40,558 research outputs found
High magnetic field pulsars and magnetars: a unified picture
We propose a unified picture of high magnetic field radio pulsars and
magnetars by arguing that they are all rotating high-field neutron stars, but
have different orientations of their magnetic axes with respective to their
rotation axes. In strong magnetic fields where photon splitting suppresses pair
creation near the surface, the high-field pulsars can have active inner
accelerators while the anomalous X-ray pulsars cannot. This can account for the
very different observed emission characteristics of the anomalous X-ray pulsar
1E 2259+586 and the high field radio pulsar PSR J1814-1744. A predicted
consequence of this picture is that radio pulsars having surface magnetic field
greater than about G should not exist.Comment: 5 pages, emulateapj style, accepted for publication in the ApJ
Letter
Spectral Line Broadening and Angular Blurring due to Spacetime Geometry Fluctuations
We treat two possible phenomenological effects of quantum fluctuations of
spacetime geometry: spectral line broadening and angular blurring of the image
of a distance source. A geometrical construction will be used to express both
effects in terms of the Riemann tensor correlation function. We apply the
resulting expressions to study some explicit examples in which the fluctuations
arise from a bath of gravitons in either a squeezed state or a thermal state.
In the case of a squeezed state, one has two limits of interest: a coherent
state which exhibits classical time variation but no fluctuations, and a
squeezed vacuum state, in which the fluctuations are maximized.Comment: 21 pages, 2 figures. Dedicated to Raphael Sorkin on the occasion of
his 60th birthday. (v2: several references added and some minor errors
corrected
The relationship between induced fluid structure and boundary slip in nanoscale polymer films
The molecular mechanism of slip at the interface between polymer melts and
weakly attractive smooth surfaces is investigated using molecular dynamics
simulations. In agreement with our previous studies on slip flow of
shear-thinning fluids, it is shown that the slip length passes through a local
minimum at low shear rates and then increases rapidly at higher shear rates. We
found that at sufficiently high shear rates, the slip flow over atomically flat
crystalline surfaces is anisotropic. It is demonstrated numerically that the
friction coefficient at the liquid-solid interface (the ratio of viscosity and
slip length) undergoes a transition from a constant value to the power-law
decay as a function of the slip velocity. The characteristic velocity of the
transition correlates well with the diffusion velocity of fluid monomers in the
first fluid layer near the solid wall at equilibrium. We also show that in the
linear regime, the friction coefficient is well described by a function of a
single variable, which is a product of the magnitude of surface-induced peak in
the structure factor and the contact density of the adjacent fluid layer. The
universal relationship between the friction coefficient and induced fluid
structure holds for a number of material parameters of the interface: fluid
density, chain length, wall-fluid interaction energy, wall density, lattice
type and orientation, thermal or solid walls.Comment: 33 pages, 14 figure
Magnetars and pulsars: a missing link
There is growing evidence that soft gamma-ray repeaters (SGRs) and anomalous
X-ray pulsars (AXPs) are isolated neutron stars with superstrong magnetic
fields, i.e., magnetars, marking them a distinguished species from the
conventional species of spindown-powered isolated neutron stars, i.e., radio
pulsars. The current arguments in favor of the magnetar interpretation of
SGR/AXP phenomenology will be outlined, and the two energy sources in
magnetars, i.e. a magnetic dissipation energy and a spindown energy, will be
reviewed. I will then discuss a missing link between magnetars and pulsars,
i.e., lack of the observational evidence of the spindown-powered behaviors in
known magnetars. Some recent theoretical efforts in studying such behaviors
will be reviewed along with some predictions testable in the near future.Comment: Invited talk at the Sixth Pacific Rim Conference on Stellar
Astrophysics, a tribute to Helmut A. Abt, July 11-17, 2002, Xi'an. To appear
in the proceedings (eds. K. S. Cheng, K. C. Leung & T. P. Li
Slip boundary conditions for shear flow of polymer melts past atomically flat surfaces
Molecular dynamics simulations are carried out to investigate the dynamic
behavior of the slip length in thin polymer films confined between atomically
smooth thermal surfaces. For weak wall-fluid interactions, the shear rate
dependence of the slip length acquires a distinct local minimum followed by a
rapid growth at higher shear rates. With increasing fluid density, the position
of the local minimum is shifted to lower shear rates. We found that the ratio
of the shear viscosity to the slip length, which defines the friction
coefficient at the liquid/solid interface, undergoes a transition from a nearly
constant value to the power law decay as a function of the slip velocity. In a
wide range of shear rates and fluid densities, the friction coefficient is
determined by the product of the value of surface induced peak in the structure
factor and the contact density of the first fluid layer near the solid wall.Comment: 27 pages, 11 figure
Electrodynamics of Magnetars: Implications for the Persistent X-ray Emission and Spindown of the Soft Gamma Repeaters and Anomalous X-ray Pulsars
(ABBREVIATED) We consider the structure of neutron star magnetospheres
threaded by large-scale electrical currents, and the effect of resonant Compton
scattering by the charge carriers (both electrons and ions) on the emergent
X-ray spectra and pulse profiles. In the magnetar model for the SGRs and AXPs,
these currents are maintained by magnetic stresses acting deep inside the star.
We construct self-similar, force-free equilibria of the current-carrying
magnetosphere with a power-law dependence of magnetic field on radius, B ~
r^(-2-p), and show that a large-scale twist softens the radial dependence to p
< 1. The spindown torque acting on the star is thereby increased in comparison
with a vacuum dipole. We comment on the strength of the surface magnetic field
in the SGR and AXP sources, and the implications of this model for the narrow
measured distribution of spin periods. A magnetosphere with a strong twist,
B_\phi/B_\theta = O(1) at the equator, has an optical depth ~ 1 to resonant
cyclotron scattering, independent of frequency (radius), surface magnetic field
strength, or charge/mass ratio of the scattering charge. When electrons and
ions supply the current, the stellar surface is also heated by the impacting
charges at a rate comparable to the observed X-ray output of the SGR and AXP
sources, if B_{dipole} ~ 10^{14} G. Redistribution of the emerging X-ray flux
at the ion and electron cyclotron resonances will significantly modify the
emerging pulse profile and, through the Doppler effect, generate a non-thermal
tail to the X-ray spectrum. The sudden change in the pulse profile of SGR
1900+14 after the 27 August 1998 giant flare is related to an enhanced optical
depth to electron cyclotron scattering, resulting from a sudden twist imparted
to the external magnetic field.Comment: 31 January 2002, minor revisions, new section 5.4.
Magnetic Photon Splitting: Computations of Proper-time Rates and Spectra
The splitting of photons in the presence of an intense magnetic field has
recently found astrophysical applications in polar cap models of gamma-ray
pulsars and in magnetar scenarios for soft gamma repeaters. Numerical
computation of the polarization-dependent rates of this third order QED process
for arbitrary field strengths and energies below pair creation threshold is
difficult: thus early analyses focused on analytic developments and simpler
asymptotic forms. The recent astrophysical interest spurred the use of the
S-matrix approach by Mentzel, Berg and Wunner to determine splitting rates. In
this paper, we present numerical computations of a full proper-time expression
for the rate of splitting that was obtained by Stoneham, and is exact up to the
pair creation threshold. While the numerical results derived here are in accord
with the earlier asymptotic forms due to Adler, our computed rates still differ
by as much as factors of 3 from the S-matrix re-evaluation of Wilke and Wunner,
reflecting the extreme difficulty of generating accurate S-matrix numerics for
fields below about \teq{4.4\times 10^{13}}Gauss. We find that our proper-time
rates appear very accurate, and exceed Adler's asymptotic specializations
significantly only for photon energies just below pair threshold and for
supercritical fields, but always by less than a factor of around 2.6. We also
provide a useful analytic series expansion for the scattering amplitude valid
at low energies.Comment: 13 pages, AASTeX format, including 3 eps figures, ApJ in pres
Interferometric Observations of V838 Monocerotis
We have used long-baseline near-IR interferometry to resolve the peculiar
eruptive variable V838 Mon and to provide the first direct measurement of its
angular size. Assuming a uniform disk model for the emission we derive an
apparent angular diameter at the time of observations (November-December 2004)
of milli-arcseconds. For a nominal distance of kpc,
this implies a linear radius of . However, the data are
somewhat better fit by elliptical disk or binary component models, and we
suggest that the emission may be strongly affected by ejecta from the outburst.Comment: 12 pages, 1 two-part encapsulated postscript figure. Accepted by
ApJL. Added a table of observation
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