Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 446, Issue 1, Pages 1115-1120Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stu2225
Keywords
gravitation; nuclear reactions, nucleosynthesis, abundances; binaries: close; stars: neutron; supernovae: general
Categories
Funding
- NSF [AST-1410950]
- Alfred P. Sloan Foundation
- University of Washington Institute for Nuclear Theory workshop 'The R-Process: Status and Challenges'
- Curie Intra-European [IEF 331873]
- COST Action [MP1304]
- FNRS (Belgium)
- Department of Energy Office of Nuclear Physics Early Career Award
- Office of Energy Research, Office of High Energy and Nuclear Physics, Divisions of Nuclear Physics, of the US Department of Energy [DE-AC02-05CH11231]
- NSF Division of Astronomical Sciences [AST-1206097]
- Direct For Mathematical & Physical Scien [1206097] Funding Source: National Science Foundation
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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 fewper cent, or similar to 10(-4)M(circle dot)) 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 beta-decay of these free neutrons in the outermost ejecta powers a 'precursor' to the main kilonova emission, which peaks on a time-scale of similar to few hours following merger at U-band magnitude similar to 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.
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