21 research outputs found
On the origin of intrinsic alignment in cosmic shear measurements: an analytic argument
Galaxy intrinsic alignment can be a severe source of error in weak-lensing
studies. The problem has been widely studied by numerical simulations and with
heuristic models, but without a clear theoretical justification of its origin
and amplitude. In particular, it is still unclear whether intrinsic alignment
of galaxies is dominated by formation and accretion processes or by the effects
of the instantaneous tidal field acting upon them. We investigate this question
by developing a simple model of intrinsic alignment for elliptical galaxies,
based on the instantaneous tidal field. Making use of the galaxy stellar
distribution function, we estimate the intrinsic alignment signal and find that
although it has the expected dependence on the tidal field, it is too weak to
account for the observed signal. This is an indirect validation of the standard
view that intrinsic alignment is caused by formation and/or accretion
processes.Comment: 11 pages, 4 figures, accepted for publication on Astronomy &
Astrophysic
Disk formation in the collapse of supramassive neutron stars
Short gamma-ray bursts (sGRBs) show a large diversity in their properties.
This suggests that the observed phenomenon can be caused by different "central
engines" or that the engine produces a variety of outcomes depending on its
parameters, or possibly both. The most popular engine scenario, the merger of
two neutron stars, has received support from the recent Fermi and INTEGRAL
detection of a burst of gamma rays (GRB170817A) following the neutron star
merger GW170817, but at the moment it is not clear how peculiar this event
potentially was. Several sGRBs engine models involve the collapse of a
supramassive neutron star that produces a black hole plus an accretion disk. We
study this scenario for a variety of equations of states both via angular
momentum considerations based on equilibrium models and via fully dynamical
Numerical Relativity simulations. We obtain a broader range of disk forming
configurations than earlier studies but we agree with the latter that none of
these configurations is likely to produce a phenomenon that would be classified
as an sGRB.Comment: accepted by MNRA
Early evolution of newly born proto-neutron stars
A proto-neutron star (PNS) is the first phase of life of a neutron star, and is likely to origin from a core-collapse supernova. After about 200 ms from core-collapse, the PNS evolution may be modeled as a sequence of quasi-stationary configurations. These configurations depend on the PNS thermodynamic profiles, whose evolution largely depends upon the neutrino diffusion.
We developed a new PNS evolutionary code that solves by iteration the neutrino number and energy transport equations together with the relativistic stellar structure equations assuming spherical symmetry. The neutrino cross sections are determined consistently with the underlying equation of state (EoS). To include the EoSs in the evolution, we devised and tested a new fitting formula for the interacting part of the baryon free-energy, valid at finite temperature and arbitrary degeneracy.
Using our code, we provide estimates for the neutrino signal in the Super-Kamiokande III detector and the frequencies of the gravitational waves due to stellar oscillations, for three stellar masses and three nucleonic EoSs. For the first time we evolve a PNS with a nuclear many-body theory EoS in a consistent way, that is, we take into account realistic nuclear interactions in the computation of the neutrino cross sections. By including rotation in an effective way, we have also determined the time variation of the rotation frequency due to PNS contraction and neutrino angular momentum loss, and the gravitational wave signal due to rotation. We find that the mass shedding limit restricts the initial angular momentum. Consequently, the final rotation frequency has to be smaller than about 300 Hz for a PNS of about 1.6 solar masses whose EoS is described by the GM3 mean-field model
Modeling Neutron Star Matter in the Age of Multimessenger Astrophysics
The interpretation of the available and forthcoming data obtained from
multimessenger astrophysical observations -- potentially providing
unprecedented access to neutron star properties -- will require the development
of novel, accurate theoretical models of dense matter. Of great importance, in
this context, will be the capability to devise a description of thermal effects
applicable to the study of quantities other than the equation of state, such as
the transport coefficients and the neutrino mean free path in the nuclear
medium. The formalism based on correlated basis states and the cluster
expansion technique has been previously employed to derive a well-behaved
effective interaction -- suitable for use in standard perturbation theory --
from a state-of-the-art nuclear Hamiltonian, including phenomenological two-
and three-nucleon potentials. Here, we provide a comprehensive and
self-contained account of the extension of this approach to the treatment of
finite-temperature effects, and report the results of numerical calculations of
a number of properties of nuclear matter with arbitrary neutron excess and
temperature up to 50 MeV.Comment: 20 pages, 11 figures, typeset using aastex63
Spin evolution of a proto-neutron star
We study the evolution of the rotation rate of a proto-neutron star, born in a core-collapse supernova, in the first seconds of its life. During this phase, the star evolution can be described as a sequence of stationary configurations, which we determine by solving the neutrino transport and the stellar structure equations in general relativity. We include in our model the angular momentum loss due to neutrino emission. We find that the requirement of a rotation rate not exceeding the mass-shedding limit at the beginning of the evolution implies a strict bound on the rotation rate at later times. Moreover, assuming that the proto-neutron star is born with a finite ellipticity, we determine the emitted gravitational wave signal and estimate its detectability by present and future ground-based interferometric detectors.This work was partially supported by “NewCompStar” (COST Action MP1304), and by the H2020-MSCA-RISE-2015 Grant No. StronGrHEP-690904. J. A. P. acknowledges support by the MINECO Grants No. AYA2013-42184-P and No. AYA2015-66899-C2-2-P
Simulating bulk viscosity in neutron stars I: formalism
The faithful inclusion of the effects of bulk viscosity induced by the presence of chemical reactions is an important issue for simulations of core-collapse supernovae, binary neutron star mergers and neutron star oscillations, where particle abundances are locally pushed out of chemical equilibrium by rarefaction and compression of the fluid elements. In this work, we discuss three different approaches that can be used to implement bulk viscosity in general relativistic hydrodynamic simulations of neutron stars: the exact multi-component reacting fluid, and two MĂĽller-Israel-Stewart theories, namely the second order Hiscock-Lindblom model and its linear limit, the Maxwell-Cattaneo model. After discussing the theory behind the three approaches, we specialize their dynamics equations to spherical symmetry in the radial gauge-polar slicing (i.e., Schwarzschild) coordinates. We also discuss a particular choice for the equation of state of the fluid and the associated neutrino emission rates, which are used in a companion paper for the numerical comparison of the three frameworks, and we obtain the effective sound speed for the Hiscock-Lindblom theory in the non-linear regime
Simulating bulk viscosity in neutron stars I: formalism
The faithful inclusion of the effects of bulk viscosity induced by the presence of chemical reactions is an important issue for simulations of core-collapse supernovae, binary neutron star mergers and neutron star oscillations, where particle abundances are locally pushed out of chemical equilibrium by rarefaction and compression of the fluid elements. In this work, we discuss three different approaches that can be used to implement bulk viscosity in general relativistic hydrodynamic simulations of neutron stars: the exact multi-component reacting fluid, and two MĂĽller-Israel-Stewart theories, namely the second order Hiscock-Lindblom model and its linear limit, the Maxwell-Cattaneo model. After discussing the theory behind the three approaches, we specialize their dynamics equations to spherical symmetry in the radial gauge-polar slicing (i.e., Schwarzschild) coordinates. We also discuss a particular choice for the equation of state of the fluid and the associated neutrino emission rates, which are used in a companion paper for the numerical comparison of the three frameworks, and we obtain the effective sound speed for the Hiscock-Lindblom theory in the non-linear regime
Simulating bulk viscosity in neutron stars II: evolution in spherical symmetry
Out-of-equilibrium reactions between different particle species are the main process contributing to bulk viscosity in neutron stars. In this work, we numerically compare three different approaches to the modeling of bulk viscosity: the multi-component fluid with reacting particle species and two bulk stress formalism based on the MĂĽller-Israel-Stewart theory, namely the Hiscock-Lindblom and the Maxwell-Cattaneo models, whose flux-conservative formulation in radial gauge-polar slicing coordinates and spherical symmetry is derived in a companion paper. In our knowledge, this is the first time that a neutron star is simulated with the complete Hiscock-Lindblom model of bulk viscosity. We find that the Hiscock-Lindblom and Maxwell-Cattaneo models are good approximations of the multi-component fluid for small perturbations and when the non-equilibrium equation of state of the fluid depends on only one independent particle fraction. For more than one independent particle fraction and for large perturbations, the bulk stress approximation is still valid but less accurate. In addition, we include the energy loss due to the luminosity of the reactions in the bulk stress formulation. We find that the energy loss due to bulk viscosity has a larger effect on the dynamics than the bulk stress or the variation in particle composition per se. The new one-dimensional, general-relativistic hydrodynamic code developed for this work, hydro-bulk-1D, is publicly available