GW170817−-the first observed neutron star merger and its kilonova: implications for the astrophysical site of the r-process

The first neutron star (NS) merger observed by advanced LIGO and Virgo, GW170817, and its fireworks of electromagnetic counterparts across the entire electromagnetic spectrum marked the beginning of multi-messenger astronomy and astrophysics with gravitational waves. The ultraviolet, optical, and near-infrared emission was consistent with being powered by the radioactive decay of nuclei synthesized in the merger ejecta by the rapid neutron capture process (r-process). Starting from an outline of the inferred properties of this 'kilonova' emission, I discuss possible astrophysical sites for r-process nucleosynthesis in NS mergers, arguing that the heaviest r-process elements synthesized in this event most likely originated in outflows from a post-merger accretion disk. I compare the inferred properties of r-process element production in GW170817 to current observational constraints on galactic heavy r-process nucleosynthesis and discuss challenges merger-only models face in explaining the r-process content of our galaxy. Based on the observational properties of GW170817 and recent theoretical progress on r-process nucleosynthesis in collapsars, I then show how GW170817 points to collapsars as the dominant source of r-process enrichment in the Milky Way. These rare core-collapse events arguably better satisfy existing constraints and overcome problems related to r-process enrichment in various environments that NS mergers face. Finally, I comment on the universality of the r-process and on how variations in light r-process elements can be obtained both in NS mergers and collapsars.Comment: 16 pages, 8 figures. Invited contribution to the EPJA Topical Issue "First joint gravitational wave and electromagnetic observations: Implications for nuclear and particle physics

Magnetic field amplification in hypermassive neutron stars via the magnetorotational instability

Mergers of binary neutron stars likely lead to the formation of a hypermassive neutron star (HMNS), which is metastable and eventually collapses to a black hole. This merger scenario is thought to explain the phenomenology of short gamma-ray bursts (SGRBs). The very high energies observed in SGRBs have been suggested to stem from neutrino-antineutrino annihilation and/or from very strong magnetic fields created during or after the merger by mechanisms like the magnetorotational instability (MRI). Here, we report on results that show for the first time the development of the MRI in HMNSs in three-dimensional, fully general-relativistic magnetohydrodynamic simulations. This instability amplifies magnetic fields exponentially and could be a vital ingredient in solving the SGRB puzzle.Comment: 6 pages, 3 figures. Proceedings of the Karl Schwarzschild Meeting 201

An upper bound from helioseismology on the stochastic background of gravitational waves

The universe is expected to be permeated by a stochastic background of gravitational radiation of astrophysical and cosmological origin. This background is capable of exciting oscillations in solar-like stars. Here we show that solar-like oscillators can be employed as giant hydrodynamical detectors for such a background in the muHz to mHz frequency range, which has remained essentially unexplored until today. We demonstrate this approach by using high-precision radial velocity data for the Sun to constrain the normalized energy density of the stochastic gravitational-wave background around 0.11 mHz. These results open up the possibility for asteroseismic missions like CoRoT and Kepler to probe fundamental physics.Comment: 6 pages, 2 figures. Updated to match published versio

Electromagnetic emission from long-lived binary neutron star merger remnants II: lightcurves and spectra

Recent observations indicate that in a large fraction of binary neutron star (BNS) mergers a long-lived neutron star (NS) may be formed rather than a black hole. Unambiguous electromagnetic (EM) signatures of such a scenario would strongly impact our knowledge on how short gamma-ray bursts (SGRBs) and their afterglow radiation are generated. Furthermore, such EM signals would have profound implications for multimessenger astronomy with joint EM and gravitational-wave (GW) observations of BNS mergers, which will soon become reality with the ground-based advanced LIGO/Virgo GW detector network starting its first science run this year. Here we explore such EM signatures based on the model presented in a companion paper, which provides a self-consistent evolution of the post-merger system and its EM emission starting from an early baryonic wind phase and resulting in a final pulsar wind nebula that is confined by the previously ejected material. Lightcurves and spectra are computed for a wide range of post-merger physical properties and particular attention is paid to the emission in the X-ray band. In the context of SGRB afterglow modeling, we present X-ray lightcurves corresponding to the 'standard' and the recently proposed 'time-reversal' scenario (SGRB prompt emission produced at the time of merger or at the time of collapse of the long-lived NS). The resulting afterglow lightcurve morphologies include, in particular, single and two-plateau features with timescales and luminosities that are in good agreement with the observations by the Swift satellite. Furthermore, we compute the X-ray signal that should precede the SGRB in the time-reversal scenario. If found, such a signal would represent smoking-gun evidence for this scenario. Finally, we find a bright, highly isotropic EM transient signal peaking in the X-ray band ...Comment: 20 pages, 16 figure

Electromagnetic emission from long-lived binary neutron star merger remnants I: formulation of the problem

Binary neutron star (BNS) mergers are the leading model to explain the phenomenology of short gamma-ray bursts (SGRBs), which are among the most luminous explosions in the universe. Recent observations of long-lasting X-ray afterglows of SGRBs challenge standard paradigms and indicate that in a large fraction of events a long-lived neutron star (NS) may be formed rather than a black hole. Understanding the mechanisms underlying these afterglows is necessary in order to address the open questions concerning the nature of SGRB central engines. However, recent theoretical progress has been hampered by the fact that the timescales of interest for the afterglow emission are inaccessible to numerical relativity simulations. Here we present a detailed model to bridge the gap between numerical simulations of the merger process and the relevant timescales for the afterglows, assuming that the merger results in a long-lived NS. This model is formulated in terms of a set of coupled differential equations that follow the evolution of the post-merger system and predict its electromagnetic (EM) emission in a self-consistent way, starting from initial data that can be extracted from BNS merger simulations and taking into account the most relevant radiative processes. Moreover, the model can accomodate the collapse of the remnant NS at any time during the evolution as well as different scenarios for the prompt SGRB emission. A second major reason of interest for BNS mergers is that they are considered the most promising source of gravitational waves (GWs) for detection with the advanced ground-based detector network LIGO/Virgo coming online this year. Multimessenger astronomy with joint EM and GW observations of the merger and post-merger phase can greatly enhance the scientific output of either type of observation. However, the actual benefit depends on ...Comment: 27 pages, 3 figures, 4 appendice

Magnetically-induced outflows from binary neutron star merger remnants

Recent observations by the Swift satellite have revealed long-lasting ($\sim 10^2-10^5\,\mathrm{s}$), "plateau-like" X-ray afterglows in the vast majority of short gamma-ray bursts events. This has put forward the idea of a long-lived millisecond magnetar central engine being generated in a binary neutron star (BNS) merger and being responsible for the sustained energy injection over these timescales ("magnetar model"). We elaborate here on recent simulations that investigate the early evolution of such a merger remnant in general-relativistic magnetohydrodynamics. These simulations reveal very different conditions than those usually assumed for dipole spin-down emission in the magnetar model. In particular, the surrounding of the newly formed NS is polluted by baryons due to a dense, highly magnetized and isotropic wind from the stellar surface that is induced by magnetic field amplification in the interior of the star. The timescales and luminosities of this wind are compatible with early X-ray afterglows, such as the "extended emission". These isotropic winds are a generic feature of BNS merger remnants and thus represent an attractive alternative to current models of early X-ray afterglows. Further implications to BNS mergers and short gamma-ray bursts are discussed.Comment: 4 pages, 2 figures. To appear in proceedings of "Swift: 10 Years of Discovery

The Value of Waiting to Invest

This paper studies the optimal timing of investment in an irreversible project where the benefits from the project and the investment cost follow continuous-time stochastic processes. The optimal time to invest and an explicit formula for the value of the option to invest are derived. The rule "invest if benefits exceed costs" does not properly account for the option value of waiting.Simulations show that this option value can be significant, and that for surprisingly reasonable parameter values it may be optimal to wait until benefits are twice the investment cost. Finally, we perform comparative static analysis on the valuation formula and on the rule for when to invest.