Living Reviews in Relativity https://doi.org/10.1007/s41114-020-00028-7(0123456789().,-volV)(0123456789().,-volV) REVIEW ARTICLE Neutron star mergers and how to study them Eric Burns1 Received: 5 January 2020 / Accepted: 12 October 2020 Ó The Author(s) 2020 Abstract Neutron star mergers are the canonical multimessenger events: they have been observed through photons for half a century, gravitational waves since 2017, and are likely to be sources of neutrinos and cosmic rays. Studies of these events enable unique insights into astrophysics, particles in the ultrarelativistic regime, the heavy element enrichment history through cosmic time, cosmology, dense matter, and fundamental physics. Uncovering this science requires vast observational resources, unparalleled coordination, and advancements in theory and simulation, which are constrained by our current understanding of nuclear, atomic, and astroparticle physics. This review begins with a summary of our current knowledge of these events, the expected observational signatures, and estimated detection rates for the next decade. I then present the key observations necessary to advance our under- standing of these sources, followed by the broad science this enables. I close with a discussion on the necessary future capabilities to fully utilize these enigmatic sources to understand our universe. Keywords Gravitational waves Á Neutron stars Á Black holes Á Nucleosynthesis Á Cosmology Á Equation of state Abbreviations BAO Baryon acoustic oscillation BAT Burst Alert Telescope BATSE Burst and Transient Source Experiment BBN Big Bang Nucleosynthesis BH Black Hole BBH Binary Black Hole BNS Binary Neutron Star CBC Compact Binary Coalescence & Eric Burns [email protected] 1 Louisiana State University, Baton Rouge, LA 70803, USA 123 E. Burns CMB Cosmic Microwave Background CCSNe Core-Collapse Supernova explosion CGRO Compton Gamma-Ray Observatory CTA Cherenkov Telescope Array CO Compact Object CR Cosmic Ray EM Electromagnetic FAR False Alarm Rate FoV Field of View EOS Equation of State GBM Gamma-ray Burst Monitor GCN Gamma-ray Coordinates Network GR General Relativity GRB Gamma-ray burst GW Gravitational wave HAWC High-Altitude Water Cherenkov HMNS HyperMassive Neutron Star IACT Imaging Atmospheric Cherenkov Telescope INTEGRAL INTErnational Gamma-Ray Astrophysics Laboratory IPN Interplanetary Network IR Infrared ISCO Innermost Stable Circular Orbit ISM Interstellar medium JWST James Webb Space Telescope KAGRA Kamioka Gravitational Wave Detector KNR Kilonova remnant LAT Large Area Telescope LEO Low Earth Orbit LGRB Long Gamma-Ray Burst LIGO Laser Interferometer Gravitational-wave Observatory LIV Lorentz Invariance Violation LSST Large Synoptic Survey Telescope LVC The LIGO Scientific Collaboration and Virgo Collaboration LISA Laser Interferometer Space Antenna NIR Near infrared MAGIC Major Atmospheric Gamma Imaging Cherenkov Telescopes MCMC Markov Chain Monte Carlo MW Milky Way NS Neutron Star NSBH Neutron Star–Black Hole PPN Parametrized Post-Newtonian PTA Pulsar Timing Arrays QCD Quantum Chromodynamics QFT Quantum Field Theory QG Quantum Gravity 123 Neutron star mergers and how to study them RAVEN Rapid VOEvent Coincidence Monitor SAA South Atlantic Anomaly SFR Star Formation Rate SGRB Short Gamma-Ray Burst SMBH SuperMassive Black Hole SMNS Supramassive Neutron Star SNR Signal-to-Noise Ratio SME Standard Model Extension SNe Supernova explosion SNEWS Supernova Early Warning System SPI-ACS SPectrometer onboard INTEGRAL-Anti-Coincidence Shield SR Special Relativity SSC Synchrotron Self Compton TOV Tolman–Oppenheimer–Volkoff TTE Time-Tagged Event UHECR Ultra-High Energy Cosmic Ray UV Ultraviolet UVOIR Ultraviolet, Optical, and Infrared UVOT Ultraviolet/Optical Telescope VHE Very High Energy WEP Weak Equivalence Principle XRT X-ray telescope ZTF Zwicky Transient Facility Contents 1 Introduction............................................................................................................................... 2 Neutron star mergers................................................................................................................ 2.1 Overview.......................................................................................................................... 2.1.1 System formation .............................................................................................. 2.1.2 Inspiral ............................................................................................................... 2.1.3 Merger................................................................................................................ 2.1.4 Jets ..................................................................................................................... 2.1.5 Quasi-isotropic outflows ................................................................................... 2.1.6 Aftermath........................................................................................................... 2.2 Intrinsic event rates ........................................................................................................... 2.3 Gravitational waves ........................................................................................................... 2.4 Prompt gamma-ray bursts ................................................................................................. 2.5 Statistical association and joint searches.......................................................................... 2.6 Joint GW-GRB detection rates ......................................................................................... 2.7 Follow-up searches ............................................................................................................ 2.8 Gamma-ray burst afterglows ............................................................................................. 2.9 Kilonovae ........................................................................................................................... 2.10 Other signatures ................................................................................................................. 2.10.1 MeV neutrinos................................................................................................... 123 E. Burns 2.10.2 Other observed non-thermal signatures............................................................ 2.10.3 High-energy neutrinos....................................................................................... 2.10.4 Very-high energy electromagnetic detections.................................................. 2.10.5 Neutron precursors to kilonova and additional energy injection .................... 2.10.6 Late-time radio emission .................................................................................. 2.10.7 Gamma-ray detections of prompt kilonova and kilonova remnants ............... 2.11 Detections summary .......................................................................................................... 3 Astrophysical inferences ............................................................................................................ 3.1 Progenitor classification and the existence of neutron star–black hole systems ............ 3.2 The immediate remnant object in binary neutron star mergers....................................... 3.3 The time delay from merger to prompt gamma-ray burst emission ............................... 3.4 The origin of early ultraviolet emission ........................................................................... 3.5 Host galaxy, redshift, and where neutron star mergers occur ......................................... 4 Short gamma-ray bursts and ultrarelativistic jets...................................................................... 4.1 The progenitors of gamma-gay bursts .............................................................................. 4.2 The central engines of short gamma-ray bursts ............................................................... 4.3 Ultrarelativistic jet formation............................................................................................ 4.4 Propagation and structure.................................................................................................. 4.5 Gamma-ray burst jet composition and ultra high energy cosmic rays ........................... 4.6 The prompt emission mechanism(s) of gamma-ray bursts .............................................. 4.7 The origin of other non-thermal signatures...................................................................... 4.7.1 Short gamma-ray burst precursors.................................................................... 4.7.2 Extended emission and X-ray plateaus ............................................................ 4.7.3 X-ray flares in the afterglow ............................................................................ 4.7.4 Synchrotron self Compton ................................................................................ 5 Kilonovae and the origin of heavy elements ............................................................................ 5.1 Heavy element production in candidate r-process sites................................................... 5.2 On-going heavy element nucleosynthesis in the Milky Way.......................................... 5.3 The heavy element enrichment history of the Universe.................................................. 6 Standard sirens and cosmology.................................................................................................