Mergers of binary neutron stars and black hole-neutron star binaries are among the most promising sources for ground-based gravitational-wave (GW) detectors and are also high-energy astrophysical ...phenomena, as illustrated by the observations of GWs and electromagnetic (EM) waves in the event of GW170817. Mergers of these neutron star binaries are also the most promising sites for r-process nucleosynthesis. Numerical simulation in full general relativity (numerical relativity) is a unique approach to the theoretical prediction of the merger process, GWs emitted, mass ejection process, and resulting EM emission. We summarize the current understanding of the processes of neutron star mergers and subsequent mass ejection based on the results of the latest numerical-relativity simulations. We emphasize that the predictions of the numerical-relativity simulations agree broadly with the optical and IR observations of GW170817.
Mergers of binary neutron stars (NSs) are among the most promising gravitational wave (GW) sources. Next generation GW detectors are expected to detect signals from NS mergers within about 200 Mpc. ...The detection of electromagnetic wave (EM) counterparts is crucial to understanding the nature of GW sources. Among the possible EM emission from the NS merger, emission powered by radioactive r-process nuclei is one of the best targets for follow-up observations. However, predictions so far have not taken into account detailed r-process element abundances in the ejecta. We perform for the first time radiative transfer simulations of the NS merger ejecta including all the r-process elements from Ga to U. We show that the opacity of the NS merger ejecta is about kappa = 10 cm super(2) g super(-1), which is higher than that of Fe-rich Type Ia supernova ejecta by a factor of ~100. As a result, the emission is fainter and lasts longer than previously expected. The spectra are almost featureless due to the high expansion velocity and bound-bound transitions of many different r-process elements. We demonstrate that the emission is brighter for a higher mass ratio of the two NSs and a softer equation of state adopted in the merger simulations. Because of the red color of the emission, follow-up observations in red optical and near-infrared (NIR) wavelengths will be the most efficient. At 200 Mpc, the expected brightness of the emission is i = 22-25 AB mag, z = 21-23 AB mag, and 21-24 AB mag in the NIR JHK bands. Thus, observations with wide-field 4 m- and 8 m-class optical telescopes and wide-field NIR space telescopes are necessary. We also argue that the emission powered by radioactive energy can be detected in the afterglow of nearby short gamma-ray bursts.
We study the heating rate of r-process nuclei and thermalization of decay products in neutron star merger ejecta and macronova (kilonova) light curves. Thermalization of charged decay products, i.e., ...electrons, -particles, and fission fragments, is calculated according to their injection energy. The γ-ray thermalization processes are also properly calculated by taking the γ-ray spectrum of each decay into account. We show that the β-decay heating rate at later times approaches a power-law decline as ∝t−2.8, which agrees with the result of Waxman et al. We present a new analytic model to calculate macronova light curves, in which the density structure of the ejecta is accounted for. We demonstrate that the observed bolometric light curve and temperature evolution of the macronova associated with GW170817 are reproduced well by the β-decay heating rate with the solar r-process abundance pattern. We interpret the break in the observed bolometric light curve around a week as a result of the diffusion wave crossing a significant part of the ejecta rather than a thermalization break. We also show that the time-weighted integral of the bolometric light curve (Katz integral) is useful to provide an estimate of the total r-process mass from the observed data, which is independent of the highly uncertain radiative transfer. For the macronova in GW170817, the ejecta mass is robustly estimated as 0.05 M for Amin ≤ 72 and 85 ≤ Amin ≤ 130 with the solar r-process abundance pattern. The code for computation of the heating rate and light curve for given initial nuclear abundances is publicly available.
In addition to producing a strong gravitational signal, a short gamma-ray burst (GRB), and a compact remnant, neutron star mergers eject significant masses (up to a few per cent of M⊙) at significant ...kinetic energies. The different components of the ejected mass include a dynamical ejected mass, a GRB jet and also a shock-breakout material, a cocoon resulting from the interaction of the jet with other ejecta, and viscous- and neutrino-driven winds. The interaction of these ejecta with the surrounding interstellar medium will produce a long-lasting radio flare. We estimate here the expected radio flares arising from these outflows. The flares are rather weak and uncertainties in the kinetic energy, the velocity, and the external density make exact estimates of these signals difficult. The relative strength of the different signals depends strongly on the viewing angle. An observer along the jet axis or close to it will detect a strong signal at a few dozen days from the radio afterglow (or the orphan radio afterglow) produced by the highly relativistic GRB jet. A generic observer at larger viewing angles will generally observe the dynamical ejecta, whose contribution peaks a year or so after the event. Depending on the observed frequency and the external density, other components may also give rise to a significant contribution. If the short GRB 130603B was a merger event, its radio flare from the dynamical ejecta might be detectable with the EVLA and the LOFAR for the higher range of external densities n ≳ 0.5 cm−3
We explore the implications of the low-spin components along the orbital axis observed in an Advanced LIGO O1 run on binary black hole (BBH) merger scenarios in which the merging BBHs have evolved ...from field binaries. The coalescence time determines the initial orbital separation of BBHs. This, in turn, determines whether the stars are synchronized before collapse, and hence determines their projected spins. Short coalescence times imply synchronization and large spins. Among known stellar objects, Wolf-Rayet (WR) stars seem to be the only progenitors consistent with the low aligned spins observed in LIGO's O1, provided that the orbital axis maintains its direction during the collapse. We calculate the spin distribution of BBH mergers in the local universe, and its redshift evolution for WR progenitors. Assuming that the BBH formation rate peaks around a redshift of ∼2-3, we show that BBH mergers in the local universe are dominated by low-spin events. The high-spin population starts to dominate at a redshift of ∼0.5-1.5. WR stars are also progenitors of long gamma-ray bursts that take place at a comparable rate to BBH mergers. We discuss the possible connection between the two phenomena. Additionally, we show that hypothetical Population III star progenitors are also possible. Although WR and Population III progenitors are consistent with the current data, both models predict a non-vanishing fraction of high positive values of the BBHs' aligned spin. If those are not detected within the coming LIGO/Virgo runs, it will be unlikely that the observed BBHs formed via field binaries.
We find, using high-resolution numerical relativistic simulations, that the tail of the dynamical ejecta of neutron star mergers extends to mildly relativistic velocities faster than 0.7c. The ...kinetic energy of this fast tail is ∼1047-1049 erg, depending on the neutron star equation of state and on the binary masses. The synchrotron flare arising from the interaction of this fast tail with the surrounding interstellar medium (ISM) can power the observed nonthermal emission that followed GW170817, provided that the ISM density is , the two neutron stars had roughly equal masses and the neutron star equation of state is soft (small neutron star radii). One of the generic predictions of this scenario is that the cooling frequency crosses the X-ray band on a timescale of a few months to a year, leading to a cooling break in the X-ray light curve. While the recent observation of the superluminal motion resolved by very long baseline interferometry (VLBI) rules out the dynamical ejecta scenario, the model described in this paper is generic and can be applied for future neutron star merger events.
ABSTRACT
We study turbulent gas diffusion affects on r-process abundances in Milky Way stars, by a combination of an analytical approach and a Monte Carlo simulation. Higher r-process event rates and ...faster diffusion, lead to more efficient mixing corresponding to a reduced scatter of r-process abundances and causing r-process enriched stars to start appearing at lower metallicities. We use three independent observations to constrain the model parameters: (i) the scatter of radioactively stable r-process element abundances, (ii) the largest r-process enrichment values observed in any solar neighborhood stars, and (iii) the isotope abundance ratios of different radioactive r-process elements (244Pu/238U and 247Cm/238U) at the early Solar system as compared to their formation. Our results indicate that the Galactic r-process rate and the diffusion coefficient are respectively r < 4 × 10−5 yr−1, D > 0.1 kpc2 Gyr−1 (r < 4 × 10−6 yr−1, D > 0.5 kpc2 Gyr−1 for collapsars or similarly prolific r-process sources) with allowed values satisfying an approximate anticorrelation such that D ≈ r−2/3, implying that the time between two r-process events that enrich the same location in the Galaxy, is τmix ≈ 100−200 Myr. This suggests that a fraction of ∼0.8 (∼0.5) of the observed 247Cm (244Pu) abundance is dominated by one r-process event in the early Solar system. Radioactively stable element abundances are dominated by contributions from ∼10 different events in the early Solar system. For metal poor stars (with Fe/H ≲ −2), their r-process abundances are dominated by either a single or several events, depending on the star formation history.
ABSTRACT
The nebular phase of lanthanide-rich ejecta of a neutron star merger (NSM) is studied by using a one-zone model, in which the atomic properties a represented by a single species, neodymium ...(Nd). Under the assumption that β-decay of r-process nuclei is the heat and ionization source, we solve the ionization and thermal balance of the ejecta under non-local thermodynamic equilibrium. The atomic data including energy levels, radiative transition rates, collision strengths, and recombination rate coefficients are obtained by using atomic structure codes, grasp2k
and hullac. We find that both permitted and forbidden lines roughly equally contribute to the cooling rate of Nd ii and Nd iii at the nebular temperatures. We show that the kinetic temperature and ionization degree increase with time in the early stage of the nebular phase, while these quantities become approximately independent of time after the thermalization break of the heating rate because the processes relevant to the ionization and thermalization balance are attributed to two-body collision between electrons and ions at later times. As a result, in spite of the rapid decline of the luminosity, the shape of the emergent spectrum does not change significantly with time after the break. We show that the emission-line nebular spectrum of the pure Nd ejecta consists of a broad structure from $0.5$ to $20\, {\rm \mu m}$ with two distinct peaks around $1$ and $10\, {\rm \mu m}$.
Abstract
Theory and observations suggest that single-star evolution is not able to produce black holes with masses in the range 3–5
M
⊙
and above ∼45
M
⊙
, referred to as the lower mass gap and the ...upper mass gap, respectively. However, it is possible to form black holes in these gaps through mergers of compact objects in, e.g., dense clusters. This implies that if binary mergers are observed in gravitational waves with at least one mass-gap object, then either clusters are effective in assembling binary mergers, or our single-star models have to be revised. Understanding how effective clusters are at populating both mass gaps have therefore major implications for both stellar and gravitational wave astrophysics. In this paper we present a systematic study of how efficient stellar clusters are at populating both mass gaps through in-cluster mergers. For this, we derive a set of closed form relations for describing the evolution of compact object binaries undergoing dynamical interactions and mergers inside their cluster. By considering both static and time-evolving populations, we find in particular that globular clusters are clearly inefficient at populating the lower mass gap in contrast to the upper mass gap. We further describe how these results relate to the characteristic mass, time, and length scales associated with the problem.
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