decline and red optical/NIR colors. Instead, models with two components consistent with lanthanide-poor and lanthanide-rich ejecta provide a good fit to the data; the resulting "blue" component has » . These ejecta masses are broadly consistent with the estimated r-process production rate required to explain the Milky Way r-process abundances, providing the first evidence that binary neutron star (BNS) mergers can be a dominant site of r-process enrichment.

We present the first effort to aggregate, homogenize, and uniformly model the combined ultraviolet, optical, and near-infrared dataset for the electromagnetic counterpart of the binary neutron star merger GW170817. By assembling all of the available data from 18 different papers and 46 different instruments, we are able to identify and mitigate systematic offsets between individual datasets, and to identify clear outlying measurements, with the resulting pruned and adjusted dataset offering an opportunity to expand the study of the kilonova. The unified dataset includes 647 individual flux measurements, spanning 0.45 to 29.4 days post-merger, and thus has greater constraining power for physical models than any single dataset. We test a number of semianalytical models and find that the data are well modeled with a three-component kilonova model: a "blue" lanthanide-poor component (κ = 0.5 cm 2 g −1 ) with M ej ≈ 0.020 M and v ej ≈ 0.27c; an intermediate opacity "purple" component (κ = 3 cm 2 g −1 ) with M ej ≈ 0.047 M and v ej ≈ 0.15c; and a "red" lanthanide-rich component (κ = 10 cm 2 g −1 ) with M ej ≈ 0.011 M and v ej ≈ 0.14c. We further explore the possibility of ejecta asymmetry and its impact on the estimated parameters. From the inferred parameters we draw conclusions about the physical mechanisms responsible for the various ejecta components, the properties of the neutron stars, and, combined with an up-to-date merger rate, the implications for r-process enrichment via this channel. To facilitate future studies of this keystone event we make the unified dataset and our modeling code public.

We present a near-infrared spectral sequence of the electromagnetic counterpart to the binary neutron star merger GW170817 detected by Advanced LIGO/Virgo. Our dataset comprises 7 epochs of J + H spectra taken with FLAMINGOS-2 on Gemini-South between 1.5-10.5 days after the merger. In the initial epoch, the spectrum is dominated by a smooth blue continuum due to a high-velocity, lanthanide-poor blue kilonova component. Starting the following night, all subsequent spectra instead show features that are similar to those predicted in model spectra of material with a high concentration of lanthanides, including spectral peaks near 1.07 and 1.55 µm. Our fiducial model with 0.04 M of ejecta, an ejection velocity of v = 0.1c, and a lanthanide concentration of X lan =10 −2 provides a good match to the spectra taken in the first five days, although it over-predicts the late-time fluxes. We also explore models with multiple fitting components, in each case finding that a significant abundance of lanthanide elements is necessary to match the broad spectral peaks that we observe starting at 2.5 d after the merger. These data provide direct evidence that binary neutron star mergers are significant production sites of even the heaviest r-process elements.

We report deep Chandra, HST and VLA observations of the binary neutron star event GW170817 at t < 160 d after merger. These observations show that GW170817 has been steadily brightening with time and might have now reached its peak, and constrain the emission process as non-thermal synchrotron emission where the cooling frequency ν c is above the X-ray band and the synchrotron frequency ν m is below the radio band. The very simple power-law spectrum extending for eight orders of magnitude in frequency enables the most precise measurement of the index p of the distribution of non-thermal relativistic electrons N(γ) ∝ γ −p accelerated by a shock launched by a NS-NS merger to date. We find p = 2.17 ± 0.01, which indicates that radiation from ejecta with Γ ∼ 3 − 10 dominates the observed emission. While constraining the nature of the emission process, these observations do not constrain the nature of the relativistic ejecta. We employ simulations of explosive outflows launched in NS ejecta clouds to show that the spectral and temporal evolution of the nonthermal emission from GW170817 is consistent with both emission from radially stratified quasi-spherical ejecta traveling at mildly relativistic speeds, and emission from off-axis collimated ejecta characterized by a narrow cone of ultra-relativistic material with slower wings extending to larger angles. In the latter scenario, GW170817 harbored a normal SGRB directed away from our line of sight. Observations at t ≤ 200 days are unlikely to settle the debate as in both scenarios the observed emission is effectively dominated by radiation from mildly relativistic material.

We report the discovery of rising X-ray emission from the binary neutron star (BNS) merger event GW170817. This is the first detection of X-ray emission from a gravitational-wave source. Observations acquired with the Chandra X-ray Observatory (CXO) at t ≈ 2.3 days post merger reveal no significant emission, with L x 3.2 × 10 38 erg s −1 (isotropic-equivalent). Continued monitoring revealed the presence of an X-ray source that brightened with time, reaching L x ≈ 9 × 10 39 erg s −1 at ≈ 15.1 days post merger. We interpret these findings in the context of isotropic and collimated relativistic outflows (both on-and off-axis). We find that the broad-band X-ray to radio observations are consistent with emission from a relativistic jet with kinetic energy E k ∼ 10 49−50 erg, viewed off-axis with θ obs ∼ 20 − 40• . Our models favor a circumbinary density n ∼ 10 −4 − 10 −2 cm −3 , depending on the value of the microphysical parameter B = 10 −4 − 10 −2 . A central-engine origin of the X-ray emission is unlikely. Future X-ray observations at t 100 days, when the target will be observable again with the CXO, will provide additional constraints to solve the model degeneracies and test our predictions. Our inferences on θ obs are testable with gravitational wave information on GW170817 from Advanced LIGO/Virgo on the binary inclination.

We present optical and ultraviolet spectra of the first electromagnetic counterpart to a gravitational wave (GW) source, the binary neutron star merger GW170817. Spectra were obtained nightly between 1.5 and 9.5 days post-merger, using the SOAR and Magellan telescopes; the UV spectrum was obtained with the Hubble Space Telescope at 5.5 days. Our data reveal a rapidly-fading blue component (T ≈ 5500 K at 1.5 days) that quickly reddens; spectra later than 4.5 days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at ∼ 7900 Å at t 4.5 days. The colours, rapid evolution and featureless spectrum are consistent with a "blue" kilonova from polar ejecta comprised mainly of light r-process nuclei with atomic mass number A 140. This indicates a sight-line within θ obs 45 • of the orbital axis. Comparison to models suggests ∼ 0.03 M of blue ejecta, with a velocity of ∼ 0.3c. The required lanthanide fraction is ∼ 10 −4 , but this drops to < 10 −5 in the outermost ejecta. The large velocities point to a dynamical origin, rather than a disk wind, for this blue component, suggesting that both binary constituents are neutron stars (as opposed to a binary consisting of a neutron star and a black hole). For dynamical ejecta, the high mass favors a small neutron star radius of 12 km. This mass also supports the idea that neutron star mergers are a major contributor to r-process nucleosynthesis. arXiv:1710.05456v1 [astro-ph.HE] 16 Oct 2017 2 NICHOLL ET AL.