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Published December 15, 2016 | Published
Journal Article Open

Impact of an improved neutrino energy estimate on outflows in neutron star merger simulations


Binary neutron star mergers are promising sources of gravitational waves for ground-based detectors such as Advanced LIGO. Neutron-rich material ejected by these mergers may also be the main source of r-process elements in the Universe, while radioactive decays in the ejecta can power bright electromagnetic postmerger signals. Neutrino-matter interactions play a critical role in the evolution of the composition of the ejected material, which significantly impacts the outcome of nucleosynthesis and the properties of the associated electromagnetic signal. In this work, we present a simulation of a binary neutron star merger using an improved method for estimating the average neutrino energies in our energy-integrated neutrino transport scheme. These energy estimates are obtained by evolving the neutrino number density in addition to the neutrino energy and flux densities. We show that significant changes are observed in the composition of the polar ejecta when comparing our new results with earlier simulations in which the neutrino spectrum was assumed to be the same everywhere in optically thin regions. In particular, we find that material ejected in the polar regions is less neutron rich than previously estimated. Our new estimates of the composition of the polar ejecta make it more likely that the color and time scale of the electromagnetic signal depend on the orientation of the binary with respect to an observer's line of sight. These results also indicate that important observable properties of neutron star mergers are sensitive to the neutrino energy spectrum, and may need to be studied through simulations including a more accurate, energy-dependent neutrino transport scheme.

Additional Information

© 2016 American Physical Society. (Received 28 July 2016; published 29 December 2016) The authors thank Matthew Duez, Dan Hemberger, and the members of the SxS Collaboration for their input and support during this project; Dan Kasen and Rodrigo Fernandez for regular discussions on binary mergers and outflows; and Brett Deaton for his comments on an earlier version of this manuscript. Support for this work was provided by National Aeronautics and Space Administration (NASA) through Einstein Postdoctoral Fellowship Grants No. PF4-150122 (F. F.) and No. PF3-140114 (L. R.) awarded by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for NASA under Contract No. NAS8-03060; and through Hubble Fellowship Grant No. 51344.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under Contract No. NAS 5-26555. The authors at the Canadian Institute for Theoretical Astrophysics (CITA) gratefully acknowledge support from the Natural Sciences and Engineering Research Council of Canada (NSERC). L. K. acknowledges support from National Science Foundation (NSF) Grants No. PHY-1306125 and No. AST-1333129 at Cornell, while the authors at Caltech acknowledge support from NSF Grants No. PHY-1404569, No. AST-1333520, No. NSF-1440083, and NSF CAREER Grant No. PHY-1151197. Authors at both Cornell and Caltech also thank the Sherman Fairchild Foundation for their support. Computations were performed on the supercomputer Briarée from the Université de Montréal, and Guillimin from McGill University, both managed by Calcul Québec and Compute Canada. The operation of these supercomputers is funded by the Canada Foundation for Innovation (CFI), NanoQuébec, Réseau de médecine génétique appliquée (RMGA) and the Fonds de recherche du Québec–Nature et Technologie (FRQ-NT). Computations were also performed on the Zwicky cluster at Caltech, supported by the Sherman Fairchild Foundation and by NSF Grant No. PHY-0960291. This work also used the Extreme Science and Engineering Discovery Environment (XSEDE) through allocation No. TGPHY990007N, supported by NSF Grant No. ACI-1053575.

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