Daniel M. Siegel

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.

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.

AN UPPER BOUND FROM HELIOSEISMOLOGY ON THE STOCHASTIC BACKGROUND OF GRAVITATIONAL WAVES

The universe is expected to be permeated by a stochastic background of gravitational radiation of astrophysical and cosmological origin. This background is capable of exciting oscillations in solar-like stars. Here we show that solar-like oscillators can be employed as giant hydrodynamical detectors for such a background in the μHz to mHz frequency range, which has remained essentially unexplored until today. We demonstrate this approach by using high-precision radial velocity data for the Sun to constrain the normalized energy density of the stochastic gravitational-wave background around 0.11 mHz. These results open up the possibility for asteroseismic missions like CoRoT and Kepler to probe fundamental physics.

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 ∼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.

Magnetically-induced outflows from binary neutron star merger remnants

Recent observations by the Swift satellite have revealed long-lasting (

An upper bound from helioseismology on the stochastic background of gravitational waves [Erratum (2014, ApJ, 784, 88)]

sim 10^2-10^5,mathrm{s}

Recovery Schemes for Primitive Variables in General-relativistic Magnetohydrodynamics

), 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.

Magnetic Field Amplification in Hypermassive Neutron Stars via the Magnetorotational Instability

We would like to point out that prior to this publication several authors have considered excitation of solar/stellar oscillations by gravitational waves. The very idea of employing the Sun as a detector for gravitational waves dates back at least to the discussion on the excitation of a 160 min solar oscillation by gravitational radiation (Walgate 1983). This discussion triggered a series of papers that refuted this speculative idea (Bonazzola et al. 1984; Kuhn & Boughn 1984; Carroll et al. 1984; Fabian & Gough 1984; Kosovichev 1984; Deruelle 1984). Furthermore, Boughn & Kuhn (1984) and, more recently, Khosroshahi & Sobouti (1997) studied the excitation of solar/stellar oscillations by gravitational waves. While Khosroshahi & Sobouti (1997) focused on computing energy absorption cross sections for polytropic models and did not report upper limits on a background of gravitational waves, Boughn & Kuhn (1984) did present such limits (as the only ones of the aforementioned authors) assuming an upper bound on the mean squared velocity of a few solar gand p-mode oscillations. The latter fact was not pointed out in the published version of this paper and we would like to correct this here. For ease of comparison, Figure 1 shows the upper limits of Boughn & Kuhn (1984) together with the upper bounds of the published paper. The upper limits presented by Boughn & Kuhn (1984) were given for individual frequencies in the range of 105.8–2116.7 Hz m . The estimate at 105.8 Hz m agrees well with our estimates around this frequency. We note that damping rates are uncertain (up to orders of magnitude) in the frequency range 110 » –1500 Hz m (cf. grey shaded area in Figure 1) and cannot be reliably calculated due to convection–pulsation interactions (Dupret 2002; Belkacem et al. 2009; Chaplin et al. 1997; Houdek 2006; Dupret et al. 2006; see also the published version of this paper). As shown by the present paper, visibility effects can influence the resulting upper bounds by The Astrophysical Journal, 810:84 (2pp), 2015 September 1 doi:10.1088/0004-637X/810/1/84

On the feasibility of employing solar-like oscillators as detectors for the stochastic background of gravitational waves

General-relativistic magnetohydrodynamic (GRMHD) simulations are an important tool to study a variety of astrophysical systems such as neutron star mergers, core-collapse supernovae, and accretion onto compact objects. A conservative GRMHD scheme numerically evolves a set of conservation equations for conserved quantities and requires the computation of certain primitive variables at every time step. This recovery procedure constitutes a core part of any conservative GRMHD scheme and it is closely tied to the equation of state (EOS) of the fluid. In the quest to include nuclear physics, weak interactions, and neutrino physics, state-of-the-art GRMHD simulations employ finite-temperature, composition-dependent EOSs. While different schemes have individually been proposed, the recovery problem still remains a major source of error, failure, and inefficiency in GRMHD simulations with advanced microphysics. The strengths and weaknesses of the different schemes when compared to each other remain unclear. Here we present the first systematic comparison of various recovery schemes used in different dynamical spacetime GRMHD codes for both analytic and tabulated microphysical EOSs. We assess the schemes in terms of (i) speed, (ii) accuracy, and (iii) robustness. We find large variations among the different schemes and that there is not a single ideal scheme. While the computationally most efficient schemes are less robust, the most robust schemes are computationally less efficient. More robust schemes may require an order of magnitude more calls to the EOS, which are computationally expensive. We propose an optimal strategy of an efficient three-dimensional Newton-Raphson scheme and a slower but more robust one-dimensional scheme as a fall-back.