3,703 research outputs found
On gravitational-wave echoes from neutron-star binary coalescences
A tentative detection of gravitational-wave echoes in the post-merger signal
of GW170817 has been recently claimed at significance level. It has
been speculated that the signal might provide evidence for near-horizon quantum
structures in the remnant exotic object. We point out that if the remnant
object is an ultracompact neutron star, echoes are expected for objects with
radius only slightly smaller than that of an ordinary neutron star. The
reported echoes at ~72 Hz are compatible with a toy model of incompressible
star with mass approximately and radius close to the
Buchdahl limit, R~9GM/(4c^2). If confirmed, low-frequency gravitational-wave
echoes would be in tension with all current neutron-star models and would have
dramatic implications for nuclear physics and gravity.Comment: v3: 4 pages, 2 figures, extended discussion, results unchanged.
Version accepted in CQG Letter
Tidal Love numbers of a slowly spinning neutron star
By extending our recent framework to describe the tidal deformations of a
spinning compact object, we compute for the first time the tidal Love numbers
of a spinning neutron star to linear order in the angular momentum. The spin of
the object introduces couplings between electric and magnetic distortions and
new classes of spin-induced ("rotational") tidal Love numbers emerge. We focus
on stationary tidal fields, which induce axisymmetric perturbations. We present
the perturbation equations for both electric-led and magnetic-led rotational
Love numbers for generic multipoles and explicitly solve them for various
tabulated equations of state and for a tidal field with an electric (even
parity) and magnetic (odd parity) component with . For a binary
system close to the merger, various components of the tidal field become
relevant. In this case we find that an octupolar magnetic tidal field can
significantly modify the mass quadrupole moment of a neutron star. Preliminary
estimates, assuming a spin parameter , show modifications
relative to the static case, at an orbital distance of five
stellar radii. Furthermore, the rotational Love numbers as functions of the
moment of inertia are much more sensitive to the equation of state than in the
static case, where approximate universal relations at the percent level exist.
For a neutron-star binary approaching the merger, we estimate that the
approximate universality of the induced mass quadrupole moment deteriorates
from in the static case to roughly when . Our
results suggest that spin-tidal couplings can introduce important corrections
to the gravitational waveforms of spinning neutron-star binaries approaching
the merger.Comment: v1: 16+11 pages, 6 appendices, 11 figures. v2: improved estimates of
the tidal-spin corrections to the quadrupole moment of spinning neutron-star
binaries approaching the merger. v3: version published in PR
Exotic Compact Objects and How to Quench their Ergoregion Instability
Gravitational-wave astronomy can give us access to the structure of black
holes, potentially probing microscopic or even Planckian corrections at the
horizon scale, as those predicted by some quantum-gravity models of exotic
compact objects. A generic feature of these models is the replacement of the
horizon by a reflective surface. Objects with these properties are prone to the
so-called ergoregion instability when they spin sufficiently fast. We
investigate in detail a simple model consisting of scalar perturbations of a
Kerr geometry with a reflective surface near the horizon. The instability
depends on the spin, on the compactness, and on the reflectivity at the
surface. The instability time scale increases only logarithmically in the
black-hole limit and, for a perfectly reflecting object, this is not enough to
prevent the instability from occurring on dynamical time scales. However, we
find that an absorption rate at the surface as small as 0.4% (reflectivity
coefficient as large as ) is sufficient to quench the
instability completely. Our results suggest that exotic compact objects are not
necessarily ruled out by the ergoregion instability.Comment: v3: 14 pages, 9 figures; further clarifications added, new appendix
on the superspinar case, results unchanged. Accepted in Phys. Rev.
Deriving item features relevance from collaborative domain knowledge
An Item based recommender system works by computing a similarity between
items, which can exploit past user interactions (collaborative filtering) or
item features (content based filtering). Collaborative algorithms have been
proven to achieve better recommendation quality then content based algorithms
in a variety of scenarios, being more effective in modeling user behaviour.
However, they can not be applied when items have no interactions at all, i.e.
cold start items. Content based algorithms, which are applicable to cold start
items, often require a lot of feature engineering in order to generate useful
recommendations. This issue is specifically relevant as the content descriptors
become large and heterogeneous. The focus of this paper is on how to use a
collaborative models domain-specific knowledge to build a wrapper feature
weighting method which embeds collaborative knowledge in a content based
algorithm. We present a comparative study for different state of the art
algorithms and present a more general model. This machine learning approach to
feature weighting shows promising results and high flexibility
Tidal deformations of a spinning compact object
The deformability of a compact object induced by a perturbing tidal field is
encoded in the tidal Love numbers, which depend sensibly on the object's
internal structure. These numbers are known only for static,
spherically-symmetric objects. As a first step to compute the tidal Love
numbers of a spinning compact star, here we extend powerful perturbative
techniques to compute the exterior geometry of a spinning object distorted by
an axisymmetric tidal field to second order in the angular momentum. The spin
of the object introduces couplings between electric and magnetic deformations
and new classes of induced Love numbers emerge. For example, a spinning object
immersed in a quadrupolar, electric tidal field can acquire some induced mass,
spin, quadrupole, octupole and hexadecapole moments to second order in the
spin. The deformations are encoded in a set of inhomogeneous differential
equations which, remarkably, can be solved analytically in vacuum. We discuss
certain subtleties in defining the multipole moments of the central object,
which are due to the difficulty in separating the tidal field from the linear
response of the object in the solution. By extending the standard procedure to
identify the linear response in the static case, we prove analytically that the
Love numbers of a Kerr black hole remain zero to second order in the spin. As a
by-product, we provide the explicit form for a slowly-rotating,
tidally-deformed Kerr black hole to quadratic order in the spin, and discuss
its geodesic and geometrical properties.Comment: 27 pages, 1 figure, 6 appendices; v2: improvements and
clarifications, version to appear in PR
Of Earth, Stone and Wood: The Restoration and Conservation of a Buddhist Temple in Ladakh, Indian Himalayas
[EN] The Dukhang Yokma is a small Buddhist temple part of the Ensa monastery in the Nubra river valley in Ladakh. The Dukhang was severely damaged by water infiltration soon after its construction at the be- ginning of the 20th century. Water seepage through its stone and mud mortar plinth caused a gradual bulging of the foundations which was followed by a steady shift of the whole structure. In the course of several decades this shift became irreversible and gradually damaged most of the masonry structure. The building had been neglected for several years before an active interest in its preservation emerged. Dur- ing this time, several parts of the buildings were dismantled and the temple’s inner chamber on two sto- reys tilted almost to the point of collapse. The conservation, consolidation and restoration of the temple has been undertaken by Achi Association India from 2018. This article analyses the restoration project and its many challenges, including wall painting stabilization. It explains in detail the issues faced by Achi team members and the way these problems were resolved through making use of local resources in this remote hermitage. One of the most complicated issues was to bring the inner temple’s structural elements back to their original straight position, avoiding any collapse and damage of the wall paintings. The complexity of the task was due to the very fragile mixed structure on two storeys made of wood, mud bricks and stone.Ferrari, EP. (2022). Of Earth, Stone and Wood: The Restoration and Conservation of a Buddhist Temple in Ladakh, Indian Himalayas. En Proceedings HERITAGE 2022 - International Conference on Vernacular Heritage: Culture, People and Sustainability. Editorial Universitat Politècnica de València. 839-846. https://doi.org/10.4995/HERITAGE2022.2022.1437783984
Equation-of-state-independent relations in neutron stars
Neutron stars are extremely relativistic objects which abound in our universe
and yet are poorly understood, due to the high uncertainty on how matter
behaves in the extreme conditions which prevail in the stellar core. It has
recently been pointed out that the moment of inertia I, the Love number lambda
and the spin-induced quadrupole moment Q of an isolated neutron star, are
related through functions which are practically independent of the equation of
state. These surprising universal I-lambda-Q relations pave the way for a
better understanding of neutron stars, most notably via gravitational-wave
emission. Gravitational-wave observations will probe highly-dynamical binaries
and it is important to understand whether the universality of the I-lambda-Q
relations survives strong-field and finite-size effects. We apply a
Post-Newtonian-Affine approach to model tidal deformations in compact binaries
and show that the I-lambda relation depends on the inspiral frequency, but is
insensitive to the equation of state. We provide a fit for the universal
relation, which is valid up to a gravitational wave frequency of ~900 Hz and
accurate to within a few percent. Our results strengthen the universality of
I-lambda-Q relations, and are relevant for gravitational-wave observations with
advanced ground-based interferometers. We also discuss the possibility of using
the Love-compactness relation to measure the neutron-star radius with an
uncertainty of about 10% or smaller from gravitational-wave observations.Comment: 5 pages, 2 figures, 2 table
Testing Gravity with Quasi Periodic Oscillations from accreting Black Holes: the Case of Einstein-Dilaton-Gauss-Bonnet Theory
Quasi-Periodic Oscillations (QPOs) observed in the X-ray flux emitted by
accreting black holes, are associated to phenomena occurring near the horizon.
Future very large area X-ray instruments will be able to measure QPO
frequencies with very high precision, thus probing this strong-field region. By
using the relativistic precession model, we show the way in which QPO
frequencies could be used to test general relativity against those alternative
theories of gravity which predict deviations from the classical theory in the
strong-field regime. We consider one of the best motivated strong-curvature
corrections to general relativity, namely the Einstein-Dilaton-Gauss-Bonnet
theory, and show that a detection of QPOs with the expected sensitivity of the
proposed ESA M-class mission LOFT would set the most stringent constraints on
the parameter space of this theory.Comment: 10 pages, 5 figures, 1 table; minor changes to match the version
appearing on Ap
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