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

Abstract: Gravitational waves were discovered with the detection of binary black hole mergers and they should also be detectable from lower mass neutron star mergers. These are predicted to eject material rich in heavy radioactive isotopes that can power an electromagnetic signal called a kilonova. The gravitational wave source GW170817 arose from a binary neutron star merger in the nearby Universe with a relatively well confined sky position and distance estimate. Here we report observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC4993, which is spatially coincident with GW170817 and a weak short gamma-ray burst. The transient has physical parameters broadly matching the theoretical predictions of blue kilonovae from neutron star mergers. The emitted electromagnetic radiation can be explained with an ejected mass of 0.04 +/- 0.01 Msol, with an opacity of kappa <= 0.5 cm2/gm at a velocity of 0.2 +/- 0.1c. The power source is constrained to have a power law slope of beta = -1.2 +/- 0.3, consistent with radioactive powering from r-process nuclides. We identify line features in the spectra that are consistent with light r-process elements (90 < A < 140). As it fades, the transient rapidly becomes red, and emission may have contribution by a higher opacity, lanthanide-rich ejecta component. This indicates that neutron star mergers produce gravitational waves, radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements.

• The paper demonstrates that joint GW and EM observations from neutron star mergers confirm kilonova emissions as indicators of r-process nucleosynthesis.
• It utilizes multi-observatory data and radiative-transfer modeling to derive key parameters such as ejecta mass, opacity (≤0.5 cm²/g), and velocities (~0.2c).
• The findings substantiate theoretical predictions, guiding future observational strategies for detecting and modeling transient high-energy events in the cosmos.

Electromagnetic Counterpart of Gravitational Waves from Neutron Star Mergers: Kilonova Analysis

The detection of gravitational waves originating from neutron star mergers has offered an unparalleled insight into cosmic events contributing to the synthesis of heavy elements. The paper under review examines such a phenomenon, specifically focusing on the gravitational wave source GW170817, which was attributed to a binary neutron star merger. This event was remarkable not only for the gravitational waves it generated, but also for the elusive electromagnetic counterpart it produced, known as a kilonova.

Observations and Modeling

The research effort described in the paper utilized a collaboration of multiple observatories and instruments, including the Advanced LIGO and Virgo detectors, Pan-STARRS, and the ePESSTO survey on the New Technology Telescope. The transient event AT2017gfo, coinciding with GW170817 in the galaxy NGC 4993, exemplified kilonova characteristics. The joint detection of gravitational waves and electromagnetic signals confirmed theoretical predictions regarding neutron star mergers as progenitors of r-process nucleosynthesis.

Key to the study were observations capturing a rapidly fading electromagnetic transient that exhibited properties consistent with theoretical models of blue kilonovae. This transient, when spatially aligned with a weak gamma-ray burst, reinforced the association with the merger event. The modeling indicated that the emitted light could be explained by ejected material with an opacity of $\kappa \leq 0.5 \, \text{cm}^2/\text{g}$, expelled at velocities of approximately 0.2 $c$.

Analytical Insights and Implications

The models tested in the paper incorporated advanced radiative-transfer techniques, assessing both opacity and compositional influences of the ejected material. The findings suggest that early emission is compatible with light r-process elements (atomic mass 90 < A < 140), while the fading red signature implies contributions from heavier lanthanide-rich ejecta. The deduced quantities of ejected mass and velocities were consistent with expected values for kilonova emissions.

A notable conclusion is that neutron star mergers are confirmed not only as sources of gravitational waves but also crucial sites of heavy element synthesis through r-process nucleosynthesis. This research further substantiates neutron star mergers as significant contributors to the cosmic abundance of elements beyond the iron peak.

Theoretical and Practical Implications

The implications of these findings are manifold. Theoretically, they advance the understanding of nucleosynthesis in high-energy astrophysical events, providing constraints on models of r-process element production. Practically, they inform the design and operational strategies of observational facilities aiming to detect and characterize future kilonovae. The study enhances predictive models, aiding in the anticipation of kilonova light curves and their spectral evolution.

Future Prospects

The research invites further investigations into the diversity of kilonova emissions, influenced by variations in merger parameters and the neutron star equation of state. Prospective observations, ideally augmenting data across the electromagnetic spectrum, would calibrate and refine existing models.

The synthesis of heavy elements via cosmic events remains a cornerstone of astrophysical research. This study underscores the critical role interdisciplinary collaboration and technology play in deciphering the origins of the building blocks constituting the observable universe. Future studies might harness these insights, delving deeper into the amalgam of physics driving some of the most luminous and energetic phenomena in the cosmos.