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

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 ..

Modelling the Magnetic Field Configuration of Neutron Stars

The properties of the extremely strong magnetic fields of neutron stars affect in a unique way their evolution and the associated phenomenology. Due to the lack of constraints from direct observations, our understanding of the magnetic field configuration in neutron star interiors depends on the progress in theoretical modelling. Here we discuss the effort in building models of magnetized neutron stars focussing on some of the recent results. In particular, we comment on the instability of purely poloidal and purely toroidal magnetic field configurations and on the evidence in favour of the so-called twisted-torus solutions. We conclude with an outlook on the present status of the field and future directions

Identification of some o-aminophenones as secondary metabolites of Saccharomyces cerevisiae

Research NoteDuring fermentation of a peculiar model medium a strain of Saccharomyces cerevisiae var. cerevisiae (S1C) yeast from our collection was able to produce o-aminoacetophenone as well as other metabolites tentatively identified as o-aminopropiophenone and 3-(o-aminophenyl)-prop-1-en-3-one

Structure and deformations of strongly magnetized neutron stars with twisted torus configurations

We construct general relativistic models of stationary, strongly magnetized neutron stars. The magnetic field configuration, obtained by solving the relativistic Grad-Shafranov equation, is a generalization of the twisted torus model recently proposed in the literature; the stellar deformations induced by the magnetic field are computed by solving the perturbed Einstein's equations; stellar matter is modeled using realistic equations of state. We find that in these configurations the poloidal field dominates over the toroidal field and that, if the magnetic field is sufficiently strong during the first phases of the stellar life, it can produce large deformations.Comment: 10 pages, 5 figures. Minor changes to match the version published on MNRA

Short gamma-ray bursts from binary neutron star mergers: the time-reversal scenario

After decades of observations the physical mechanisms that generate short gamma-ray bursts (SGRBs) still remain unclear. Observational evidence provides support to the idea that SGRBs originate from the merger of compact binaries, consisting of two neutron stars (NSs) or a NS and a black hole (BH). Theoretical models and numerical simulations seem to converge to an explanation in which the central engine of SGRBs is given by a spinning BH surrounded by a hot accretion torus. Such a BH-torus system can be formed in compact binary mergers and is able to launch a relativistic jet, which can then produce the SGRB. This basic scenario, however, has recently been challenged by Swift satellite observations, which have revealed long-lasting X-ray afterglows in association with a large fraction of SGRB events. The long durations of these afterglows (from minutes to several hours) cannot be explained by the $\sim\text{s}$ accretion timescale of the torus onto the BH, and, instead, suggest a long-lived NS as the persistent source of radiation. Yet, if the merger results in a massive NS the conditions to generate a relativistic jet and thus the prompt SGRB emission are hardly met. Here we consider an alternative scenario that can reconcile the two aspects and account for both the prompt and the X-ray afterglow emission. Implications for future observations, multi-messenger astronomy and for constraining NS properties are discussed, as well as potential challenges for the model

Implementing a new recovery scheme for primitive variables in the general relativistic magnetohydrodynamic code Spritz

General relativistic magnetohydrodynamic (GRMHD) simulations represent a fundamental tool to probe various underlying mechanisms at play during binary neutron star (BNS) and neutron star (NS) - black hole (BH) mergers. Contemporary flux-conservative GRMHD codes numerically evolve a set of conservative equations based on `conserved' variables which then need to be converted back into the fundamental (`primitive') variables. The corresponding conservative-to-primitive variable recovery procedure, based on root-finding algorithms, constitutes one of the core elements of such GRMHD codes. Recently, a new robust, accurate and efficient recovery scheme called RePrimAnd was introduced, which has demonstrated the ability to always converge to a unique solution. The scheme provides fine-grained error policies to handle invalid states caused by evolution errors, and also provides analytical bounds for the error of all primitive variables. In this work, we describe the technical aspects of implementing the RePrimAnd scheme into the GRMHD code Spritz. To check our implementation as well as to assess the various features of the scheme, we perform a number of GRMHD tests in three dimensions. Our tests, which include critical cases such as a NS collapse to a BH as well as the early evolution (~50 ms) of a Fishbone-Moncrief BH-accrection disk system, show that RePrimAnd is able to support magnetized, low density environments with magnetic-to-fluid pressure ratios as high as 10^4, in situations where the previously used recovery scheme fails

Relativistic models of magnetars: the twisted-torus magnetic field configuration

We find general relativistic solutions of equilibrium magnetic field configurations in magnetars, extending previous results of Colaiuda et al. (2008). Our method is based on the solution of the relativistic Grad-Shafranov equation, to which Maxwell's equations can be reduced in some limit. We obtain equilibrium solutions with the toroidal magnetic field component confined into a finite region inside the star, and the poloidal component extending to the exterior. These so-called twisted-torus configurations have been found to be the final outcome of dynamical simulations in the framework of Newtonian gravity, and appear to be more stable than other configurations. The solutions include higher order multipoles, which are coupled to the dominant dipolar field. We use arguments of minimal energy to constrain the ratio of the toroidal to the poloidal field.Comment: 13 pages, 12 figures. Minor changes to match the version published on MNRA