ABSTRACT
The connection between short gamma-ray bursts (SGRBs) and binary neutron star (BNS) mergers was recently confirmed by the association of GRB 170817A with the merger event GW170817. However, ...no conclusive indications were obtained on whether the merger remnant that powered the SGRB jet was an accreting black hole (BH) or a long-lived massive neutron star (NS). Here, we explore the latter case via BNS merger simulations covering up to 250 ms after merger. We report, for the first time in a full merger simulation, the formation of a magnetically driven collimated outflow along the spin axis of the NS remnant. For the system at hand, the properties of such an outflow are found largely incompatible with an SGRB jet. With due consideration of the limitations and caveats of our present investigation, our results favour a BH origin for GRB 170817A and SGRBs in general. Even though this conclusion needs to be confirmed by exploring a larger variety of physical conditions, we briefly discuss possible consequences of all SGRB jets being powered by accreting BHs.
The first multimessenger observation of a binary neutron star (BNS) merger in August 2017 demonstrated the huge scientific potential of these extraordinary events. This breakthrough led to a number ...of discoveries and provided the best evidence that BNS mergers can launch short gamma-ray burst (SGRB) jets and are responsible for a copious production of heavy r-process elements. On the other hand, the details of the merger and post-merger dynamics remain only poorly constrained, leaving behind important open questions. Numerical relativity simulations are a powerful tool to unveil the physical processes at work in a BNS merger and as such they offer the best chance to improve our ability to interpret the corresponding gravitational wave (GW) and electromagnetic emission. Here, we review the current theoretical investigation on BNS mergers based on general relativistic magnetohydrodynamics simulations, paying special attention to the magnetic field as a crucial ingredient. First, we discuss the evolution, amplification, and emerging structure of magnetic fields in BNS mergers. Then, we consider their impact on various critical aspects: (i) jet formation and the connection with SGRBs, (ii) matter ejection, r-process nucleosynthesis, and radioactively-powered kilonova transients, and (iii) post-merger GW emission.
ABSTRACT X-ray flashes (XRFs) are a class of high-energy transients whose nature is still open to question. Similar in many aspects to common gamma-ray bursts (GRBs), their strong X-ray emission is ...accompanied by very low or absent emission in the gamma-ray band. Despite this key difference, a number of indications have consolidated the idea that XRFs and GRBs share a common origin, including a number of potential XRF/supernova associations and the consistency of some XRFs with the Amati relation for long GRBs. However, the difficulties in explaining XRFs as off-axis or intrinsically weak GRBs still cast doubts on this interpretation. Here we explore the possibility that some XRFs are instead powered by the spindown of a long-lived neutron star (NS) formed in a binary NS (BNS) merger or, possibly, in a core-collapse supernova. Focusing on XRF 020903 and a few other cases observed by HETE-2, we show that their lack of gamma-ray emission, spectral properties, duration and X-ray luminosity find a natural explanation within our hypothesis. Moreover, we point out that the agreement of XRF 020903 with the Amati and Ghirlanda relations for long GRBs is respectively only marginal and problematic. Assuming a BNS merger origin for the long-lived NS, we use XRF observations to estimate a lower limit on the rate of BNS mergers accompanied by a potentially observable XRF signal. Within the reach of the advanced LIGO and Virgo gravitational wave detectors, we find . Finally, we discuss the implications of a supernova association for the XRF events considered.
The observation of a radioactively powered kilonova associated with the first binary neutron star (BNS) merger detected in gravitational waves proved that these events are ideal sites for the ...production of heavy r-process elements. However, the physical origin of the ejected material responsible for the early ("blue") and late ("red") components of this kilonova is still debated. Here, we investigate the possibility that the early/blue kilonova originated from the magnetically driven baryon wind launched after merger by the metastable neutron star remnant. Exploiting a magnetized BNS merger simulation with over 250 ms of post-merger evolution, we can follow for the first time the full mass-ejection process up to its final decline. We find that the baryon wind carries 0.010-0.028 M of unbound material, proving that the high mass estimated for the blue kilonova can be achieved. We also find expansion velocities of up to ∼0.2c, consistent with the lower end of the observational estimates, and we discuss possible effects neglected here that could further increase the final ejecta velocity. Overall, our results show that the magnetically driven baryon wind represents a viable channel to explain the blue kilonova.
The first combined detection of gravitational waves and electromagnetic signals from a binary neutron star (BNS) merger in August 2017 (an event named GW170817) represents a major landmark in the ...ongoing investigation of these extraordinary systems. In this short review, we discuss BNS mergers as events of utmost importance for astrophysics and fundamental physics and survey the main discoveries enabled by this first multimessenger observation, including compelling evidence that such mergers produce a copious amount of heavy r-process elements and can power short gamma-ray bursts. We further discuss some remaining key open questions regarding this event and BNS mergers in general, focusing on the current status and limitations of theoretical models and numerical simulations.
ABSTRACT Binary neutron star (BNS) mergers are the leading model to explain the phenomenology of short gamma-ray bursts (SGRBs). 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. The model presented here also allows us to search for suitable EM counterparts for multimessenger astronomy, which is expected to become reality within the next few years thanks to ground-based GW detectors such as advanced LIGO and Virgo. This paper discusses the formulation and implementation of the model. In a companion paper, we employ this model to predict the EM emission from to after a BNS merger and discuss the implications in the context of SGRBs and multimessenger astronomy.
ABSTRACT 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 thanks to the ground-based advanced LIGO/Virgo GW detector network. 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 up to ∼107 s. Light curves and spectra are computed for a wide range of post-merger physical properties. We present X-ray afterglow light curves corresponding to the "standard" and the "time-reversal" scenario for SGRBs (prompt emission associated with the merger or with the collapse of the long-lived NS). The light curve morphologies include single and two-plateau features with timescales and luminosities that are in good agreement with Swift observations. Furthermore, we compute the X-ray signal that should precede the SGRB in the time-reversal scenario, the detection of which would represent smoking-gun evidence for this scenario. Finally, we find a bright, highly isotropic EM transient peaking in the X-ray band at ∼102-104 s after the BNS merger with luminosities of LX ∼ 1046-1048 erg s−1. This signal represents a very promising EM counterpart to the GW emission from BNS mergers.
Modern simulation codes for general relativistic ideal magnetohydrodynamics are all facing a long-standing technical problem given by the need to recover fundamental variables from those variables ...that are evolved in time. In the relativistic case, this requires the numerical solution of a system of nonlinear equations. Although several approaches are available, none has proven completely reliable. A recent study comparing different methods showed that all can fail, in particular for the important case of strong magnetization and moderate Lorentz factors. Here, we propose a new robust, efficient, and accurate solution scheme, along with a proof for the existence and uniqueness of a solution, and analytic bounds for the accuracy. Further, the scheme allows us to reliably detect evolution errors leading to unphysical states and automatically applies corrections for typical harmless cases. A reference implementation of the method is made publicly available as a software library. The aim of this library is to improve the reliability of binary neutron star merger simulations, in particular in the investigation of jet formation and magnetically driven winds.
Short gamma-ray bursts (SGRBs) are among the most luminous explosions in the universe and their origin still remains uncertain. Observational evidence favors the association with binary neutron star ...or neutron star-black hole (NS-BH) binary mergers. Leading models relate SGRBs to a relativistic jet launched by the BH-torus system resulting from the merger. However, recent observations have revealed a large fraction of SGRB events accompanied by X-ray afterglows with durations ~10 super(2)-10 super(5) s, suggesting continuous energy injection from a long-lived central engine, which is incompatible with the short (<, ~1 s) accretion timescale of a BH-torus system. The formation of a supramassive NS, resisting the collapse on much longer spin-down timescales, can explain these afterglow durations, but leaves serious doubts on whether a relativistic jet can be launched at the merger. Here we present a novel scenario accommodating both aspects, where the SGRB is produced after the collapse of a supramassive NS. Early differential rotation and subsequent spin-down emission generate an optically thick environment around the NS consisting of a photon-pair nebula and an outer shell of baryon-loaded ejecta. While the jet easily drills through this environment, spin-down radiation diffuses outward on much longer timescales and accumulates a delay that allows the SGRB to be observed before (part of) the long-lasting X-ray signal. By analyzing diffusion timescales for a wide range of physical parameters, we find delays that can generally reach ~10 super(5) s, compatible with observations. The success of this fundamental test makes this "time-reversal" scenario an attractive alternative to current SGRB models.
Merging binary neutron stars (BNSs) represent the ultimate targets for multimessenger astronomy, being among the most promising sources of gravitational waves (GWs), and, at the same time, likely ...accompanied by a variety of electromagnetic counterparts across the entire spectrum, possibly including short gamma-ray bursts (SGRBs) and kilonova/macronova transients. Numerical relativity simulations play a central role in the study of these events. In particular, given the importance of magnetic fields, various aspects of this investigation require general relativistic magnetohydrodynamics (GRMHD). So far, most GRMHD simulations focused the attention on BNS mergers leading to the formation of a hypermassive neutron star (NS), which, in turn, collapses within few tens of ms into a black hole surrounded by an accretion disk. However, recent observations suggest that a significant fraction of these systems could form a long-lived NS remnant, which will either collapse on much longer time scales or remain indefinitely stable. Despite the profound implications for the evolution and the emission properties of the system, a detailed investigation of this alternative evolution channel is still missing. Here, we follow this direction and present a first detailed GRMHD study of BNS mergers forming a long-lived NS. We consider magnetized binaries with different mass ratios and equations of state and analyze the structure of the NS remnants, the rotation profiles, the accretion disks, the evolution and amplification of magnetic fields, and the ejection of matter. Moreover, we discuss the connection with the central engine of SGRBs and provide order-of-magnitude estimates for the kilonova/macronova signal. Finally, we study the GW emission, with particular attention to the post-merger phase.
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