Kilonovae

Abstract: The coalescence of double neutron star (NS-NS) and black hole (BH)-NS binaries are prime sources of gravitational waves (GW) for Advanced LIGO/Virgo and future ground-based detectors. Neutron-rich matter released from such events undergo rapid neutron capture (r-process) nucleosynthesis as it decompresses into space, enriching our universe with rare heavy elements like gold and platinum. Radioactive decay of these unstable nuclei powers a rapidly evolving, approximately isotropic thermal transient known as a ``kilonova', which probes the physical conditions during the merger and its aftermath. Here I review the history and physics of kilonovae, leading to the current paradigm of day-timescale emission at optical wavelengths from lanthanide-free components of the ejecta, followed by week-long emission with a spectral peak in the near-infrared (NIR). These theoretical predictions, as compiled in the original version of this review, were largely confirmed by the transient optical/NIR counterpart discovered to the first NS-NS merger, GW170817, discovered by LIGO/Virgo. Using a simple light curve model to illustrate the essential physical processes and their application to GW170817, I then introduce important variations about the standard picture which may be observable in future mergers. These include ~hours-long UV precursor emission, powered by the decay of free neutrons in the outermost ejecta layers or shock-heating of the ejecta by a delayed ultra-relativistic outflow; and enhancement of the luminosity from a long-lived central engine, such as an accreting BH or millisecond magnetar. Joint GW and kilonova observations of GW170817 and future events provide a new avenue to constrain the astrophysical origin of the r-process elements and the equation of state of dense nuclear matter.

• The paper presents a comprehensive review of kilonovae as electromagnetic counterparts driven by r-process nucleosynthesis in compact binary mergers.
• The paper details numerical simulations showing how ejecta mass, velocity, and electron fraction govern distinct blue and red kilonova light curves.
• The paper discusses the implications for r-process element synthesis and dense matter properties, bridging theoretical models with multi-messenger observations.

Overview of the Paper: "Kilonovae" by Brian D. Metzger

The paper "Kilonovae," authored by Brian D. Metzger, provides a comprehensive review of the electromagnetic counterparts known as kilonovae, which are expected to accompany gravitational wave events from the mergers of neutron star (NS) binaries and black hole (BH)-NS binaries. These astronomical phenomena are pivotal for Advanced LIGO/Virgo and future ground-based gravitational wave detectors. As the paper outlines, kilonovae result from the radioactive decay of heavy elements synthesized by the rapid neutron capture process (r-process) nucleosynthesis. Such occurrences during the mergers contribute significantly to the enrichment of the universe with high atomic number elements like gold and platinum.

Kilonova Emission Mechanisms

Metzger elaborates on the theoretical backbone and observational advances concerning kilonovae, emphasizing the relevance of these thermal transients for understanding the aftermath of NS-NS and BH-NS mergers. The paper dissects the evolution of kilonova models, where emission is typically theorized to occur on a timescale of days in optical wavelengths due to lanthanide-free ejecta components, transitioning to a week-long near-infrared (NIR) emission from lanthanide-rich ejecta. The predictions for the emission from these components were substantively validated by the transient counterpart observed following the seminal NS-NS merger event GW170817, detected by the LIGO/Virgo observatories.

Major Numerical Results and Predictions

A key component of the paper involves theoretical predictions grounded in numerical simulations illustrating that the macroscopic ejecta properties such as mass, velocity, and electron fraction ($Y_e$) significantly influence the kilonova light curves. For instance, the distinction between 'blue' and 'red' kilonovae is predicated on the $Y_e$ of the ejecta. Blue kilonovae originate from high-$Y_e$ matter (lanthanide-free), leading to lower opacity and bluer emission, while red kilonovae arise from lanthanide-bearing material, marked by higher opacity and predominantly NIR emission. These differing opacities ($$\kappa \sim 1 \mathrm{to} 30 \, \mathrm{cm}^2 \, \mathrm{g}^{-1}$$) impact the peak luminosity and spectrum, with blue kilonovae manifesting brighter and earlier light curve peaks compared to their red counterparts.

Implications and Future Developments

The theoretical framework Metzger presents holds significant potential for deducing the astrophysical sites of r-process nucleosynthesis, constraining the nuclear equation of state for dense matter, and refining models of stellar evolution in compact binary systems. Aspects such as free neutron emission in the outer layers of the ejecta or variations driven by a central engine (such as a nascent magnetar) could introduce early blue or ultraviolet emission that is currently speculative but might be observable with future instruments.

Research into kilonovae follows a trajectory enriched by multidisciplinary studies melding electromagnetic observations with gravitational wave data. It opens avenues for future explorations that aim to fine-tune our understanding of r-process elements through late-time kilonova light curve analysis, potentially offering insights equivalent to the characterization of supernovae from radioisotope decay lines. Moreover, beyond individual phenomena, a broader statistical understanding across many events will likely facilitate constraints on neutron star properties and merger dynamics through joint EM-GW analyses.

In conclusion, Metzger's paper underscores kilonovae as a pivotal element in the study of cosmic nucleosynthesis, merging the fields of nuclear astrophysics with observational cosmology, and effectively bridging theoretical predictions with empirical evidence through multi-messenger astronomy. The continued disentangling of these signals is poised to deepen our comprehension of both the microphysics of dense matter and the macroscopic tapestry of the cosmos.