276 research outputs found
Echoes of ECOs: gravitational-wave signatures of exotic compact objects and of quantum corrections at the horizon scale
Gravitational waves from binary coalescences provide one of the cleanest
signatures of the nature of compact objects. It has been recently argued that
the post-merger ringdown waveform of exotic ultracompact objects is initially
identical to that of a black-hole, and that putative corrections at the horizon
scale will appear as secondary pulses after the main burst of radiation. Here
we extend this analysis in three important directions: (i) we show that this
result applies to a large class of exotic compact objects with a photon sphere
for generic orbits in the test-particle limit; (ii) we investigate the
late-time ringdown in more detail, showing that it is universally characterized
by a modulated and distorted train of "echoes" of the modes of vibration
associated with the photon sphere; (iii) we study for the first time
equal-mass, head-on collisions of two ultracompact boson stars and compare
their gravitational-wave signal to that produced by a pair of black-holes. If
the initial objects are compact enough as to mimic a binary black-hole
collision up to the merger, the final object exceeds the maximum mass for boson
stars and collapses to a black-hole. This suggests that - in some
configurations - the coalescence of compact boson stars might be almost
indistinguishable from that of black-holes. On the other hand, generic
configurations display peculiar signatures that can be searched for in
gravitational-wave data as smoking guns of exotic compact objects.Comment: 13 pages, RevTex4. v2: typo in equation 7 corrected, references
added, to appear in PR
Large Eddy Simulations of Magnetized Mergers of Neutron Stars with Neutrinos
Neutron star mergers are very violent events involving extreme physical
processes: dynamic, strong-field gravity, large magnetic field, very hot, dense
matter, and the copious production of neutrinos. Accurate modeling of such a
system and its associated multi-messenger signals, such as gravitational waves,
short gamma ray burst, and kilonova, requires the inclusion of all these
processes, and is increasingly important in light of advancements in
multi-messenger astronomy generally, and in gravitational wave astronomy in
particular (such as the development of third-generation detectors). Several
general relativistic codes have been incorporating some of these elements with
different levels of realism. Here, we extend our code MHDuet, which can perform
large eddy simulations of magnetohydrodynamics to help capture the magnetic
field amplification during the merger, and to allow for realistic equations of
state and neutrino cooling via a leakage scheme. We perform several tests
involving isolated and binary neutron stars demonstrating the accuracy of the
code.Comment: 20 pages, 11 figures (typos corrected
Head-on collisions of boson stars
We study head-on collisions of boson stars in three dimensions. We consider
evolutions of two boson stars which may differ in their phase or have opposite
frequencies but are otherwise identical. Our studies show that these phase
differences result in different late time behavior and gravitational wave
output
Robustness of the Blandford-Znajek mechanism
The Blandford-Znajek mechanism has long been regarded as a key ingredient in
models attempting to explain powerful jets in AGNs, quasars, blazzars etc. In
such mechanism, energy is extracted from a rotating black hole and dissipated
at a load at far distances. In the current work we examine the behaviour of the
BZ mechanism with respect to different boundary conditions, revealing the
mechanism robustness upon variation of these conditions. Consequently, this
work closes a gap in our understanding of this important scenario.Comment: 7 pages, accepted in CQ
Simulating binary neutron stars: dynamics and gravitational waves
We model two mergers of orbiting binary neutron stars, the first forming a
black hole and the second a differentially rotating neutron star. We extract
gravitational waveforms in the wave zone. Comparisons to a post-Newtonian
analysis allow us to compute the orbital kinematics, including trajectories and
orbital eccentricities. We verify our code by evolving single stars and
extracting radial perturbative modes, which compare very well to results from
perturbation theory. The Einstein equations are solved in a first order
reduction of the generalized harmonic formulation, and the fluid equations are
solved using a modified convex essentially non-oscillatory method. All
calculations are done in three spatial dimensions without symmetry assumptions.
We use the \had computational infrastructure for distributed adaptive mesh
refinement.Comment: 14 pages, 16 figures. Added one figure from previous version;
corrected typo
Intense Electromagnetic Outbursts from Collapsing Hypermassive Neutron Stars
We study the gravitational collapse of a magnetized neutron star using a
novel numerical approach able to capture both the dynamics of the star and the
behavior of the surrounding plasma. In this approach, a fully general
relativistic magnetohydrodynamics implementation models the collapse of the
star and provides appropriate boundary conditions to a force-free model which
describes the stellar exterior. We validate this strategy by comparing with
known results for the rotating monopole and aligned rotator solutions and then
apply it to study both rotating and non-rotating stellar collapse scenarios,
and contrast the behavior with what is obtained when employing the
electrovacuum approximation outside the star. The non-rotating electrovacuum
collapse is shown to agree qualitatively with a Newtonian model of the
electromagnetic field outside a collapsing star. We illustrate and discuss a
fundamental difference between the force-free and electrovacuum solutions,
involving the appearance of large zones of electric-dominated field in the
vacuum case. This provides a clear demonstration of how dissipative
singularities appear generically in the non-linear time-evolution of force-free
fluids. In both the rotating and non-rotating cases, our simulations indicate
that the collapse induces a strong electromagnetic transient. In the case of
sub-millisecond rotation, the magnetic field experiences strong winding and the
transient carries much more energy. This result has important implications for
models of gamma-ray bursts.Comment: 28 pages, 20 figures (quality lowered to reduce sizes). Improved
initial data and matching condition results in a lower, but still important,
energy emission. Added appendix with a discussion on effects of transition
laye
Critical Phenomena in Neutron Stars I: Linearly Unstable Nonrotating Models
We consider the evolution in full general relativity of a family of linearly
unstable isolated spherical neutron stars under the effects of very small,
perturbations as induced by the truncation error. Using a simple ideal-fluid
equation of state we find that this system exhibits a type-I critical
behaviour, thus confirming the conclusions reached by Liebling et al. [1] for
rotating magnetized stars. Exploiting the relative simplicity of our system, we
are able carry out a more in-depth study providing solid evidences of the
criticality of this phenomenon and also to give a simple interpretation of the
putative critical solution as a spherical solution with the unstable mode being
the fundamental F-mode. Hence for any choice of the polytropic constant, the
critical solution will distinguish the set of subcritical models migrating to
the stable branch of the models of equilibrium from the set of subcritical
models collapsing to a black hole. Finally, we study how the dynamics changes
when the numerically perturbation is replaced by a finite-size, resolution
independent velocity perturbation and show that in such cases a nearly-critical
solution can be changed into either a sub or supercritical. The work reported
here also lays the basis for the analysis carried in a companion paper, where
the critical behaviour in the the head-on collision of two neutron stars is
instead considered [2].Comment: 15 pages, 9 figure
Method to estimate ISCO and ring-down frequencies in binary systems and consequences for gravitational wave data analysis
Recent advances in the description of compact binary systems have produced
gravitational waveforms that include inspiral, merger and ring-down phases.
Comparing results from numerical simulations with those of post-Newtonian (PN),
and related, expansions has provided motivation for employing PN waveforms in
near merger epochs when searching for gravitational waves and has encouraged
the development of analytic fits to full numerical waveforms. The models and
simulations do not yet cover the full binary coalescence parameter space. For
these yet un-simulated regions, data analysts can still conduct separate
inspiral, merger and ring-down searches. Improved knowledge about the end of
the inspiral phase, the beginning of the merger, and the ring-down frequencies
could increase the efficiency of both coherent inspiral-merger-ring-down (IMR)
searches and searches over each phase separately. Insight can be gained for all
three cases through a recently presented theoretical calculation, which,
corroborated by the numerical results, provides an implicit formula for the
final spin of the merged black holes, accurate to within 10% over a large
parameter space. Knowledge of the final spin allows one to predict the end of
the inspiral phase and the quasinormal mode ring-down frequencies, and in turn
provides information about the bandwidth and duration of the merger. In this
work we will discuss a few of the implications of this calculation for data
analysis.Comment: Added references to section 3 14 pages 5 figures. Submitted to
Classical and Quantum Gravit
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