A kilonova as the electromagnetic counterpart to a gravitational-wave source

Area of Science:

• Astrophysics
• Nuclear Astrophysics
• Gravitational Wave Astronomy

Background:

• Gravitational waves (GWs) were initially detected from binary black-hole mergers.
• Neutron-star mergers are predicted to produce GWs and eject radioactive material.
• This ejected material can power a luminous electromagnetic signal known as a kilonova.

Purpose of the Study:

• To report observations of an electromagnetic transient coincident with a binary neutron-star merger (GW170817).
• To model the physical parameters of the transient and compare them with theoretical kilonova predictions.
• To investigate the nucleosynthetic origin of heavy elements produced in neutron-star mergers.

Main Methods:

• Observations of the transient event in the galaxy NGC 4993, spatially coincident with GW170817.
• Physical modeling of the observed electromagnetic radiation, including ejected mass, opacity, velocity, and power source.
• Spectroscopic analysis to identify elemental composition and compare with r-process nucleosynthesis predictions.

Main Results:

• Detection of a rapidly fading electromagnetic transient (kilonova) spatially coincident with GW170817 and a weak gamma-ray burst.
• Physical parameters (ejected mass ~0.04 solar masses, low opacity, high velocity) broadly match blue kilonova predictions.
• Power-law slope of the light curve (-1.2 ± 0.3) is consistent with radioactive powering from r-process nuclides.
• Spectral features suggest the presence of light r-process elements (atomic masses 90-140).
• Observed rapid fading and reddening indicate a possible contribution from lanthanide-rich ejecta.

Conclusions:

• Binary neutron-star mergers generate both gravitational waves and radioactively powered kilonovae.
• These mergers are confirmed as a significant nucleosynthetic source of r-process elements, heavier than iron.
• The observed kilonova provides direct evidence for the production of heavy elements in these cosmic events.