Optical emission from a kilonova following a gravitational-wave-detected neutron-star merger (1710.05843v1)

Abstract: The merger of two neutron stars has been predicted to produce an optical-infrared transient (lasting a few days) known as a 'kilonova', powered by the radioactive decay of neutron-rich species synthesized in the merger. Evidence that short gamma-ray bursts also arise from neutron-star mergers has been accumulating. In models of such mergers a small amount of mass ($10^{-4}$-$10^{-2}$ solar masses) with a low electron fraction is ejected at high velocities (0.1-0.3 times light speed) and/or carried out by winds from an accretion disk formed around the newly merged object. This mass is expected to undergo rapid neutron capture (r-process) nucleosynthesis, leading to the formation of radioactive elements that release energy as they decay, powering an electromagnetic transient. A large uncertainty in the composition of the newly synthesized material leads to various expected colours, durations and luminosities for such transients. Observational evidence for kilonovae has so far been inconclusive as it was based on cases of moderate excess emission detected in the afterglows of gamma-ray bursts. Here we report optical to near-infrared observations of a transient coincident with the detection of the gravitational-wave signature of a binary neutron-star merger and of a low-luminosity short-duration gamma-ray burst. Our observations, taken roughly every eight hours over a few days following the gravitational-wave trigger, reveal an initial blue excess, with fast optical fading and reddening. Using numerical models, we conclude that our data are broadly consistent with a light curve powered by a few hundredths of a solar mass of low-opacity material corresponding to lanthanide-poor (a fraction of $10^{-4.5}$ by mass) ejecta.

• The paper presents dense, multi-band imaging of kilonova AT 2017gfo following GW170817, revealing ejecta velocities up to 0.3c and low lanthanide opacity.
• The paper employs analytical and numerical models to capture a swift brightness peak and rapid fading, supporting predictions of radioactive decay-powered transients.
• The paper demonstrates how multi-messenger astronomy refines theoretical frameworks for neutron-star mergers and enhances our understanding of r-process nucleosynthesis.

Observations of Optical Emission from a Kilonova Following a Neutron-Star Merger

This paper provides a detailed analysis of the optical emission observed from a kilonova resulting from a neutron-star merger detected through gravitational waves. The observations support theoretical models predicting that such cosmic events produce electromagnetic transients, characterized by their optical and infrared emissions, driven by the radioactive decay of heavy elements synthesized during the merger process.

Key Observations and Results

The detection of GW170817 on August 17, 2017, by LIGO and Virgo, offered a unique opportunity to observe its associated electromagnetic counterpart, GRB170817A, detected by the Fermi satellite shortly thereafter. An intense observational campaign was triggered, utilizing the Las Cumbres Observatory (LCO) network, to scrutinize a region of the sky localized by these detectors. Within hours, a new transient source—designated AT 2017gfo—was identified in the galaxy NGC 4993, positioned about 40 Mpc away.

Throughout several days, the LCO pursued multi-band imaging from multiple global sites, capturing the rapid evolution of the kilonova. The resulting densely sampled light curve revealed atypical properties distinct from standard supernovae, featuring a swift peak in brightness followed by rapid fading across various optical bands. This behavior aligns with established kilonova predictions, wherein a few hundredths of a solar mass of ejected material with low opacity emits energy as it decays, predominantly composing lanthanide-poor r-process isotopes.

Analytical and Numerical Comparisons

The observational data were juxtaposed with different analytical and numerical models of kilonovae. Analyses of the luminosity peak, as well as temporal evolutions, align with models predicting low-ejecta mass compositions and velocities up to 0.3c, with correspondingly low lanthanide mass fractions resulting in reduced opacity. Noteworthy discrepancies were noted in the g-band data, suggesting potential variations in the ejecta composition and highlighting the possible contribution of an additional radiation source beyond radioactive decay—perhaps linked to the gamma-ray burst engine.

Implications and Future Prospects

The concordance between the empirical data and kilonova models substantiates the hypothesis that binary neutron-star mergers are indeed progenitors of such optical transients. The detection of these phenomena in conjunction with gravitational waves and gamma-ray bursts validates theoretical grounds, offering pivotal insights into the processes underpinning r-process nucleosynthesis and the evolution of neutron-star mergers.

The implications for the paper of astrophysical transients are substantial, with anticipated advancements from upcoming projects such as the Large Synoptic Survey Telescope (LSST), which promise to register a multitude of kilonovae annually. These future surveys will provide further windows into kilonovae phenomena, offering stricter constraints on theoretical models and refining our understanding of the fundamental physics driving these cosmic events.

Conclusion

This work showcases a milestone in multi-messenger astronomy, demonstrating how cooperative observations from gravitational-wave detectors and traditional electromagnetic methods can elucidate the intricate aftermath of neutron-star mergers. As methodologies and technologies evolve, enriched observations will further enhance comprehension of kilonovae, shedding light on matter's complex behavior under extreme conditions and contributing to the broader understanding of cosmic nucleosynthesis pathways.