Neutron star mergers have been long considered as promising sites of heavy r-process nucleosynthesis. We overview observational evidence supporting this scenario including: the total amount of r-process elements in the Galaxy, extreme metal poor stars, geological radioactive elemental abundances, dwarf galaxies, and short gamma-ray bursts (sGRBs). Recently, the advanced LIGO and Virgo observatories discovered a gravitational-wave signal of a neutron star merger, GW170817, as well as accompanying multi-wavelength electromagnetic (EM) counterparts. The ultra-violet, optical, and near infrared observations point to r-process elements that have been synthesized in the merger ejecta. The rate and ejected mass inferred from GW170817 and the EM counterparts are consistent with other observations. We find however that, within simple one zone chemical evolution models (based on merger rates with reasonable delay time distributions as expected from evolutionary models, or from observations of sGRBs), it is difficult to reconcile the current observations of the europium abundance history of Galactic stars for [Fe/H] −1. This implies that to account for the role of mergers in the Galactic chemical evolution, we need a Galactic model with multiple populations that have different spatial distributions and/or varying formation rates.

We consider a sample of ten GRBs with long lasting ( 10 2 sec) emission detected by Fermi/LAT and for which X-ray data around 1 day are also available. We assume that both the X-rays and the GeV emission are produced by electrons accelerated at the external forward shock, and show that the X-ray and the GeV fluxes lead to very different estimates of the initial kinetic energy of the blast wave. The energy estimated from GeV is on average ∼ 50 times larger than the one estimated from X-rays. We model the data (accounting also for optical detections around 1 day, if available) to unveil the reason for this discrepancy and find that good modelling within the forward shock model is always possible and leads to two possibilities: either the X-ray emitting electrons (unlike the GeV emitting electrons) are in the slow cooling regime or ii) the X-ray synchrotron flux is strongly suppressed by Compton cooling, whereas, due to the Klein-Nishina suppression, this effect is much smaller at GeV energies. In both cases the X-ray flux is no longer a robust proxy for the blast wave kinetic energy. On average, both cases require weak magnetic fields (10 −6 B 10 −3 ) and relatively large isotropic kinetic blast wave energies 10 53 erg < E 0,kin < 10 55 erg corresponding to large lower limits on the collimated energies, in the range 10 52 erg < E θ,kin < 5 × 10 52 erg for an ISM environment with n ∼ 1cm −3 and 10 52 erg < E θ,kin < 10 53 erg for a wind environment with A * ∼ 1. These energies are larger than those estimated from the Xray flux alone, and imply smaller inferred values of the prompt efficiency mechanism, reducing the efficiency requirements on the still uncertain mechanism responsible for prompt emission.

Rapidly spinning, strongly magnetized proto-neutron stars ("millisecond proto-magnetars") are candidate central engines of long-duration gamma-ray bursts (GRB), superluminous supernovae (SLSNe), and binary neutron star mergers. Magnetar birth may be accompanied by the fall-back of stellar debris, lasting for seconds or longer following the explosion. Accretion alters the magnetar evolution by (1) providing an additional source of rotational energy (or a potential sink, if the propeller mechanism operates); (2) enhancing the spin-down luminosity above the dipole rate by compressing the magnetosphere and expanding the polar cap region of open magnetic field lines; (3) supplying an additional accretion-powered neutrino luminosity that sustains the wind baryon-loading, even after the magnetar's internal neutrino luminosity has subsided. The more complex evolution of the jet power and magnetization of an accreting magnetar more readily accounts for the high 56 Ni yields GRB SNe and irregular time evolution of some GRB light curves (e.g. bursts with precursors followed by a long quiescent interval before the main emission episode). Additional baryon-loading from accretion-powered neurino irradiation of the polar cap lengthens the timeframe over which the jet magnetization is in the requisite range σ 10 3 for efficient gamma-ray emission, thereby accommodating GRBs with ultra-long durations. Though accretion does not significantly raise the maximum energy budget from the limit of f ew × 10 52 ergs for an isolated magnetar, it greatly expands the range of magnetic field strengths and birth spin periods capable of powering GRB jets, reducing the differences between the magnetar properties normally invoked to explain GRBs versus SLSNe.

Double Neutron Stars (DNS) have to survive two supernovae and still remain bound. This sets strong limits on the nature of the second collapse in these systems. We consider the masses and orbital parameters of the DNS population and constrain the two distributions of mass ejection and kick velocities directly from observations with no a-priori assumptions regarding evolutionary models and/or the types of the supernovae involved. We show that there is strong evidence for two distinct types of supernovae in these systems, where the second collapse in the majority of the observed systems involved small mass ejection (∆M 0.5M ) and a corresponding low-kick velocity (v k 30km s −1 ). This formation scenario is compatible, for example, with an electron capture supernova. Only a minority of the systems have formed via the standard SN scenario involving larger mass ejection of ∼ 2.2M and kick velocities of up to 400km s −1 . Due to the typically small kicks in most DNS (which are reflected by rather low proper motion), we predict that most of these systems reside close to the galactic disc. In particular, this implies that more NS-NS mergers occur close to the galactic plane. This may have non-trivial implications to the estimated merger rates of DNS and to the rate of LIGO / VIRGO detections.

A luminous radio burst was recently detected in temporal coincidence with a hard X-ray flare from the Galactic magnetar SGR 1935+2154with a time and frequency structure consistent with cosmological fast radio bursts (FRBs) and a fluence within a factor of 10 of the least energetic extragalactic FRB previously detected. Although active magnetars are commonly invoked FRB sources, several distinct mechanisms have been proposed for generating the radio emission that make different predictions for the accompanying higher-frequency radiation. We show that the properties of the coincident radio and X-ray flares from SGR 1935+2154, including their approximate simultaneity and relative fluence~-E E 10 radio X 5 , as well as the duration and spectrum of the X-ray emission, are consistent with extant predictions for the synchrotron maser shock model. Rather than arising from the inner magnetosphere, the X-rays are generated by (incoherent) synchrotron radiation from thermal electrons heated at the same internal shocks that produce the coherent maser emission as ultrarelativistic flare ejecta collides with a slower particle outflow (e.g., as generated by earlier flaring activity) on a radial scale of~10 11 cm. Although the rate of SGR 1935+2154-like bursts in the local universe is not sufficient to contribute appreciably to the extragalactic FRB rate, the inclusion of an additional population of more active magnetars with stronger magnetic fields than the Galactic population can explain both the FRB rate and the repeating fraction, but only if the population of active magnetars are born at a rate that is at least 2 orders of magnitude lower than that of the SGR 1935+2154-like magnetars. This may imply that the more active magnetar sources are not younger magnetars formed in a similar way to the Milky Way population (e.g., via ordinary supernovae) but are instead formed through more exotic channels, such as superluminous supernovae, accretion-induced collapse, or neutron star mergers.