204 research outputs found
Partially ionized atmospheres of neutron stars with strong magnetic fields
We construct hydrogen atmosphere models for strongly magnetized neutron stars
in thermodynamic equilibrium, taking into account partial ionization. The
presence of bound states affects the equation of state, absorption
coefficients, and polarizability tensor of a strongly magnetized plasma.
Therefore the partial ionization influences the polarization vectors and
opacities of normal electromagnetic waves, and thus the spectra of outgoing
radiation. Here we review a model suitable for the most typical neutron-star
atmospheres and focus on the problems that remain to be solved for its
extension to other atmospheric parameters.Comment: 7 pages, 1 figure, Adv. Sp. Res., in pres
Inconsistency in Fermi's probability of the quantum states
We point out an important hidden inconsistency in Fermi's probability of the
quantum states that engendered inconsistent/inaccurate equations-of-state
extensively used in the literature to model nonideal plasma systems. The
importance of this amendment goes beyond rectifying our comprehension and
foundation of an important physical problem to influencing contemporary
research results.Comment: Accepted for Publicatio
Cooling of Neutron Stars with Strong Toroidal Magnetic Fields
We present models of temperature distribution in the crust of a neutron star in the presence of a strong toroidal component superposed to the poloidal component of the magnetic field. The presence of such a toroidal field hinders heat flow toward the surface in a large part of the crust. As a result, the neutron star surface presents two warm regions surrounded by extended cold regions and has a thermal luminosity much lower than in the case the magnetic field is purely poloidal. We apply these models to calculate the thermal evolution of such neutron stars and show that the lowered photon luminosity naturally extends their life-time as detectable thermal X-ray sources
Neutron star cooling: Theoretical aspects and observational constraints
The cooling theory of isolated neutron stars is reviewed. The main cooling
regulators are discussed, first of all, operation of direct Urca process (or
similar processes in exotic phases of dense matter) and superfluidity in
stellar interiors. The prospects to constrain gross parameters of supranuclear
matter in neutron-star interiors by confronting cooling theory with
observations of isolated neutron stars are outlined. A related problem of
thermal states of transiently accreting neutron stars with deep crustal heating
of accreted matter is discussed in application to soft X-ray transients.Comment: 10 pages, 3 figures, Proceedings of the 34th COSPAR Scientific
Assembly (Adv. Sp. Res., accepted
Gauge Coupling Variation in Brane Models
We consider the space-time variation of gauge couplings in brane-world models
induced by the coupling to a bulk scalar field. A variation is generated by the
running of the gauge couplings with energy and a conformal anomaly while going
from the Jordan to the Einstein frame. We indicate that the one-loop
corrections cancel implying that one obtains a variation of the fine structure
constant by either directly coupling the gauge fields to the bulk scalar field
or having bulk scalar field dependent Yukawa couplings. Taking into account the
cosmological dynamics of the bulk scalar field, we constrain the strength of
the gauge coupling dependence on the bulk scalar field and relate it to
modifications of gravity at low energy.Comment: 4 pages, 1 figur
QED can explain the non-thermal emission from SGRs and AXPs : Variability
Owing to effects arising from quantum electrodynamics (QED),
magnetohydrodynamical fast modes of sufficient strength will break down to form
electron-positron pairs while traversing the magnetospheres of strongly
magnetised neutron stars. The bulk of the energy of the fast mode fuels the
development of an electron-positron fireball. However, a small, but potentially
observable, fraction of the energy ( ergs) can generate a
non-thermal distribution of electrons and positrons far from the star. This
paper examines the cooling and radiative output of these particles. Small-scale
waves may produce only the non-thermal emission. The properties of this
non-thermal emission in the absence of a fireball match those of the quiescent,
non-thermal radiation recently observed non-thermal emission from several
anomalous X-ray pulsars and soft-gamma repeaters. Initial estimates of the
emission as a function of angle indicate that the non-thermal emission should
be beamed and therefore one would expect this emission to be pulsed as well.
According to this model the pulsation of the non-thermal emission should be
between 90 and 180 degrees out of phase from the thermal emission from the
stellar surface.Comment: 7 pages, 5 figures, to appear in the proceedings of the conference
"Isolated Neutron Stars: from the Interior to the Surface" (April 2006,
London), eds. D. Page, R. Turolla, & S. Zane, Astrophysics & Space Scienc
Time Variations in the Scale of Grand Unification
We study the consequences of time variations in the scale of grand
unification, , when the Planck scale and the value of the unified coupling
at the Planck scale are held fixed. We show that the relation between the
variations of the low energy gauge couplings is highly model dependent. It is
even possible, in principle, that the electromagnetic coupling varies,
but the strong coupling does not (to leading approximation). We
investigate whether the interpretation of recent observations of quasar
absorption lines in terms of time variation in can be accounted for by
time variation in . Our formalism can be applied to any scenario where a
time variation in an intermediate scale induces, through threshold corrections,
time variations in the effective low scale couplings.Comment: 14 pages, revtex4; Updated observational results and improved
statistical analysis (section IV); added reference
Limits on Cosmological Variation of Strong Interaction and Quark Masses from Big Bang Nucleosynthesis, Cosmic, Laboratory and Oklo Data
Recent data on cosmological variation of the electromagnetic fine structure
constant from distant quasar (QSO) absorption spectra have inspired a more
general discussion of possible variation of other constants. We discuss
variation of strong scale and quark masses. We derive the limits on their
relative change from (i) primordial Big-Bang Nucleosynthesis (BBN); (ii)
Oklo natural nuclear reactor, (iii) quasar absorption spectra, and (iv)
laboratory measurements of hyperfine intervals.Comment: 10 pages 2 figurs: second version have several references added and
some new comment
Recent Advances in Modeling Stellar Interiors
Advances in stellar interior modeling are being driven by new data from
large-scale surveys and high-precision photometric and spectroscopic
observations. Here we focus on single stars in normal evolutionary phases; we
will not discuss the many advances in modeling star formation, interacting
binaries, supernovae, or neutron stars. We review briefly: 1) updates to input
physics of stellar models; 2) progress in two and three-dimensional evolution
and hydrodynamic models; 3) insights from oscillation data used to infer
stellar interior structure and validate model predictions (asteroseismology).
We close by highlighting a few outstanding problems, e.g., the driving
mechanisms for hybrid gamma Dor/delta Sct star pulsations, the cause of giant
eruptions seen in luminous blue variables such as eta Car and P Cyg, and the
solar abundance problem.Comment: Proceedings for invited talk at conference High Energy Density
Laboratory Astrophysics 2010, Caltech, March 2010, submitted for special
issue of Astrophysics and Space Science; 7 pages; 5 figure
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