1,007 research outputs found
Breakdown of the linear approximation in the perturbative analysis of heat conduction in relativistic systems
We analyze the effects of thermal conduction in a relativistic fluid just
after its departure from spherical symmetry, on a time scale of the order of
relaxation time. Using first order perturbation theory, it is shown that, as in
spherical systems, at a critical point the effective inertial mass density of a
fluid element vanishes and becomes negative beyond that point. The impact of
this effect on the reliability of causality conditions is discussed.Comment: 11 pages (Latex2.09) To appear in Physics Letters
On Scaling Solutions with a Dissipative Fluid
We study the asymptotic behaviour of scaling solutions with a dissipative
fluid and we show that, contrary to recent claims, the existence of stable
accelerating attractor solution which solves the `energy' coincidence problem
depends crucially on the chosen equations of state for the thermodynamical
variables. We discuss two types of equations of state, one which contradicts
this claim, and one which supports it.Comment: 8 pages and 5 figures; to appear in Class. Quantum Gra
Thermal Conduction in Systems out of Hydrostatic Equilibrium
We analyse the effects of thermal conduction in a relativistic fluid, just
after its departure from hydrostatic equilibrium, on a time scale of the order
of thermal relaxation time. It is obtained that the resulting evolution will
critically depend on a parameter defined in terms of thermodynamic variables,
which is constrained by causality requirements.Comment: 16 pages, emTex (LaTex 2.09). To appear in Classical and Quantum
Gravit
Optical pulsations from a transitional millisecond pulsar
Weakly magnetic, millisecond spinning neutron stars attain their very fast
rotation through a 1E8-1E9 yr long phase during which they undergo
disk-accretion of matter from a low mass companion star. They can be detected
as accretion-powered millisecond X-ray pulsars if towards the end of this phase
their magnetic field is still strong enough to channel the accreting matter
towards the magnetic poles. When mass transfer is much reduced or ceases
altogether, pulsed emission generated by particle acceleration in the
magnetosphere and powered by the rotation of the neutron star is observed,
preferentially in the radio and gamma-ray bands. A few transitional millisecond
pulsars that swing between an accretion-powered X-ray pulsar regime and a
rotationally-powered radio pulsar regime in response to variations of the mass
in-flow rate have been recently identified. Here we report the detection of
optical pulsations from a transitional pulsar, the first ever from a
millisecond spinning neutron star. The pulsations were observed when the pulsar
was surrounded by an accretion disk and originated inside the magnetosphere or
within a few hundreds of kilometres from it. Energy arguments rule out
reprocessing of accretion-powered X-ray emission and argue against a process
related to accretion onto the pulsar polar caps; synchrotron emission of
electrons in a rotation-powered pulsar magnetosphere seems more likely.Comment: 32 pages, 7 figures. The first two authors contributed equally to
this wor
Viscous Quark-Gluon Plasma in the Early Universe
We consider the evolution of a flat, isotropic and homogeneous
Friedmann-Robertson-Walker Universe, filled with a causal bulk viscous
cosmological fluid, that can be characterized by an ultra-relativistic equation
of state and bulk viscosity coefficient obtained from recent lattice QCD
calculations. The basic equation for the Hubble parameter is derived under the
assumption that the total energy in the Universe is conserved. By assuming a
power law dependence of bulk viscosity coefficient, temperature and relaxation
time on energy density, an approximate solution of the field equations has been
obtained, in which we utilized equations of state from recent lattice QCD
simulations QCD and heavy-ion collisions to derive an evolution equation. In
this treatment for the viscous cosmology, we found no evidence for singularity.
For example, both Hubble parameter and scale factor are finite at , is
the comoving time. Furthermore, their time evolution essentially differs from
the one associated with non-viscous and ideal gas. Also thermodynamic
quantities, like temperature, energy density and bulk pressure remain finite as
well. In order to prove that the free parameter in our model does influence the
final results, qualitatively, we checked out other articular solutions.Comment: 19 pages, 4 figures including 7 eps graph
An XMM-Newton study of the 401 Hz accreting pulsar SAX J1808.4-3658 in quiescence
SAX J1808.4-3658 is a unique source being the first Low Mass X-ray Binary
showing coherent pulsations at a spin period comparable to that of millisecond
radio pulsars. Here we present an XMM-Newton observation of SAX J1808.4-3658 in
quiescence, the first which assessed its quiescent luminosity and spectrum with
good signal to noise. XMM-Newton did not reveal other sources in the vicinity
of SAX J1808.4-3658 likely indicating that the source was also detected by
previous BeppoSAX and ASCA observations, even if with large positional and flux
uncertainties. We derive a 0.5-10 keV unabsorbed luminosity of L_X=5x10^{31}
erg/s, a relatively low value compared with other neutron star soft X-ray
transient sources. At variance with other soft X-ray transients, the quiescent
spectrum of SAX J1808.4-3658 was dominated by a hard (Gamma~1.5) power law with
only a minor contribution (<10%) from a soft black body component. If the power
law originates in the shock between the wind of a turned-on radio pulsar and
matter outflowing from the companion, then a spin-down to X-ray luminosity
conversion efficiency of eta~10^{-3} is derived; this is in line with the value
estimated from the eclipsing radio pulsar PSR J1740-5340. Within the deep
crustal heating model, the faintness of the blackbody-like component indicates
that SAX J1808.4-3658 likely hosts a massive neutronstar (M>1.7 solar masses).Comment: Paper accepted for publication in ApJ
On the stability of the shear-free condition
The evolution equation for the shear is reobtained for a spherically
symmetric anisotropic, viscous dissipative fluid distribution, which allows us
to investigate conditions for the stability of the shear-free condition. The
specific case of geodesic fluids is considered in detail, showing that the
shear-free condition, in this particular case, may be unstable, the departure
from the shear-free condition being controlled by the expansion scalar and a
single scalar function defined in terms of the anisotropy of the pressure, the
shear viscosity and the Weyl tensor or, alternatively, in terms of the
anisotropy of the pressure, the dissipative variables and the energy density
inhomogeneity.Comment: 19 pages Latex. To appear in Gen. Rel. Gra
Expansion-Free Cavity Evolution: Some exact Analytical Models
We consider spherically symmetric distributions of anisotropic fluids with a
central vacuum cavity, evolving under the condition of vanishing expansion
scalar. Some analytical solutions are found satisfying Darmois junction
conditions on both delimiting boundary surfaces, while some others require the
presence of thin shells on either (or both) boundary surfaces. The solutions
here obtained model the evolution of the vacuum cavity and the surrounding
fluid distribution, emerging after a central explosion. This study complements
a previously published work where modeling of the evolution of such kind of
systems was achieved through a different kinematical condition.Comment: 9 pages, Revtex. Typos corrected. Published in Int. J. Mod. Phys.
Hyperbolic theories of dissipation: Why and when do we need them?
We illustrate and emphasize the relevance of hyperbolic theories of
dissipation in different physical scenarios. Particular attention is paid to
self-gravitating systems where the relaxation time may become large enough as
to require a description of the transient regime. It is argued that even
outside that regime, hyperbolic theories may be needed to provide an accurate
description of dissipative processes.Comment: 15 pages Latex, to appear in Physica
On the dual interpretation of zero-curvature Friedmann-Robertson-Walker models
Two possible interpretations of FRW cosmologies (perfect fluid or dissipative
fluid)are considered as consecutive phases of the system. Necessary conditions
are found, for the transition from perfect fluid to dissipative regime to
occur, bringing out the conspicuous role played by a particular state of the
system (the ''critical point '').Comment: 13 pages Latex, to appear in Class.Quantum Gra
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