Red or blue? A potential kilonova imprint of the delay until black hole formation following a neutron star merger

Abstract: Mergers of binary neutron stars (NSs) usually result in the formation of a hypermassive neutron star (HMNS). Whether- and when this remnant collapses to a black hole (BH) depends primarily on the equation of state and on angular momentum transport processes, both of which are uncertain. Here we show that the lifetime of the merger remnant may be directly imprinted in the radioactively powered kilonova emission following the merger. We employ axisymmetric, time-dependent hydrodynamic simulations of remnant accretion disks orbiting a HMNS of variable lifetime, and characterize the effect of this delay to BH formation on the disk wind ejecta. When BH formation is relatively prompt (~ 100 ms), outflows from the disk are sufficiently neutron rich to form heavy r-process elements, resulting in ~ week-long emission with a spectral peak in the near-infrared (NIR), similar to that produced by the dynamical ejecta. In contrast, delayed BH formation allows neutrinos from the HMNS to raise the electron fraction in the polar direction to values such that potentially Lanthanide-free outflows are generated. The lower opacity would produce a brighter, bluer, and shorter-lived ~ day-long emission (a `blue bump') prior to the late NIR peak from the dynamical ejecta and equatorial wind. This new diagnostic of BH formation should be useful for events with a signal to noise lower than that required for direct detection of gravitational waveform signatures.

• The paper reveals that a delayed black hole formation significantly alters ejecta composition, producing an early blue bump observable in kilonova light curves.
• The paper employs axisymmetric, time-dependent hydrodynamic simulations to detail how neutrino-driven changes in accretion disk winds affect ejecta properties.
• The paper shows that early blue emission in kilonovae serves as a diagnostic tool for HMNS lifetime, refining models of neutron star equations of state.

Diagnosing Black Hole Formation with Kilonovae

The paper by Metzger and Fernandez investigates the aftermath of neutron star mergers (NSM) by examining the kilonovae emissions that follow these astronomical events. Specifically, it explores the dynamics leading to black hole (BH) formation following the merger and the imprint this formation leaves on the kilonova emissions. The research employs axisymmetric, time-dependent hydrodynamic simulations to study the behavior of accretion disks around hypermassive neutron stars (HMNS) and examines how the delay to BH formation impacts the disk wind ejecta.

Main Findings

1. HMNS Lifetime Impacts on Ejecta Composition: The study reveals that delayed black hole formation significantly alters the composition and dynamics of the post-merger ejecta. When BH formation occurs promptly, within approximately 100 milliseconds, the outflows remain heavily neutron-rich, favoring the production of heavy r-process elements, resulting in a near-infrared spectral peak lasting about a week. However, if BH formation is delayed, neutrino emissions from the HMNS can increase the electron fraction of the ejecta, particularly in the polar regions, to a level that potentially corresponds to lanthanide-free outflows. This leads to a brighter, bluer, and shorter-lived emission visible as an "early blue bump" in the kilonova light curve.
2. Diagnostic Potential of Kilonovae: The presence or absence of this blue emission can act as an indirect diagnostic tool for the lifetime of the HMNS. This could be particularly valuable for events where gravitational waveform signals are too weak to be detected directly. The finding emphasizes that kilonovae can offer more than just a companion signal to gravitational waves; they can provide pertinent details about the state and evolution of the merger remnants.
3. Implications for Astrophysics: The research suggests predictions that can be tested with future observations. The presence of a blue component prior to the late-time NIR peak can help distinguish whether a NSM remnant collapses into a BH promptly or acts as a long-lived HMNS. Observing these signals can help refine models of neutron star equations of state, BH formation and inform searches across large sky error regions provided by current and forthcoming gravitational-wave detectors.

Implications

On a practical level, this research aids the interpretation of kilonova observations to infer the nature and timing of black hole formation in neutron star mergers. It highlights the role of kilonovae beyond acting merely as counterparts to gravitational wave signals, suggesting they can provide unique insights into the elemental synthesis and evolution of accretion disks in the early stages following a merger.

Theoretically, the findings contribute to ongoing debates surrounding the equation of state for neutron stars by influencing the expected delay timescales for BH formation. The results emphasize the sensitivity of neutron star mergers to initial conditions such as the total binary mass and the remnant's equation of state.

Future Directions

As detector technologies advance, continued observation of NSMs and their resultant kilonovae will be critical. Future work will benefit from increased sensitivity to detect these faint, early blue emissions which will help refine models for accretion disk evolution and provide critical tests of the theories behind r-process nucleosynthesis. Furthermore, the model outlined presents a framework from which extensions can be made to explore a wider parameter space, potentially informing cosmological studies of heavy element distribution.

This research is a step towards a more nuanced understanding of how multi-messenger astronomy, combining electromagnetic signals with gravitational waves, can unravel the complexities of cosmic events such as neutron star mergers.