81 research outputs found
Robust Recovery of Primitive Variables in Relativistic Ideal Magnetohydrodynamics
No full textModern 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
Effects of chiral effective field theory equation of state on binary neutron star mergers
No full textWe present fully general relativistic simulations of binary neutron star mergers, employing a new zero-temperature chiral effective field theory equation of state (EOS), the BL EOS. We offer a comparison with respect to the older GM3 EOS, which is based on standard relativistic mean-field theory, and separately determine the impact of the mass. We provide a detailed analysis of the dynamics, with focus on the postmerger phase. For all models, we extract the gravitational wave strain and the postmerger frequency spectrum. Further, we determine the amount, velocity, and polar distribution of ejected matter and provide estimates for the resulting kilonova signals. We also study the evolution of the disk while it is interacting with the hypermassive remnant and discuss the merits of different disk mass definitions applicable before collapse, with regard to the mass remaining after black hole formation. Finally, we investigate the radial mass distribution and rotation profile of the remnants, which validate previous results and also corroborate a recently proposed stability criterion
First 100 ms of a long-lived magnetized neutron star formed in a binary neutron star merger
No full textThe recent multimessenger observation of the short gamma-ray burst (SGRB) GRB 170817A together with the gravitational wave (GW) event GW170817 provides evidence for the long-standing hypothesis associating SGRBs with binary neutron star (BNS) mergers. The nature of the remnant object powering the SGRB, which could have been either an accreting black hole (BH) or a long-lived magnetized neutron star (NS), is, however, still uncertain. General relativistic magnetohydrodynamic (GRMHD) simulations of the merger process represent a powerful tool to unravel the jet launching mechanism, but so far most simulations focused the attention on a BH as the central engine, while the long-lived NS scenario remains poorly investigated. Here, we explore the latter by performing a GRMHD BNS merger simulation extending up to ~100 ms after merger, much longer than any previous simulation of this kind. This allows us to (i) study the emerging structure and amplification of the magnetic field and observe a clear saturation at magnetic energy erg, (ii) follow the magnetically supported expansion of the outer layers of the remnant NS and its evolution into an ellipsoidal shape without any surrounding torus, and (iii) monitor density, magnetization, and velocity along the axis, observing no signs of jet formation. We also argue that the conditions at the end of the simulation disfavor later jet formation on subsecond timescales if no BH is formed. Furthermore, we examine the rotation profile of the remnant, the conversion of rotational energy associated with differential rotation, the overall energy budget of the system, and the evolution of the GW frequency spectrum. Finally, we perform an additional simulation where we induce the collapse to a BH ~70 ms after merger, in order to gain insights on the prospects for massive accretion tori in case of a late collapse. We find that..
Implementing a new recovery scheme for primitive variables in the general relativistic magnetohydrodynamic code Spritz
No full textGeneral 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.Comment: 18 pages, 15 figures, matching accepted versio
Finite tidal effects in GW170817: observational evidence or model assumptions?
No full textAfter the detection of gravitational waves caused by the coalescence of compact objects in the mass range of neutron stars, GW170817, several studies have searched for an imprint of tidal effects in the signal, employing different model assumptions. One important distinction is whether or not to assume that both objects are neutron stars and obey the same equation of state. Some studies assumed independent properties. Others assume a universal equation of state, and in addition that the tidal deformability follows certain phenomenological relations. An important question is whether the gravitational-wave data alone constitute observational evidence for finite tidal effects. All studies provide Bayesian credible intervals, often without sufficiently discussing the impact of prior assumptions, especially in the case of lower limits on the neutron-star tidal deformability or radius. In this article, we scrutinize the implicit and explicit prior assumptions made in those studies. Our findings strongly indicate that existing lower credible bounds are mainly a consequence of prior assumptions combined with information gained about the system's masses. Importantly, those lower bounds are typically not informed by the observation of tidal effects in the gravitational-wave signal. Thus, regarding them as observational evidence might be misleading without a more detailed discussion. Further, we point out technical problems and conceptual inconsistencies in existing studies. We also assess the limitations due to systematic waveform model uncertainties in a novel way, demonstrating that differences between existing models are not guaranteed to be small enough for an unbiased estimation of lower bounds on the tidal deformability. Finally, we propose strategies for gravitational-wave data analysis designed to avoid some of the problems we uncovered
Fundamental oscillation modes of neutron stars: validity of universal relations
No full textCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORWe study the f-mode frequencies and damping times of nonrotating neutron stars (NS) in general relativity by solving the linearized perturbation equations, with the aim to establish "universal" relations that depend only weakly on the equations of state (EOS). Using a more comprehensive set of EOSs, we reexamine some proposed empirical relations that describe the f-mode parameters in terms of mass and radius of the NS, and we test a more recent proposal for expressing the f-mode parameters as quadratic functions of the effective compactness. Our extensive results for each equation of state considered allow us to study the accuracy of each proposal. In particular, the empirical relation proposed in the literature for the damping time in terms of the mass and radius deviates considerably from our results. We introduce a new universal relation for the product of the f-mode frequency and damping time as a function of the (ordinary) compactness, which proved to be more accurate. The more recently proposed relations using the effective compactness, on the other hand, also fit our data accurately. Our results show that the maximum oscillation frequency depends strongly on the EOS, such that the measurement of a high oscillation frequency would rule out several EOSs. Lastly, we compare the exact mode frequencies to those obtained in the Cowling approximation, and also to results obtained with a nonlinear evolution code, validating the implementations of the different approaches.We study the f-mode frequencies and damping times of nonrotating neutron stars (NS) in general relativity by solving the linearized perturbation equations, with the aim to establish "universal" relations that depend only weakly on the equations of state (EOS). Using a more comprehensive set of EOSs, we reexamine some proposed empirical relations that describe the f-mode parameters in terms of mass and radius of the NS, and we test a more recent proposal for expressing the f-mode parameters as quadratic functions of the effective compactness. Our extensive results for each equation of state considered allow us to study the accuracy of each proposal. In particular, the empirical relation proposed in the literature for the damping time in terms of the mass and radius deviates considerably from our results. We introduce a new universal relation for the product of the f-mode frequency and damping time as a function of the (ordinary) compactness, which proved to be more accurate. The more recently proposed relations using the effective compactness, on the other hand, also fit our data accurately. Our results show that the maximum oscillation frequency depends strongly on the EOS, such that the measurement of a high oscillation frequency would rule out several EOSs. Lastly, we compare the exact mode frequencies to those obtained in the Cowling approximation, and also to results obtained with a nonlinear evolution code, validating the implementations of the different approaches.914110CAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORSem informaçãoMiller, M.C., arXiv:1312.0029Akmal, A., Pandharipande, V.R., Ravenhall, D.G., Equation of state of nuclear matter and neutron star structure (1998) Phys. Rev. C, 58, p. 1804. , PRVCAN 0556-2813Balberg, S., Farrar, G.R., Piran, T., Neutron stars with a stable, light supersymmetric baryon (2001) Astrophys. J., 548, p. 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J., 181, p. 181. , ASJOAB 0004-637XChirenti, C., Silveira, P.R., Aguiar, O.D., Non-radial oscillations of neutron stars and the detection of gravitational waves (2012) Int. J. Mod. Phys. Conf. Ser., 18, p. 48. , IJMPJT 2010-1945Galeazzi, F., Kastaun, W., Rezzolla, L., Font, J.A., Implementation of a simplified approach to radiative transfer in general relativity (2013) Phys. Rev. D, 88, p. 064009. , PRVDAQ 1550-7998Dimmelmeier, H., Stergioulas, N., Font, J.A., Non-linear axisymmetric pulsations of rotating relativistic stars in the conformal flatness approximation (2006) Mon. Not. R. Astron. Soc., 368, p. 1609. , MNRAA4 0035-8711Kastaun, W., Inertial modes of rigidly rotating neutron stars in cowling approximation (2008) Phys. Rev. D, 77, p. 124019. , PRVDAQ 1550-7998Yoshida, S., Eriguchi, Y., Neutral points of oscillation modes along equilibrium sequences of rapidly rotating polytropes in general relativity: Application of the cowling approximation (1997) Astrophys. J., 490, p. 779. , ASJOAB 0004-637XBrown, D., Diener, P., Sarbach, O., Schnetter, E., Tiglio, M., Turduckening black holes: An analytical and computational study (2009) Phys. Rev. D, 79, p. 044023. , PRVDAQ 1550-7998Alic, D., Bona-Casas, C., Bona, C., Rezzolla, L., Palenzuela, C., Conformal and covariant formulation of the Z4 system with constraint-violation damping (2012) Phys. Rev. D, 85, p. 064040. , PRVDAQ 1550-7998Alic, D., Kastaun, W., Rezzolla, L., Constraint damping of the conformal and covariant formulation of the Z4 system in simulations of binary neutron stars (2013) Phys. Rev. D, 88, p. 064049. , PRVDAQ 1550-7998This work was supported by CAPES and the Max Planck Society. Some of the computations were carried out on the Datura cluster of the AEI. We thank F. Galeazzi for preparing the LS220 and SHT EOS tables. C. Chirenti wishes to thank L. Rezzolla and S. Yoshida for useful discussions
Properties of hypermassive neutron stars formed in mergers of spinning binaries
No full textWe present numerical simulations of binary neutron star mergers, comparing irrotational binaries to binaries of NSs rotating aligned to the orbital angular momentum. For the first time, we study spinning BNSs employing nuclear physics equations of state, namely the ones of Lattimer and Swesty as well as Shen, Horowitz, and Teige. We study mainly equal mass systems leading to a hypermassive neutron star (HMNS), and analyze in detail its structure and dynamics. In order to exclude gauge artifacts, we introduce a novel coordinate system used for post-processing. The results for our equal mass models show that the strong radial oscillations of the HMNS modulate the instantaneous frequency of the gravitational wave (GW) signal to an extend that leads to separate peaks in the corresponding Fourier spectrum. In particular, the high frequency peaks which are often attributed to combination frequencies can also be caused by the modulation of the m=2 mode frequency in the merger phase. As a consequence for GW data analysis, the offset of the high frequency peak does not necessarily carry information about the radial oscillation frequency. Further, the low frequency peak in our simulations is dominated by the contribution of the plunge and the first 1-2 bounces. The amplitude of the radial oscillations depends on the initial NS spin, which therefore has a complicated influence on the spectrum. Another important result is that HMNSs can consist of a slowly rotating core with an extended, massive envelope rotating close to Keplerian velocity, contrary to the common notion that a rapidly rotating core is necessary to prevent a prompt collapse. Finally, our estimates on the amount of unbound matter show a dependency on the initial NS spin, explained by the influence of the latter on the amplitude of radial oscillations, which in turn cause shock waves
Modern tools for computing neutron star properties
No full textAstronomical observations place increasingly tighter and more diverseconstraints on the properties of neutron stars (NS). Examples includeobservations of radio or gamma-ray pulsars, accreting neutron stars and x-raybursts, magnetar giant flares, and recently, the gravitational waves (GW) fromcoalescing binary neutron stars. Computing NS properties for a given EOS, suchas mass, radius, moment of inertia, tidal deformability, and innermost stablecircular orbits (ISCO), is therefore an important task. This task isunnecessarily difficult because relevant formulas are scattered throughout theliterature and publicly available software tools are far from being completeand easy to use. Further, naive implementations are unreliable in numericalcorner cases, most notably when using equations of state (EOS) with phasetransitions. To improve the situation, we provide a public library forcomputing NS properties and handling of EOS data. Further, we include acollection of EOS based on existing nuclear physics models together withprecomputed sequences of NS models. All methods are accessible via a Pythoninterface. This article collects all relevant equations and numerical methodsin full detail, including a novel formulation for the tidal deformabilityequations suitable for use with phase transitions. As a sidenote to the topicof ISCOs, we discuss the stability of non-interacting dark matter particlecircular orbits inside NSs. Finally, we present some simple applicationsrelevant for parameter estimation studies of GW data. For example, we explorethe validity of universal relations, and discuss the appearance of multiplestable branches for parametrized EOS.<br
Numerical Inside View of Hypermassive Remnant Models for GW170817
No full textThe first multimessenger observation attributed to a merging neutron star binary provided an enormous amount of observational data. Unlocking the full potential of this data requires a better understanding of the merger process and the early post-merger phase, which are crucial for the later evolution that eventually leads to observable counterparts. In this work, we perform standard hydrodynamical numerical simulations of a system compatible with GW170817. We focus on a single equation of state (EOS) and two mass ratios, while neglecting magnetic fields and neutrino radiation. We then apply newly developed postprocessing and visualization techniques to the results obtained for this basic setting. The focus lies on understanding the three-dimensional structure of the remnant, most notably the fluid flow pattern, and its evolution until collapse. We investigate the evolution of mass and angular momentum distribution up to collapse, as well as the differential rotation along and perpendicular to the equatorial plane. For the cases that we studied, the remnant cannot be adequately modeled as a differentially rotating axisymetric NS. Further, the dominant aspect leading to collapse is the GW radiation and not internal redistribution of angular momentum. We relate features of the gravitational wave signal to the evolution of the merger remnant, and make the waveforms publicly available. Finally, we find that the three-dimensional vorticity field inside the disk is dominated by medium-scale perturbances and not the orbital velocity, with potential consequences for magnetic field amplification effects
The role of weak interactions in dynamic ejecta from binary neutron star mergers
No full textWeak reactions are critical for the neutron richness of the matter dynamically ejected after the merger of two neutron stars. The neutron richness, defined by the electron fraction (Ye), determines which heavy elements are produced by the r-process and thus directly impacts the kilonova light curve. In this work, we have performed a systematic and detailed post-processing study of the impact of weak reactions on the distribution of the electron fraction and of the entropy on the dynamic ejecta obtained from an equal mass neutron star binary merger simulated in full general relativity and with microscopic equation of state. Previous investigations indicated that shocks increase Ye, however our results show that shocks can also decrease Ye, depending on their thermodynamical conditions. Moreover, we have found that neutrino absorption are key and need to be considered in future simulations. We also demonstrated that the angular dependence of the neutrino luminosity and the spatial distribution of the ejecta can lead to significant difference in the electron fraction distribution. In addition to the detailed study of the Ye evolution and its dependences, we have performed nucleosynthesis calculations. They clearly point to the necessity of improving the neutrino treatment in current simulations to be able to predict the contribution of neutron star mergers to the chemical history of the universe and to reliable calculate their kilonova light curves
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