Neutron-powered precursors of kilonovae

By B. Metzger, A. Bauswein, S. Goriely, D. Kasen. 2014

143 citations recorded by the source corpus.

The merger of binary neutron stars (NSs) ejects a small quantity of neutron-rich matter, the radioactive decay of which powers a day to week long thermal transient known as a kilonova. Most of the ejecta remains sufficiently dense during its expansion that all neutrons are captured into nuclei during the r-process. However, recent general relativistic merger simulations by Bauswein and collaborators show that a small fraction of the ejected mass (a few per cent, or ~10-4 M⊙) expands sufficiently rapidly for most neutrons to avoid capture. This matter originates from the shocked-heated interface between the merging NSs. Here, we show that the β-decay of these free neutrons in the outermost ejecta powers a 'precursor' to the main kilonova emission, which peaks on a time-scale of ~ few hours following merger at U-band magnitude ~22 (for an assumed distance of 200 Mpc). The high luminosity and blue colours of the neutron precursor render it a potentially important counterpart to the gravitational wave source, that may encode valuable information on the properties of the merging binary (e.g. NS-NS versus NS-black hole) and the NS equation of state. Future work is necessary to assess the robustness of the fast-moving ejecta and the survival of free neutrons in the face of neutrino absorptions, although the precursor properties are robust to a moderate amount of leptonization. Our results provide additional motivation for short latency gravitational wave triggers and rapid follow-up searches with sensitive ground-based telescopes.

Subfields: Astronomy and Astrophysics, Radiation

View Paper: https://www.semanticscholar.org/paper/c3e70d5bf1d17b347d33b9bc6d535e58f5464053

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In Astronomy and Astrophysics

1. B. Metzger
2. E. Berger
3. E. Quataert
4. Bing Zhang
5. David Radice
6. D. Kasen
7. R. Fern'andez
8. Luke F. Roberts

In Radiation

1. Luke F. Roberts
2. Jonas Lippuner
3. S. Smartt
4. E. Nakar

Citations in the corpus

• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-time Emission from the Kilonova Ejecta (2017, background): “3 , to which optical KN observations of GW170817 are not sensitive (since its optical/UV emission would have peaked on earlier timescales of a few hours; e.g., Metzger et al. 2015; Nakar & Piran 2017).”
• The cocoon emission - an electromagnetic counterpart to gravitational waves from neutron star mergers (2017, background): “A signal arising from the neutrino driven wind (Grossman et al. 2013; Perego et al. 2014b; Kasen et al. 2015) and a signal arising from a neutron rich outflow that may precede this wind (Metzger et al. 2015).”
• Dynamical Mass Ejection from Binary Neutron Star Mergers (2016, background): “This seems to exclude the neutron freezeout scenario proposed by Metzger et al. (2015).”
• A Magnetar Origin for the Kilonova Ejecta in GW170817 (2018, background): “The early KN emission can also be enhanced due to the radioactive decay of free neutrons in the outermost layers of the ejecta (Metzger et al. 2015; see also Kulkarni 2005).”
• Production of the entire range of r-process nuclides by black hole accretion disc outflows from neutron star mergers (2016, background): “More promising in this respect is the possible onset of a neutron-powered precursor if a small fraction of material escapes quickly enough to freeze-out before neutrons are captured by heavy seeds (Metzger et al. 2015).”
• Off-axis emission of short γ-ray bursts and the detectability of electromagnetic counterparts of gravitational-wave-detected binary mergers (2016, background): “A kilonova precursor peaking ∼1 h after the merger has also been discussed (Metzger et al. 2015), possibly outshining the cocoon optical afterglow component at early time.”
• AT 2017gfo: An Anisotropic and Three-component Kilonova Counterpart of GW170817 (2017, background): “In our analysis, we did not consider the possible presence of a small amount (∼ 10−4 M ) of free-neutron ejecta (Bauswein et al. 2013), which could contribute to the UV/visible emission a few hours after the merger (Metzger et al. 2015).”
• r-PROCESS LANTHANIDE PRODUCTION AND HEATING RATES IN KILONOVAE (2015, methodology): “These results are very similar to what Metzger et al. (2015) found, however, they found peak effective temperatures of ∼ 104 K, because they used higher opacities (κ = 30 cm2 g−1) since their trajectories still contained a significant amount of lanthanides and actinides (Metzger 2015).”
• Follow Up of GW170817 and Its Electromagnetic Counterpart by Australian-Led Observing Programmes (2017, methodology): “…to generate a relatively rapidly evolving optical/infrared transient—referred to as kilonova or macronova (e.g., Li & Paczyński 1998; Metzger et al. 2010; Roberts et al. 2011; Barnes & Kasen 2013; Tanaka & Hotokezaka 2013; Kasen, Fernández, & Metzger 2015; Metzger et al. 2015; Barnes et al. 2016).”
• A cocoon shock breakout as the origin of the γ-ray emission in GW170817 (2017, background): “Note that free neutrons can remain in the fast components of the ejecta with ve j∼>0.5c (Metzger et al. 2015).”
• Long-term GRMHD simulations of neutron star merger accretion discs: implications for electromagnetic counterparts (2018, background): “Mass with vr ∼ c can generate strong early (∼ hr timescales) thermal emission powered by the radioactive decay of free neutrons left over after the r-process completes (Metzger et al. 2015).”
• Constraints on the neutron star equation of state from AT2017gfo using radiative transfer simulations (2018, background): “…to physical effects not included in our modelling, such as the decay of free neutrons in the outermost fastest parts of the ejecta (Kulkarni 2005; Metzger et al. 2015), or additional thermal energy added to the ejecta by a relativistic jet [‘cocoon’ emission (Gottlieb et al. 2018; Kasliwal et…”
• Kilonova light curves from the disc wind outflows of compact object mergers (2014, background): “…neutrons relative to r-process nuclei may power bright ultraviolet/blue emission on a time-scale of hours following the merger, despite the presence of lanthanides (Metzger et al. 2015). outflows to estimate the critical electron fraction above which the ejecta is lanthanide free (Section 2.3).”