25,156 research outputs found
Nuclear Equation of State and Internal Structure of Magnetars
Recently, neutron stars with very strong surface magnetic fields have been
suggested as the site for the origin of observed soft gamma repeaters (SGRs).
We investigate the influence of a strong magnetic field on the properties and
internal structure of such strongly magnetized neutron stars (magnetars). The
presence of a sufficiently strong magnetic field changes the ratio of protons
to neutrons as well as the neutron appearance density. We also study the pion
production and pion condensation in a strong magnetic field. We discuss the
pion condensation in the interior of magnetars as a possible source of SGRs.Comment: 5 pages with 3 figures, To appear in the Proceedings of the 5th
Huntsville Gamma Ray Burst Symposium, Huntsville, Alabama, USA, Oct. 18-22,
199
Constraining the neutron star equation of state using XMM-Newton
We have identified three possible ways in which future XMM-Newton
observations can provide significant constraints on the equation of state of
neutron stars. First, using a long observation of the neutron star X-ray
transient CenX-4 in quiescence one can use the RGS spectrum to constrain the
interstellar extinction to the source. This removes this parameter from the
X-ray spectral fitting of the pn and MOS spectra and allows us to investigate
whether the variability observed in the quiescent X-ray spectrum of this source
is due to variations in the soft thermal spectral component or variations in
the power law spectral component coupled with variations in N_H. This will test
whether the soft thermal spectral component can indeed be due to the hot
thermal glow of the neutron star. Potentially such an observation could also
reveal redshifted spectral lines from the neutron star surface. Second,
XMM-Newton observations of radius expansion type I X-ray bursts might reveal
redshifted absorption lines from the surface of the neutron star. Third,
XMM-Newton observations of eclipsing quiescent low-mass X-ray binaries provide
the eclipse duration. With this the system inclination can be determined
accurately. The inclination determined from the X-ray eclipse duration in
quiescence, the rotational velocity of the companion star and the
semi-amplitude of the radial velocity curve determined through optical
spectroscopy, yield the neutron star mass.Comment: 4 pages, 1 figure, proceedings of the XMM-Newton workshop, June 2007,
accepted for publication in A
Detection of a 1258 Hz high-amplitude kilohertz quasi-periodic oscillation in the ultra-compact X-ray binary 1A 1246-588
We have observed the ultra-compact low-mass X-ray binary (LMXB) 1A 1246-588
with the Rossi X-ray Timing Explorer (RXTE). In this manuscript we report the
discovery of a kilohertz quasi-periodic oscillation (QPO) in 1A 1246-588. The
kilohertz QPO was only detected when the source was in a soft high-flux state
reminiscent of the lower banana branch in atoll sources. Only one kilohertz QPO
peak is detected at a relatively high frequency of 1258+-2 Hz and at a single
trial significance of more than 7 sigma. Kilohertz QPOs with a higher frequency
have only been found on two occasions in 4U 0614+09. Furthermore, the frequency
is higher than that found for the lower kilohertz QPO in any source, strongly
suggesting that the QPO is the upper of the kilohertz QPO pair often found in
LMXBs. The full-width at half maximum is 25+-4 Hz, making the coherence the
highest found for an upper kilohertz QPO. From a distance estimate of ~6 kpc
from a radius expansion burst we derive that 1A 1246-588 is at a persistent
flux of ~0.2-0.3 per cent of the Eddington flux, hence 1A 1246-588 is one of
the weakest LMXBs for which a kilohertz QPO has been detected. The
root-mean-square (rms) amplitude in the 5-60 keV band is 27+-3 per cent, this
is the highest for any kilohertz QPO source so far, in line with the general
anti-correlation between source luminosity and rms amplitude of the kilohertz
QPO peak identified before. Using the X-ray spectral information we produce a
colour-colour diagram. The source behaviour in this diagram provides further
evidence for the atoll nature of the source.Comment: 4 pages, 3 figures, accepted for publication in MNRA
A coalescence model for freely decaying two-dimensional turbulence
We propose a ballistic coalescence model (punctuated-Hamiltonian approach)
mimicking the fusion of vortices in freely decaying two-dimensional turbulence.
A temporal scaling behaviour is reached where the vortex density evolves like
. A mean-field analytical argument yielding the approximation
is shown to slightly overestimate the decay exponent whereas
Molecular Dynamics simulations give , in agreement with
recent laboratory experiments and simulations of Navier-Stokes equation.Comment: 6 pages, 1 figure, to appear in Europhysics Letter
The Berry-Keating Hamiltonian and the Local Riemann Hypothesis
The local Riemann hypothesis states that the zeros of the Mellin transform of
a harmonic-oscillator eigenfunction (on a real or p-adic configuration space)
have real part 1/2. For the real case, we show that the imaginary parts of
these zeros are the eigenvalues of the Berry-Keating hamiltonian H=(xp+px)/2
projected onto the subspace of oscillator eigenfunctions of lower level. This
gives a spectral proof of the local Riemann hypothesis for the reals, in the
spirit of the Hilbert-Polya conjecture. The p-adic case is also discussed.Comment: 9 pages, no figures; v2 included more mathematical background, v3 has
minor edits for clarit
Commutator Leavitt path algebras
For any field K and directed graph E, we completely describe the elements of
the Leavitt path algebra L_K(E) which lie in the commutator subspace
[L_K(E),L_K(E)]. We then use this result to classify all Leavitt path algebras
L_K(E) that satisfy L_K(E)=[L_K(E),L_K(E)]. We also show that these Leavitt
path algebras have the additional (unusual) property that all their Lie ideals
are (ring-theoretic) ideals, and construct examples of such rings with various
ideal structures.Comment: 24 page
Mass-Radius Relation for Magnetic White Dwarfs
Recently, several white dwarfs with very strong surface magnetic fields have
been observed. In this paper we explore the possibility that such stars could
have sufficiently strong internal fields to alter their structure. We obtain a
revised white dwarf mass-radius relation in the presence of strong internal
magnetic fields. We first derive the equation of state for a fully degenerate
ideal electron gas in a magnetic field using an Euler-MacLaurin expansion. We
use this to obtain the mass-radius relation for magnetic He, C,
and Fe white dwarfs of uniform composition.Comment: 7 pages, 7 figures and 1 table, To appear in Ap
Mean-Field Equations for Spin Models with Orthogonal Interaction Matrices
We study the metastable states in Ising spin models with orthogonal
interaction matrices. We focus on three realizations of this model, the random
case and two non-random cases, i.e.\ the fully-frustrated model on an infinite
dimensional hypercube and the so-called sine-model. We use the mean-field (or
{\sc tap}) equations which we derive by resuming the high-temperature expansion
of the Gibbs free energy. In some special non-random cases, we can find the
absolute minimum of the free energy. For the random case we compute the average
number of solutions to the {\sc tap} equations. We find that the
configurational entropy (or complexity) is extensive in the range
T_{\mbox{\tiny RSB}}. Finally we present an apparently
unrelated replica calculation which reproduces the analytical expression for
the total number of {\sc tap} solutions.Comment: 22+3 pages, section 5 slightly modified, 1 Ref added, LaTeX and
uuencoded figures now independent of each other (easier to print). Postscript
available http://chimera.roma1.infn.it/index_papers_complex.htm
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