8,936 research outputs found
Bounds for axially symmetric linear perturbations for the extreme Kerr black hole
We obtain remarkably simple integral bounds for axially symmetric linear
perturbations for the extreme Kerr black hole in terms of conserved energies.
From these estimates we deduce pointwise bounds for the perturbations outside
the horizon.Comment: 15 page
On the linear stability of the extreme Kerr black hole under axially symmetric perturbations
We prove that for axially symmetric linear gravitational perturbations of the
extreme Kerr black hole there exists a positive definite and conserved energy.
This provides a basic criteria for linear stability in axial symmetry. In the
particular case of Minkowski, using this energy we also prove pointwise
boundedness of the perturbation in a remarkable simple way.Comment: 35 pages. 2 figures. Relevant improvement in the presentation in
section 2.
Construction of the Lyapunov spectrum in a chaotic system displaying phase synchronization
We consider a three-dimensional chaotic system consisting of the suspension
of Arnold's cat map coupled with a clock via a weak dissipative interaction. We
show that the coupled system displays a synchronization phenomenon, in the
sense that the relative phase between the suspension flow and the clock locks
to a special value, thus making the motion fall onto a lower dimensional
attractor. More specifically, we construct the attractive invariant manifold,
of dimension smaller than three, using a convergent perturbative expansion.
Moreover, we compute via convergent series the Lyapunov exponents, including
notably the central one. The result generalizes a previous construction of the
attractive invariant manifold in a similar but simpler model. The main novelty
of the current construction relies in the computation of the Lyapunov spectrum,
which consists of non-trivial analytic exponents. Some conjectures about a
possible smoothening transition of the attractor as the coupling is increased
are also discussed.Comment: 24 pages, 10 figure
Mass models from high-resolution HI data of the dwarf galaxy NGC 1560
We present HI observations performed at the GMRT of the nearby dwarf galaxy
NGC 1560. This Sd galaxy is well-known for a distinct "wiggle" in its rotation
curve. Our new observations have twice the resolution of the previously
published HI data. We derived the rotation curve by taking projection effects
into account, and we verified the derived kinematics by creating model
datacubes. This new rotation curve is similar to the previously published one:
we confirm the presence of a clear wiggle. The main differences are in the
innermost ~100 arcsec of the rotation curve, where we find slightly (<~ 5 km/s)
higher velocities. Mass modelling of the rotation curve results in good fits
using the core-dominated Burkert halo (which however does not reproduce the
wiggle), bad fits using the a Navarro, Frenk & White halo, and good fits using
MOND (Modified Newtonian Dynamics), which also reproduces the wiggle.Comment: Accepted for publication in MNRAS. 11 pages, 13 figures.
High-resolution version available at
http://users.ugent.be/~ggianfra/1560_final.pd
Modified gravity models and the central cusp of dark matter haloes in galaxies
The N-body dark matter (DM) simulations point that DM density profiles, e.g. the Navarro Frenk White (NFW) halo, should be cuspy in its centre, but observations disfavour this kind of DM profile. Here we consider whether the observed rotation curves close to the galactic centre can favour modified gravity models in comparison to the NFW halo, and how to quantify such difference. Two explicit modified gravity models are considered, Modified Newtonian Dynamics (MOND) and a more recent approach renormalization group effects in general relativity (RGGR). It is also the purpose of this work to significantly extend the sample on which RGGR has been tested in comparison to other approaches. By analysing 62 galaxies from five samples, we find that (i) there is a radius, given by half the disc scale length, below which RGGR and MOND can match the data about as well or better than NFW, albeit the formers have fewer free parameters; (ii) considering the complete rotation curve data, RGGR could achieve fits with better agreement than MOND, and almost as good as a NFW halo with two free parameters (NFW and RGGR have, respectively, two and one more free parameters than MOND)
Supernova neutrino physics with a nuclear emulsion detector
The existence of the coherent neutrino-nucleus scattering reaction requires
to evaluate, for any detector devoted to WIMP searches, the irreducible
background due to conventional neutrino sources and at same time, it gives a
unique chance to reveal supernova neutrinos. We report here a detailed study
concerning a new directional detector, based on the nuclear emulsion
technology. A Likelihood Ratio test shows that, in the first years of
operations and with a detector mass of several tens of tons, the observation of
the supernova signal can be achieved. The determination of the distance of the
supernova from the neutrinos and the observation of B neutrinos are also
discussed.Comment: 22 pages, 12 figure
Remarks on the properties of elliptical galaxies in modified Newtonian dynamics
Two incorrect arguments against MOND in elliptical galaxies could be that the
equivalent circular velocity curves tend to become flat at much larger
accelerations than in spiral galaxies, and that the Newtonian dark matter halos
are more concentrated than in spirals. Here, we compare published scaling
relations for the dark halos of elliptical galaxies to the scaling relations
expected for MONDian phantom halos. We represent the baryonic content of
galaxies by spherical profiles, and their corresponding MONDian phantom halos
by logarithmic halos. We then derive the surface densities, central densities,
and phase space densities and compare them with published scaling relations. We
conclude that it is possible to get flat circular velocity curves at high
acceleration in MOND, and that this happens for baryonic distributions
described by Jaffe profiles in the region where the circular velocity curve is
flat. Moreover, the scaling relations of dark halos of ellipticals are
remarkably similar to the scaling relations of phantom halos of MOND.Comment: Accepted for publication in A and
Isolated and non-isolated dwarfs in terms of modified Newtonian dynamics
Within the framework of modified Newtonian dynamics (MOND) we investigate the
kinematics of two dwarf spiral galaxies belonging to very different
environments, namely KK 246 in the Local Void and Holmberg II in the M81 group.
A mass model of the rotation curve of KK 246 is presented for the first time,
and we show that its observed kinematics are consistent with MOND. We re-derive
the outer rotation curve of Holmberg II, by modelling its HI data cube, and
find that its inclination should be closer to face-on than previously derived.
This implies that Holmberg II has a higher rotation velocity in its outer
parts, which, although not very precisely constrained, is consistent with the
MOND prediction.Comment: Accepted in A&A as a Research Note. 6 pages, 3 figure
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