347 research outputs found
A Semi-analytic Study of Axial Perturbations of Ultra Compact Stars
Compact object perturbations, at linear order, often lead in solving one or
more coupled wave equations. The study of these equations was typically done by
numerical or semi-analytical methods. The WKB method and the associated
Bohr-Sommerfeld rule have been proved extremely useful tools in the study of
black-hole perturbations and the estimation of the related quasi-normal modes.
Here we present an extension of the aforementioned semi-analytic methods in the
study of perturbations of ultra-compact stars and gravastars.Comment: Accepted for publication in CQG, 13 pages, 3 figures, 5 table
Parameter estimation of gravitational wave echoes from exotic compact objects
Relativistic ultracompact objects without an event horizon may be able to
form in nature and merge as binary systems, mimicking the coalescence of
ordinary black holes. The postmerger phase of such processes presents
characteristic signatures, which appear as repeated pulses within the emitted
gravitational waveform, i.e., echoes with variable amplitudes and frequencies.
Future detections of these signals can shed new light on the existence of
horizonless geometries, and provide new information on the nature of gravity in
a genuine strong-field regime. In this work we analyze phenomenological
templates used to characterize echolike structures produced by exotic compact
objects, and we investigate for the first time the ability of current and
future interferometers to constrain their parameters. Using different models
with an increasing level of accuracy, we determine the features that can be
measured with the largest precision, and we span the parameter space to find
the most favorable configurations to be detected. Our analysis shows that
current detectors may already be able to extract all the parameters of the
echoes with good accuracy, and that multiple interferometers can measure
frequencies and damping factors of the signals at the level of percent.Comment: References update
Quasi-Normal Modes from Bound States: The Numerical Approach
It is known that the spectrum of quasi-normal modes of potential barriers is
related to the spectrum of bound states of the corresponding potential wells.
This property has been widely used to compute black hole quasi-normal modes,
but it is limited to a few "approximate" potentials with certain transformation
properties for which the spectrum of bound states must be known analytically.
In this work we circumvent this limitation by proposing an approach that allows
one to make use of potentials with similar transformation properties, but where
the spectrum of bound states can also be computed numerically. Because the
numerical calculation of bound states is usually more stable than the direct
computation of the corresponding quasi-normal modes, the new approach is also
interesting from a technical point of view. We apply the method to different
potentials, including the P\"oschl-Teller potential for which all steps can be
understood analytically, as well as potentials for which we are not aware of
analytic results but provide independent numerical results for comparison. As a
canonical test, all potentials are chosen to match the Regge-Wheeler potential
of axial perturbations of the Schwarzschild black hole. We find that the new
approximate potentials are more suitable to approximate the exact quasi-normal
modes than the P\"oschl-Teller potential, particularly for the first overtone.
We hope this work opens new perspectives to the computation of quasi-normal
modes and finds further improvements and generalizations in the future.Comment: 13 pages, 10 figures, 3 tables. Comments welcome
Rapidly rotating neutron stars: Universal relations and EOS inference
We provide accurate universal relations that allow to estimate the moment of
inertia and the ratio of kinetic to gravitational binding energy of
uniformly rotating neutron stars from the knowledge of mass, radius, and moment
of inertia of an associated non-rotating neutron star. Based on these, several
other fluid quantities can be estimated as well. Astrophysical neutron stars
rotate to varying degrees and although rotational effects may be neglected in
some cases, not modeling them will inevitably introduce bias when performing
parameter estimation. This is especially important for future, high-precision
measurements coming from electromagnetic and gravitational wave observations.
The proposed universal relations facilitate computationally cheap EOS inference
codes that permit the inclusion of observations of rotating neutron stars. To
demonstrate this, we deploy them into a recent Bayesian framework for equation
of state parameter estimation that is now valid for arbitrary, uniform
rotation. Our inference results are robust up to around percent level precision
for the generated neutron star observations, consisting of the mass, equatorial
radius, rotation rate, as well as co- and counter-rotating -mode
frequencies, that enter the framework as data.Comment: 16 pages, 14 figure
Constraining modifications of black hole perturbation potentials near the light ring with quasinormal modes
In modified theories of gravity, the potentials appearing in the
Schr\"odinger-like equations that describe perturbations of non-rotating black
holes are also modified. In this paper we ask: can these modifications be
constrained with high-precision gravitational-wave measurements of the black
hole's quasinormal mode frequencies? We expand the modifications in a small
perturbative parameter regulating the deviation from the general-relativistic
potential, and in powers of . We compute the quasinormal modes of the
modified potential up to quadratic order in the perturbative parameter. Then we
use Markov-chain-Monte-Carlo (MCMC) methods to recover the coefficients in the
expansion in an ``optimistic'' scenario where we vary them one at a time,
and in a ``pessimistic'' scenario where we vary them all simultaneously. In
both cases, we find that the bounds on the individual parameters are not
robust. Because quasinormal mode frequencies are related to the behavior of the
perturbation potential near the light ring, we propose a different strategy.
Inspired by Wentzel-Kramers-Brillouin (WKB) theory, we demonstrate that the
value of the potential and of its second derivative at the light ring can be
robustly constrained. These constraints allow for a more direct comparison
between tests based on black hole spectroscopy and observations of black hole
`shadows'' by the Event Horizon Telescope and future instruments.Comment: 12 pages, 7 figure
To ring or not to ring, the tale of black hole quasi-normal modes
Extracting quasi-normal modes from compact binary mergers to perform black
hole spectroscopy is one of the fundamental pillars in current and future
strong-gravity tests. Among the most remarkable findings of recent works is
that including a large number of overtones not only reduces the mismatch of the
fitted ringdown but also allows one to extract black hole parameters from a
ringdown analysis that goes well within the non-linear merger part. At the same
time, it is well understood that several details of the ringdown analysis have
important consequences for the question of whether overtones are present or
not, and subsequently, to what extent one can claim to perform black hole
spectroscopy. To clarify and tackle some aspects of overtone fitting, we
revisit the clearer problem of wave propagation in the scalar Regge-Wheeler and
P\"oschl-Teller potentials. This set-up, which is to some extent qualitatively
very similar to the non-linear merger-ringdown regime, indicates that using
even an approximate model for the overtones yields an improved extraction of
the black hole mass at early ringdown times. We find that the relevant
parameter is the number of included modes rather than using the correct model
for the overtones themselves. This further adds evidence to the proposal that
large overtone numbers may instead remove non-quasi-normal mode contributions
that are relevant at early times of a ringdown, but do not necessarily
correspond to the physical excitation of modes of the system.Comment: 9 pages, 9 figure
Tests of Pressure Gauges in High-Magnetic Fields at Forschungszentrum Karlsruhe (FZK), Institute for Technical Physics (ITP) and Department for Highest-Magnetic Fields
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