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.

This is the second paper of a series in which we present new measurements of the observed rates of supernovae (SNe) in the local Universe, determined from the Lick Observatory Supernova Search (LOSS). In this paper, a complete SN sample is constructed, and the observed (uncorrected for host-galaxy extinction) luminosity functions (LFs) of SNe are derived. These LFs solve two issues that have plagued previous rate calculations for nearby SNe: the luminosity distribution of SNe and the host-galaxy extinction. We select a volumelimited sample of 175 SNe, collect photometry for every object and fit a family of light curves to constrain the peak magnitudes and light-curve shapes. The volume-limited LFs show that they are not well represented by a Gaussian distribution. There are notable differences in the LFs for galaxies of different Hubble types (especially for SNe Ia). We derive the observed fractions for the different subclasses in a complete SN sample, and find significant fractions of SNe II-L (10 per cent), IIb (12 per cent) and IIn (9 per cent) in the SN II sample. Furthermore, we derive the LFs and the observed fractions of different SN subclasses in a magnitude-limited survey with different observation intervals, and find that the LFs are enhanced at the highluminosity end and appear more 'standard' with smaller scatter, and that the LFs and fractions of SNe do not change significantly when the observation interval is shorter than 10 d. We also discuss the LFs in different galaxy sizes and inclinations, and for different SN subclasses. Some notable results are that there is not a strong correlation between the SN LFs and the host-galaxy size, but there might be a preference for SNe IIn to occur in small, late-type spiral galaxies. The LFs in different inclination bins do not provide strong evidence for extreme extinction in highly inclined galaxies, though the sample is still small. The LFs of different SN subclasses show significant differences. We also find that SNe Ibc and IIb come from more luminous galaxies than SNe II-P, while SNe IIn come from less luminous galaxies, suggesting a possible metallicity effect. The limitations and applications of our LFs are also discussed.

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.

This is the third paper of a series in which we present new measurements of the observed rates of supernovae (SNe) in the local Universe, determined from the Lick Observatory Supernova Search (LOSS). We have considered a sample of ∼1000 SNe and used an optimal subsample of 726 SNe (274 SNe Ia, 116 SNe Ibc and 324 SNe II) to determine our rates. We study the trend of the rates as a function of a few quantities available for our galaxy sample, such as luminosity in the B and K bands, stellar mass and morphological class. We discuss different choices (SN samples, input SN luminosity functions, inclination correction factors) and their effect on the rates and their uncertainties. A comparison between our SN rates and the published measurements shows that they are consistent with each other to within the uncertainties when the rate calculations are done in the same manner. Nevertheless, our data demonstrate that the rates cannot be adequately described by a single parameter using either galaxy Hubble types or B − K colours. A secondary parameter in galaxy 'size', expressed by luminosity or stellar mass, is needed to adequately describe the rates in the rate-size relation: the galaxies of smaller sizes have higher SN rates per unit mass or per unit luminosity. The trends of the SN rates in galaxies of different Hubble types and colours are discussed. We examine possible causes for the rate-size relation. Physically, such a relation for the core-collapse SNe is probably linked to the correlation between the specific star-formation rate and the galaxy sizes, but it is not clear whether the same link can be established for SNe Ia. We discuss the two-component ('tardy' and 'prompt') model for SN Ia rates, and find that the SN Ia rates in young stellar populations might have a strong correlation with the core-collapse SN rates. We derive volumetric rates for the different SN types [e.g. for SNe Ia, a rate of (0.301 ± 0.062) × 10 −4 SN Mpc −3 yr −1 at redshift 0] and compare them to the measurements at different redshifts. Finally, we estimate the SN rate for the Milky Way Galaxy to be 2.84 ± 0.60 SNe per century (with a systematic uncertainty of a factor of ∼2), consistent with published SN rates based on several different techniques.

The merger of two neutron stars has been predicted to produce an opticalinfrared transient (lasting a few days) known as a 'kilonova', powered by the radioactive decay of neutron-rich species synthesized in the merger 1-5 .Evidence that short γ-ray bursts also arise from neutron star mergers has been accumulating 6-8 . In models 2,9 of such mergers a small amount of mass (10 − 4 -10 − 2 solar masses) with a low electron fraction is ejected at high velocities (0.1-0.3 times light speed) and/or carried out by winds from an accretion disk formed around the newly merged object 10,11 . This mass is expected to undergo rapid neutron capture (r-process) nucleosynthesis, leading to the formation of radioactive elements that release energy as they decay, powering an electromagnetic transient [1][2][3][9][10][11][12][13][14] . A large uncertainty in the composition of the newly synthesized material leads to various expected colours, durations and luminosities for such transients 11-14 . Observational evidence for kilonovae has so far been inconclusive as it was based on cases 15-19 of moderate excess emission detected in the afterglows of γ-ray bursts. Here we report optical to near-infrared observations of a transient coincident with the detection of the gravitational-wave signature of a binary neutron-star merger and of a lowluminosity short-duration γ-ray burst 20 . Our observations, taken roughly every eight hours over a few days following the gravitational-wave trigger, reveal an initial blue excess, with fast optical fading and reddening. Using numerical models 21 , we conclude that our data are broadly consistent with a light curve powered by a few hundredths of a solar mass of low-opacity material corresponding to lanthanide-poor (a fraction of 10 −4.5 by mass) ejecta.GW170817 was detected 22 by the LIGO 23 and Virgo 24 gravitational-wave detectors on 2017 August 17 12:41:04 (UT used throughout; we adopt this as the time of the merger). Approximately two seconds later, a low-luminosity shortduration γ-ray burst, GRB170817A, was detected 25 by the Gamma-ray Burst Monitor (GBM) on board the Fermi satellite. A few hours later, the gravitational wave signal was robustly identified as the signature of a binary neutron star merger 40±8 Mpc away in a region of the sky coincident with the Fermi localization of the γ-ray burst 26 (Fig. 1). Shortly after receiving the gravitational-wave localization, we activated our preapproved program to search for an optical counterpart with the Las Cumbres Observatory (LCO) global network of robotic telescopes 27 . Given the size of the LIGO-Virgo localization region (about 30 square degrees) compared to the field of view of our cameras (about 0.2 square degrees), our search strategy involved targeting specific galaxies 28 (chosen from the GLADE catalog; http://aquarius.elte.hu/glade/) at the reported distance range and location area included in the LIGO-Virgo three-dimensional localization 29 (see Methods). The fifth galaxy on our prioritized list was NGC 4993, an S0 galaxy 39.5 Mpc awa...

Stars with initial masses 10 M ⊙ < ∼ M initial < ∼ 100 M ⊙ fuse progressively heavier elements in their centres, up to inert iron. The core then gravitationally collapses to a neutron star or a black hole, leading to an explosion -an iron-core-collapse supernova (SN) [1,2]. In contrast, extremely massive stars (M initial > ∼ 140 M ⊙ ), if such exist, have oxygen cores which exceed M core = 50 M ⊙ . There, high temperatures are reached at relatively low densities. Conversion of energetic, pressure-supporting photons into electron-positron pairs occurs prior to oxygen ignition, and leads to a violent contraction that triggers a catastrophic nuclear explosion [3,4,5]. Tremendous energies ( > ∼ 10 52 erg) are released, completely unbinding the star in a pair-instability SN (PISN), with no compact remnant. Transitional objects with 100 M ⊙ < M initial < 140 M ⊙ , which end up as iron-core-collapse supernovae following violent mass ejections, perhaps due to short instances of the pair instability, may have been identified [6,7,8]. However, genuine PISNe, perhaps common in the early Universe, have not been observed to date. Here, we present our discovery of SN 2007bi, a luminous, slowly evolving supernova located within a dwarf galaxy (∼ 1% the size of the Milky Way). We measure the exploding core mass to be likely ∼ 100 M ⊙ , in which case theory unambiguously predicts a PISN outcome. We show that > 3 M ⊙ of radioactive 56 Ni were synthesized, and that our observations are well fit by PISN models [9,10]. A PISN explosion in the local Universe indicates that nearby dwarf galaxies probably host extremely massive stars, above the apparent Galactic limit [11], perhaps resulting from star formation processes similar to those that created the first stars in the Universe.

Supernovae (SNe) are stellar explosions driven by gravitational or thermonuclear energy, observed as electromagnetic radiation emitted over weeks or more. In all known SNe, this radiation comes from internal energy deposited in the outflowing ejecta by either radioactive decay of freshly-synthesized elements (typically 56Ni), stored heat deposited by the explosion shock in the envelope of a supergiant star, or interaction between the SN debris and slowly-moving, hydrogen-rich circumstellar material. Here we report on a new class of luminous SNe whose observed properties cannot be explained by any of these known processes. These include four new SNe we have discovered, and two previously unexplained events (SN 2005ap; SCP 06F6) that we can now identify as members. These SNe are all ~10 times brighter than SNe Ia, do not show any trace of hydrogen, emit significant ultra-violet (UV) flux for extended periods of time, and have late-time decay rates which are inconsistent with radioactivity. Our data require that the observed radiation is emitted by hydrogen-free material distributed over a large radius (~10^15 cm) and expanding at high velocities (>10^4 km s^-1). These long-lived, UV-luminous events can be observed out to redshifts z>4 and offer an excellent opportunity to study star formation in, and the interstellar medium of, primitive distant galaxies.Comment: Accepted to Nature. Press embargoed until 2011 June 8, 18:00 U

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