Kilonovae

Abstract: The mergers of double neutron star (NS-NS) and black hole (BH)-NS binaries are promising gravitational wave (GW) sources for Advanced LIGO and future GW detectors. The neutron-rich ejecta from such merger events undergoes rapid neutron capture (r-process) nucleosynthesis, enriching our Galaxy with rare heavy elements like gold and platinum. The radioactive decay of these unstable nuclei also powers a rapidly evolving, supernova-like transient known as a "kilonova". Kilonovae provide an approximately isotropic electromagnetic counterpart to the GW signal, which also provides a unique and direct probe of an important, if not dominant, r-process site. This handbook reviews the history and physics of kilonovae, leading to the current paradigm of week-long emission with a spectral peak at near-infrared wavelengths. Using a simple light curve model to illustrate the basic physics, I introduce potentially important variations on this canonical picture, including: ~day-long optical ("blue") emission from lanthanide-free components of the ejecta; ~hours-long precursor UV/blue emission, powered by the decay of free neutrons in the outermost ejecta layers; and enhanced emission due to energy input from a long-lived central engine, such as an accreting BH or millisecond magnetar. I assess the prospects of detecting kilonovae following future GW detections of NS-NS/BH-NS mergers in light of the recent follow-up campaign of the LIGO binary BH-BH mergers.

• The paper reviews kilonovae, highlighting their role as crucial electromagnetic counterparts to gravitational wave events from neutron star mergers and identifying them as primary sites for rapid neutron capture (r-process) nucleosynthesis.
• It details the development of kilonova emission models, emphasizing the significant impact of lanthanide opacities which cause emission to peak in the near-infrared over approximately a week.
• The review discusses variations on the standard kilonova model, the potential influence of central engines on luminosity, and prospects for future kilonova observations with advancing gravitational wave detector sensitivity.

Overview of "Kilonovae" by Brian D. Metzger

The paper "Kilonovae" authored by Brian D. Metzger provides an extensive review of kilonovae, the optical and infrared counterparts to neutron star mergers, and their connection to gravitational wave events. These astronomical phenomena are especially significant as they offer a unique electromagnetic counterpart to gravitational wave detections, contributing to the field of multi-messenger astronomy. Kilonovae are a result of radioactive decay within neutron-rich ejecta from merging neutron star (NS-NS) or black hole-neutron star (BH-NS) binaries, which undergo rapid neutron capture, known as the r-process. This process significantly enriches the universe with heavy elements, including gold and platinum.

Main Findings and Contributions

1. Gravitational Wave and Electromagnetic Counterparts: The paper highlights kilonovae as potential observables coinciding with gravitational wave (GW) events. Such synergy could not only resolve the source of GWs from NS mergers but also establish a direct r-process element production site.
2. Model of Emission and Physics: The author provides a structured narrative on the development of kilonova models, starting from rudimentary explanations of kilonova physics to modern paradigms that incorporate the detailed role of lanthanide-rich opacities on emission spectra. The presence of lanthanide-rich ejecta results in opacities that lead to emission peaking in the near-infrared over a week-long period.
3. Variations and Substructures in Kilonovae: Metzger discusses variations on the canonical kilonova model, such as potential early optical emissions from lanthanide-free ejecta and ultraviolet precursors powered by the decay of free neutrons in the outer layers of the ejecta. These components introduce significant diversity in observable kilonova signatures.
4. Central Engine Influence: The potential impact of an accreting black hole or a long-lived magnetar engine on kilonova luminosity is explored. Such central engines may significantly enhance the electromagnetic output of kilonovae, potentially observable as bright, long-lived transients.
5. Detection Prospects: In light of recent non-detections from LIGO's observations of binary black hole mergers, the paper contrasts expectations for kilonovae apparent from NS-NS/BH-NS mergers. Future GW detections provide optimized prospects for kilonova observations, which could help in understanding both galactic chemical evolution and binary evolution scenarios.

Implications and Future Developments

The research presented in this paper has profound implications for the field of astrophysics, especially concerning the study of GWs, stellar nucleosynthesis, and the role neutron star mergers play in the chemical enrichment of galaxies. As GW detector sensitivity advances, the likelihood of observing kilonovae increases, offering opportunities to probe the astrophysical conditions in extreme environments and the properties of dense matter in NS remnants.

Future developments in AI and machine learning could further enhance our ability to model the complex processes in kilonovae, correct for observational biases, and efficiently classify transients in astronomical data. Machine learning techniques could be applied to filter vast datasets from sky surveys, isolating promising kilonova candidates. Moreover, AI might assist in developing more sophisticated simulations to understand nuclear reactions and improve opacity calculations, crucial for accurate light curve modeling.

In summary, the insights from this paper chart a path forward for observing and analyzing kilonovae, heralding an era where multi-messenger astronomy combines GW and electromagnetic observations to unlock the secrets of the universe’s most violent and element-forming events.