The discovery of gravitational waves from a neutron-star merger and the detection of the event across the electromagnetic spectrum give insight into many aspects of gravity and astrophysics. See Letter p.64, p.67, p.71, p.75 & p.80
Kilonovae, short gamma-ray bursts & neutron star mergers
This Collection of research and comment from Nature Research focuses on the electromagnetic counterparts to the gravitational wave event GW 170817 from the merger of two neutron stars. LIGO’s first three gravitational wave detections, and LIGO-Virgo’s first, all originated from mergers of black holes. These momentous black-hole clashes produced gravitational waves that were audible to LIGO-Virgo but there was nothing to see. But a neutron star merger is different. Following GW 170817, a short gamma ray burst and kilonova occurred, releasing photons across a wide electromagnetic spectrum: from radio waves to infrared to visible to X-rays to gamma rays. The Research papers published in Nature and Nature Astronomy cover some of these counterpart signals. Welcome to the era of gravitational wave astrophysics.
News and comment
Stellar collision confirms theoretical predictions about the periodic table.
The detection of a gravitational wave was a historic event that heralded a new phase of astronomy. A numerical model of the Universe now allows researchers to tell the story of the black-hole system that caused the wave. See Letter p.512
Discovering gravitational waves would not only validate Einstein's theory of gravitation but also reveal aspects of the Universe's earliest moments. The hunt for these elusive ripples is now well under way.
The first detection of electromagnetic emission from a gravitational wave source bridges the gap between one of the most energetic phenomena in the Universe and their dark, difficult to detect progenitors.
Kilonovae and short gamma-ray bursts
Detection of X-ray emission at a location coincident with the kilonova transient of the gravitational-wave event GW170817 provides the missing observational link between short gamma-ray bursts and gravitational waves from neutron-star mergers.
Modelling the electromagnetic emission of kilonovae enables the mass, velocity and composition (with some heavy elements) of the ejecta from a neutron-star merger to be derived from the observations.
Optical to near-infrared observations of a transient coincident with the detection of the gravitational-wave signature of a binary neutron-star merger and a low-luminosity short-duration γ-ray burst are presented and modelled.
Observations of the transient associated with the gravitational-wave event GW170817 and γ-ray burst GRB 170817A reveal a bright kilonova with fast-moving ejecta, including lanthanides synthesized by rapid neutron capture.
Observations and modelling of an optical transient counterpart to a gravitational-wave event and γ-ray burst reveal that neutron-star mergers produce gravitational waves and radioactively powered kilonovae, and are a source of heavy elements.
A double neutron star merger gave rise to the gravitational-wave event GW 170817, with counterpart electromagnetic radiation in the optical and gamma-ray spectra. Polarization measurements of the optical emission reveal a lanthanide-rich macronova.
LIGO and fundamental physics
The astronomical event GW170817, detected in gravitational and electromagnetic waves, is used to determine the expansion rate of the Universe, which is consistent with and independent of existing measurements.
A stochastic background of gravitational waves is expected to arise from a superposition of a large number of unresolved gravitational-wave sources and should carry unique signatures from the earliest epochs of the Universe. Limits on the amplitude of the stochastic gravitational-wave background are now reported using the data from a two-year science run of the Laser Interferometer Gravitational-wave Observatory. These limits rule out certain models of early Universe evolution.
Black holes and spacetime singularities are fundamental in science. While observational proof for black holes is hard to come by, alternatives can be ruled out or confirmed to exist through precision gravitational wave observations.
Black holes present a profound challenge to our current foundations of physics, and an exciting era of astronomy is just opening in which gravitational-wave observation and very-long-baseline interferometry may provide important hints about the new principles of physics needed.