Riccardo Ciolfi

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. Our method is based on the solution of the relativistic Grad-Shafranov equation, to which Maxwells equations can be reduced. 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.

Twisted-torus configurations with large toroidal magnetic fields in relativistic stars

Understanding the properties of the internal magnetic field of neutron stars remains a theoretical challenge. Over the last years, twisted-torus geometries have been considered both in Newtonian and general-relativistic equilibrium models, as they represent a potentially good description of neutron star interiors. All of these works have found an apparent intrinsic limitation to geometries that are poloidal-field-dominated, with a toroidal-to-poloidal energy ratio inside the star that are <10%, unless surface currents are included and magnetic fields are allowed to be discontinuous. This limitation is in stark contrast with the general expectation that much higher toroidal fields should be present in the stellar interior and casts doubt about the stability and hence realism of these configurations. We here discuss how to overcome this limitation by adopting a new prescription for the azimuthal currents that leads to magnetized equilibria where the toroidal-to-total magnetic-field energy ratio can be as high as 90%, thus including geometries that are toroidal-field-dominated. Moreover, our results show that for a fixed exterior magnetic-field strength, a higher toroidal-field energy implies a much higher total magnetic energy stored in the star, with a potentially strong impact on the expected electromagnetic and gravitational-wave emission from highly magnetized neutron stars.

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 GradShafranov 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.

Late time afterglow observations reveal a collimated relativistic jet in the ejecta of the binary neutron star merger GW170817

The binary neutron star (BNS) merger GW170817 was the first astrophysical source detected in gravitational waves and multiwavelength electromagnetic radiation. The almost simultaneous observation of a pulse of gamma rays proved that BNS mergers are associated with at least some short gamma-ray bursts (GRBs). However, the gamma-ray pulse was faint, casting doubt on the association of BNS mergers with the luminous, highly relativistic outflows of canonical short GRBs. Here we show that structured jets with a relativistic, energetic core surrounded by slower and less energetic wings produce afterglow emission that brightens characteristically with time, as recently seen in the afterglow of GW170817. Initially, we only see the relatively slow material moving towards us. As time passes, larger and larger sections of the outflow become visible, increasing the luminosity of the afterglow. The late appearance and increasing brightness of the multiwavelength afterglow of GW170817 allow us to constrain the geometry of its ejecta and thus reveal the presence of an off-axis jet pointing about 30° away from Earth. Our results confirm a single origin for BNS mergers and short GRBs: GW170817 produced a structured outflow with a highly relativistic core and a canonical short GRB. We did not see the bright burst because it was beamed away from Earth. However, approximately one in 20 mergers detected in gravitational waves will be accompanied by a bright, canonical short GRB.

SHORT GAMMA-RAY BURSTS IN THE ''TIME-REVERSAL'' SCENARIO

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 2 –10 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 5 s, compatible with observations. The success of this fundamental test makes this “time-reversal” scenario an attractive alternative to current SGRB models.

INSTABILITY-DRIVEN EVOLUTION OF POLOIDAL MAGNETIC FIELDS IN RELATIVISTIC STARS

The problem of the stability of magnetic fields in stars has a long history and has been investigated in detail in perturbation theory. Here, we consider the nonlinear evolution of a nonrotating neutron star with a purely poloidal magnetic field, in general relativity. We find that an instability develops in the region of the closed magnetic field lines and over an Alfven timescale, as predicted by perturbation theory. After the initial unstable growth, our evolutions show that a toroidal magnetic field component is generated, which increases until it is locally comparable in strength with the poloidal one. On longer timescales the system relaxes to a new non-axisymmetric configuration with a reorganization of the stellar structure and large-amplitude oscillations, mostly in the fundamental mode. We discuss the energies involved in the instability and the impact they may have on the phenomenology of magnetar flares and on their detectability through gravitational-wave emission.

Poloidal-field Instability in Magnetized Relativistic Stars

We investigate the instability of purely poloidal magnetic fields in nonrotating neutron stars (NSs) by means of three-dimensional general-relativistic magnetohydrodynamics simulations, extending the work presented by Ciolfi et al. in 2011. Our aim is to draw a clear picture of the dynamics associated with the instability and to study the final configuration reached by the system, thus obtaining indications on possible equilibria in a magnetized NS. Furthermore, since the internal rearrangement of magnetic fields is a highly dynamical process and has been suggested to be behind magnetar giant flares, our simulations can provide a realistic estimate of the electromagnetic and gravitational-wave (GW) emission that should accompany the flare event. Our main findings are the following: (1) the initial development of the instability meets all the expectations of perturbative studies in terms of the location of the seed of the instability, the timescale for its growth, and the generation of a toroidal component; (2) in the subsequent nonlinear reorganization of the system, ~90% of magnetic energy is lost in few Alfven timescales mainly through electromagnetic emission, and further decreases on a much longer timescale; (3) all stellar models tend to achieve a significant amount of magnetic helicity and the equipartition of energy between poloidal and toroidal magnetic fields and evolve to a new configuration that does not show a subsequent instability on dynamical or Alfven timescales; (4) the electromagnetic emission matches the duration of the initial burst in luminosity observed in giant flares, giving support to the internal rearrangement scenario; and (5) only a small fraction of the energy released during the process is converted into f-mode oscillations and in the consequent GW emission, thus resulting in very low chances of detecting this signal with present and near-future ground-based detectors.We study the spatial and temporal behavior of fluid in fully three-dimensional, general relativistic, magnetohydrodynamical simulations of both tilted and untilted black hole accretion flows. We uncover characteristically greater variability in tilted simulations at frequencies similar to those predicted by the formalism of trapped modes, but ultimately conclude that its spatial structure is inconsistent with a modal interpretation. We find instead that previously identified, transient, over-dense clumps orbiting on roughly Keplerian trajectories appear generically in our global simulations, independent of tilt. Associated with these fluctuations are acoustic spiral waves interior to the orbits of the clumps. We show that the two nonaxisymmetric standing shock structures that exist in the inner regions of these tilted flows effectively amplify the variability caused by these spiral waves to markedly higher levels than in untilted flows, which lack standing shocks. Our identification of clumps, spirals, and spiral-shock interactions in these fully general relativistic, magnetohydrodynamical simulations suggests that these features may be important dynamical elements in models which incorporate tilt as a way to explain the observed variability in black hole accretion flows.

Magnetorotational instability in relativistic hypermassive neutron stars

A differentially rotating hypermassive neutron star (HMNS) is a metastable object which can be formed in the merger of neutron-star binaries. The eventual collapse of the HMNS into a black hole is a key element in generating the physical conditions expected to accompany the launch of a short gamma-ray burst. We investigate the influence of magnetic fields on HMNSs by performing three-dimensional simulations in general-relativistic magnetohydrodynamics. In particular, we provide direct evidence for the occurrence of the magnetorotational instability (MRI) in HMNS interiors. For the first time in simulations of these systems, rapidly-growing and spatially-periodic structures are observed to form with features like those of the channel flows produced by the MRI in other systems. Moreover, the growth time and wavelength of the fastest-growing mode are extracted and compared successfully with analytical predictions. The MRI emerges as an important mechanism to amplify magnetic fields over the lifetime of the HMNS, whose collapse to a black hole is accelerated. The evidence provided here that the MRI can actually develop in HMNSs could have a profound impact on the outcome of the merger of neutron-star binaries and on its connection to short gamma-ray bursts.

General relativistic magnetohydrodynamic simulations of binary neutron star mergers forming a long-lived neutron star

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.

Stochastic background of gravitational waves emitted by magnetars

Two classes of high-energy sources in our galaxy are believed to host magnetars, i.e. neutron stars whose emission results from the dissipation of their magnetic field. The extremely high magnetic field of magnetars distorts their shape, and causes the emission of a conspicuous gravitational wave signal if rotation is fast and takes place around a different axis than the symmetry axis of the magnetic distortion. Based on a numerical model of the cosmic star formation history, we derive the cosmological background of gravitational waves produced by magnetars, when they are very young and fast spinning. We adopt different models for the configuration and strength of the internal magnetic field (which determines the distortion) as well as different values of the external dipole field strength (which governs the spin evolution of magnetars over a wide range of parameters). We find that the expected gravitational wave background differs considerably from one model to another. The strongest signals are generated for magnetars with very intense toroidal internal fields (∼10 16 G range) and external dipole fields of ∼10 14 , as envisaged in models aimed at explaining the properties of the 2004 December giant flare from SGR 1806-20. Such signals should be easily detectable with third-generation ground-based interferometers such as the Einstein Telescope.