Optical emission from a kilonova following a gravitational-wave-detected neutron-star merger

Abstract

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...

Localization of binary neutron star mergers with second and third generation gravitational-wave detectors

“…This multimessenger data provided convincing answers to many outstanding questions. For instance, the detection of a short gamma ray burst (GRB) 1.7 seconds after GW170817 [24][25][26], and subsequent kilonova [27][28][29][30][31][36][37][38][39][40][41][42][43][44], confirmed that BNS mergers are a progenitor of these events. Lanthanide signatures in the kilonova light curves also showed BNS mergers to be a major site for nucleosynthesis of elements heavier than iron [40,44,47,48].…”

Optical Follow-up of Gravitational-wave Events with Las Cumbres Observatory

“…For the second trigger, our algorithm selected the correct host galaxy as fifth in priority from the list of 182 galaxies identified in the LIGO/Virgo localization region. This allowed us to obtain early data of the counterpart (Arcavi et al 2017a). …”

“…No obvious counterpart candidates were found in the other galaxies surveyed. The follow-up data obtained for AT 2017gfo is the topic of two companion papers (Arcavi et al 2017a;McCully et al 2017). …”

“…We summarize in §5. Our followup observations of AT 2017gfo, the optical counterpart of GW170817, are described in companion papers (Arcavi et al 2017a;McCully et al 2017). …”

Follow Up of GW170817 and Its Electromagnetic Counterpart by Australian-Led Observing Programmes

“…The 1M2H team referred to the transient with the name Swope Supernova Survey 2017a (SSS17a, Coulter et al 2017bCoulter et al , 2017a. Details about the other independent detections can be found in Allam et al (2017) for the Dark Energy Camera, Valenti et al (2017) and Yang et al (2017) for the Distance Less Than 40 Mpc survey (DLT40), Arcavi et al (2017a) and Arcavi et al (2017b) for the Las Cumbres Observatory, Tanvir et al (2017a) and for the Visible and Infrared Survey Telescope for Astronomy, Lipunov et al (2017a) and for the MASTER discoveries. Lipunov et al (2017b) offer an extensive review of the world-wide follow up.…”