334 research outputs found

### Collapse of differentially rotating neutron stars and cosmic censorship

We present new results on the dynamics and gravitational-wave emission from
the collapse of differentially rotating neutron stars. We have considered a
number of polytropic stellar models having different values of the
dimensionless angular momentum J/M^2, where J and M are the asymptotic angular
momentum and mass of the star, respectively. For neutron stars with J/M^2<1,
i.e., "sub-Kerr" models, we were able to find models that are dynamically
unstable and that collapse promptly to a rotating black hole. Both the dynamics
of the collapse and the consequent emission of gravitational waves resemble the
one seen for uniformly rotating stars, although with an overall decrease in the
efficiency of gravitational-wave emission. For stellar models with J/M^2>1,
i.e., "supra-Kerr" models, on the other hand, we were not able to find models
that are dynamically unstable and all of the computed supra-Kerr models were
found to be far from the stability threshold. For these models a gravitational
collapse is possible only after a very severe and artificial reduction of the
pressure, which then leads to a torus developing nonaxisymmetric instabilities
and eventually contracting to a stable axisymmetric stellar configuration.
While this does not exclude the possibility that a naked singularity can be
produced by the collapse of a differentially rotating star, it also suggests
that cosmic censorship is not violated and that generic conditions for a
supra-Kerr progenitor do not lead to a naked singularity.Comment: 15 pages, 15 figures. Minor changes to the text and to the
references. In press on Phys. Rev.

### First 100 ms of a long-lived magnetized neutron star formed in a binary neutron star merger

The 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 $E_\mathrm{mag} \sim 10^{51}$
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...Comment: 14 pages, 16 figures, matches published version in PR

### General-relativistic resistive magnetohydrodynamics in three dimensions: Formulation and tests

We present a new numerical implementation of the general-relativistic
resistive magnetohydrodynamics (MHD) equations within the Whisky code. The
numerical method adopted exploits the properties of implicit-explicit
Runge-Kutta numerical schemes to treat the stiff terms that appear in the
equations for large electrical conductivities. Using tests in one, two, and
three dimensions, we show that our implementation is robust and recovers the
ideal-MHD limit in regimes of very high conductivity. Moreover, the results
illustrate that the code is capable of describing scenarios in a very wide
range of conductivities. In addition to tests in flat spacetime, we report
simulations of magnetized nonrotating relativistic stars, both in the Cowling
approximation and in dynamical spacetimes. Finally, because of its
astrophysical relevance and because it provides a severe testbed for
general-relativistic codes with dynamical electromagnetic fields, we study the
collapse of a nonrotating star to a black hole. We show that also in this case
our results on the quasinormal mode frequencies of the excited electromagnetic
fields in the Schwarzschild background agree with the perturbative studies
within 0.7% and 5.6% for the real and the imaginary part of the l=1 mode
eigenfrequency, respectively. Finally we provide an estimate of the
electromagnetic efficiency of this process.Comment: 22 pages, 19 figure

### Can magnetic fields be detected during the inspiral of binary neutron stars?

Using accurate and fully general-relativistic simulations we assess the
effect that magnetic fields have on the gravitational-wave emission produced
during the inspiral and merger of magnetized neutron stars. In particular, we
show that magnetic fields have an impact after the merger, because they are
amplified by a Kelvin-Helmholtz instability, but also during the inspiral, most
likely because the magnetic tension reduces the stellar tidal deformation for
extremely large initial magnetic fields, B_0>~10^{17}G. We quantify the
influence of magnetic fields by computing the overlap, O, between the waveforms
produced during the inspiral by magnetized and unmagnetized binaries. We find
that for any realistic magnetic field strength B_0<~10^{14}G the overlap during
the inspiral is O>~0.999 and is quite insensitive to the mass of the neutron
stars. Only for unrealistically large magnetic fields like B_0~10^{17}G the
overlap does decrease noticeably, becoming at our resolutions O<~0.76/0.67 for
stars with baryon masses M_b~1.4/1.6 Msun, respectively. Because neutron stars
are expected to merge with magnetic fields ~10^{8}-10^{10}G and because present
detectors are sensitive to O<~0.995, we conclude that it is very unlikely that
the present detectors will be able to discern the presence of magnetic fields
during the inspiral of neutron stars.Comment: 5 pages, 4 figures. Small changes to text and figures. Matches
version to appear on MNRAS Letter

### Prompt Electromagnetic Transients from Binary Black Hole Mergers

Binary black hole (BBH) mergers provide a prime source for current and future
interferometric GW observatories. Massive BBH mergers may often take place in
plasma-rich environments, leading to the exciting possibility of a concurrent
electromagnetic (EM) signal observable by traditional astronomical facilities.
However, many critical questions about the generation of such counterparts
remain unanswered. We explore mechanisms that may drive EM counterparts with
magnetohydrodynamic simulations treating a range of scenarios involving
equal-mass black-hole binaries immersed in an initially homogeneous fluid with
uniform, orbitally aligned magnetic fields. We find that the time development
of Poynting luminosity, which may drive jet-like emissions, is relatively
insensitive to aspects of the initial configuration. In particular, over a
significant range of initial values, the central magnetic field strength is
effectively regulated by the gas flow to yield a Poynting luminosity of
$10^{45}-10^{46} \rho_{-13} M_8^2 \, {\rm erg}\,{\rm s}^{-1}$, with BBH mass
scaled to $M_8 \equiv M/(10^8 M_{\odot})$ and ambient density $\rho_{-13}
\equiv \rho/(10^{-13} \, {\rm g} \, {\rm cm}^{-3})$. We also calculate the
direct plasma synchrotron emissions processed through geodesic ray-tracing.
Despite lensing effects and dynamics, we find the observed synchrotron flux
varies little leading up to merger.Comment: 22 pages, 21 figures; additional reference + clarifying text added to
match published versio

### Accurate evolutions of inspiralling and magnetized neutron-stars: equal-mass binaries

By performing new, long and numerically accurate general-relativistic
simulations of magnetized, equal-mass neutron-star binaries, we investigate the
role that realistic magnetic fields may have in the evolution of these systems.
In particular, we study the evolution of the magnetic fields and show that they
can influence the survival of the hypermassive-neutron star produced at the
merger by accelerating its collapse to a black hole. We also provide evidence
that even if purely poloidal initially, the magnetic fields produced in the
tori surrounding the black hole have toroidal and poloidal components of
equivalent strength. When estimating the possibility that magnetic fields could
have an impact on the gravitational-wave signals emitted by these systems
either during the inspiral or after the merger we conclude that for realistic
magnetic-field strengths B<~1e12 G such effects could be detected, but only
marginally, by detectors such as advanced LIGO or advanced Virgo. However,
magnetically induced modifications could become detectable in the case of
small-mass binaries and with the development of gravitational-wave detectors,
such as the Einstein Telescope, with much higher sensitivities at frequencies
larger than ~2 kHz.Comment: 18 pages, 10 figures. Added two new figures (figures 1 and 7). Small
modifications to the text to match the version published on Phys. Rev.

### General Relativistic Simulations of High-Mass Binary Neutron Star Mergers: rapid formation of low-mass stellar black holes

Almost 100 compact binary mergers have been detected via gravitational waves
by the LIGO-Virgo-KAGRA collaboration in the past few years providing us with a
significant amount of new information on black holes and neutron stars. In
addition to observations, numerical simulations using newly developed modern
codes in the field of gravitational wave physics will guide us to understand
the nature of single and binary degenerate systems and highly energetic
astrophysical processes. We here present a set of new fully general
relativistic hydrodynamic simulations of high-mass binary neutron star systems
performed with the publicly available Einstein Toolkit and LORENE codes. We
considered systems with a total baryonic mass between 2.8 $M_\odot$ and 4.0
$M_\odot$ and we adopted the SLy equation of state. For all models we analyzed
the gravitational wave signal and we report potential indicators of the systems
undergoing rapid collapse into a black hole that may be observed by
future-planned detectors such as the Einstein Telescope and the Cosmic
Explorer. We also extracted the properties of the post-merger black hole, the
disk and ejecta masses and their dependence on the binary parameters. We also
compare our numerical results with recent analytical fits presented in the
literature and we also provide parameter-dependent semi-analytical relations
between the total mass and mass ratio of the systems and the resulting black
hole masses and spins, coalescence time scale, mass loss, and gravitational
wave energy.Comment: 13 pages, 5 figure, 4 tables, submitted for publicatio

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