Cyclotron emission from AM Herculis

The cyclotron absorption coefficients in the ordinary and extraordinary modes are calculated for the shock heated region of AM Her. The equations of radiative transfer are solved and the intensity of the emitted UV radiation determined as a function of angle. The average spectrum is shown to have deviations from the previously predicted Rayleigh-Jeans spectrum and the magnetic field of AM Her is deduced to be roughly 5 x 10 to the 7th power gauss

The Effect of Zinc Nutriture on Prostaglandin Synthesis and Fatty Acid Composition in Rat Testes.

To determine if zinc affects prostaglandin synthesis in the rat testis, one group of rats was fed a zinc deficient (\u3c 1 ppm) diet and two control groups were pair fed and ad lib fed a zinc sufficient (100 ppm) diet. The diets of half the zinc deficient and pair fed rats were supplemented with docosapentaenoic acid to determine if low levels of this acid, observed in the testes of zinc deficient rats, was responsible for some of the symptoms of zinc deficiency in the testis. In the tunica, 6-keto-PGF(,1(alpha)) and PGE(,2) levels (ng/gm) were lower in zinc deficient rats compared to controls, but there was no difference when PG concentration was expressed as ng/mg protein. In the testis parenchyma, zinc deficient rats had higher levels of PGs, probably due to increased levels of the precursor, arachidonic acid. PG synthesis was greater in tunica compared to parenchyma and in both tissues 6-keto-PGF(,1(alpha)) levels were much higher than those of PGE(,2). Decosapentaenoic acid supplementation had no effect on testis weight, PGs or sperm counts. It also had no effect on 22:5(omega)6 concentration in testes lipids implying that the level of supplementation was too low to have an effect, if any, on the testes. It was concluded that zinc affects PG levels in the testes, but only indirectly, through an effect on protein and fatty acid levels. It was also concluded that PGI(,2) was the major PG produced by the testis and that PG synthesis was an important function of the tunica

Evolution of Crustal Magnetic Fields in Isolated Neutron Stars : Combined Effects of Cooling and Curvature of Space-time

The ohmic decay of magnetic fields confined within the crust of neutron stars is considered by incorporating both the effect of neutron star cooling and the effect of space-time curvature produced by the intense gravitational field of the star. For this purpose a stationary and static gravitational field has been considered with the standard as well as the accelerated cooling models of neutron stars. It is shown that general relativistic effect reduces the magnetic field decay rate substantially. At the late stage of evolution when the field decay is mainly determined by the impurity-electron scattering, the effect of space-time curvature suppresses the role of the impurity content significantly and reduces the decay rate by more than an order of magnitude. Even with a high impurity content the decay rate is too low to be of observational interest if the accelerated cooling model along with the effect of space-time curvature is taken into account. It is, therefore, pointed out that if a decrease in the magnetic field strength by more than two orders of magnitude from its initial value is detected by observation then the existence of quark in the core of the neutron star would possibly be ruled out.Comment: 15 pages, AAS LATEX macros v4.0, 5 postscript figures, Accepted for publication in the Astrophysical Journal (Part I

Accretion physics of AM Herculis binaries, I. Results from one-dimensional stationary radiation hydrodynamics

We have solved the one-dimensional stationary two-fluid hydrodynamic equations for post-shock flows on accreting magnetic white dwarfs simultaneous with the fully frequency and angle-dependent radiative transfer for cyclotron radiation and bremsstrahlung. Magnetic field strengths B = 10 to 100 MG are considered. At given B, this theory relates the properties of the emission region to a single physical parameter, the mass flow density (or accretion rate per unit area). We present the normalized temperature profiles and fit formulae for the peak electron temperature, the geometrical shock height, and the column density of the post-shock flow. The results apply to pillbox-shaped emission regions. With a first-order temperature correction they can also be used for narrower columns provided they are not too tall.Comment: 10 pages with 10 Postscript figures, accepted for publication in Astronomy & Astrophysics. The source file contains Table 1a/b in ASCII forma

Pulsars With Jets May Harbor Dynamically Important Accretion Disks

For many astrophysical sources with jets, there is evidence for the contemporaneous presence of disks. In contrast, pulsars such as the Crab and Vela show jets but have not yet revealed direct evidence for accretion disks. Here we show that for such pulsars, an accretion disk radiating below detectable thresholds may simultaneously account for (1) observed deviations in the braking indices from that of the simple dipole, (2) observed pulsar timing ages, and (3) possibly even the jet morphology via a disk outflow that interacts with the pulsar wind within, collimating and/or redirecting it.Comment: 10 pages, 2 figs., in press, ApJ. Let

Global axisymmetric Magnetorotational Instability with density gradients

We examine global incompressible axisymmetric perturbations of a differentially rotating MHD plasma with radial density gradients. It is shown that the standard magnetorotational instability, (MRI) criterion drawn from the local dispersion relation is often misleading. If the equilibrium magnetic field is either purely axial or purely toroidal, the problem reduces to finding the global radial eigenvalues of an effective potential. The standard Keplerian profile including the origin is mathematically ill-posed, and thus any solution will depend strongly on the inner boundary. We find a class of unstable modes localized by the form of the rotation and density profiles, with reduced dependence on boundary conditions.Comment: 22 pages, 5 figure

Cyclotron-Synchrotron: harmonic fitting functions in the non-relativistic and trans-relativistic regimes

The present work investigates the calculation of absorption and emission cyclotron line profiles in the non-relativistic and trans-relativistic regimes. We provide fits for the ten first harmonics with synthetic functions down to 10^(-4) of the maximum flux with an accuracy of 20 per cent at worst. The lines at a given particle energy are calculated from the integration of the Schott formula over the photon and the particle solid angles relative to the magnetic field direction. The method can easily be extended to a larger number of harmonics. We also derive spectral fits of thermal emission line plasmas at non-relativistic and trans-relativistic temperatures extending previous parameterisations.Comment: 11 pages, 11 figures, accepted for publication in Astronomy & Astrophysic

High and low states of the system AM Herculis

Context: We investigate the distribution of optically high and low states of the system AM Herculis (AM Her). Aims: We determine the state duty cycles, and their relationships with the mass transfer process and binary orbital evolution of the system. Methods: We make use of the photographic plate archive of the Harvard College Observatory between 1890 and 1953 and visual observations collected by the American Association of Variable Star Observers between 1978 and 2005. We determine the statistical probability of the two states, their distribution and recurrence behaviors. Results: We find that the fractional high state duty cycle of the system AM Her is 63%. The data show no preference of timescales on which high or low states occur. However, there appears to be a pattern of long and short duty cycle alternation, suggesting that the state transitions retain memories. We assess models for the high/low states for polars (AM Her type systems). We propose that the white-dwarf magnetic field plays a key role in regulating the mass transfer rate and hence the high/low brightness states, due to variations in the magnetic-field configuration in the system.Comment: 8 pages, 5 figures, accepted for publication in A&

The Tayler instability of toroidal magnetic fields in a columnar gallium experiment

The nonaxisymmetric Tayler instability of toroidal magnetic fields due to axial electric currents is studied for conducting incompressible fluids between two coaxial cylinders without endplates. The inner cylinder is considered as so thin that even the limit of R_in \to 0 can be computed. The magnetic Prandtl number is varied over many orders of magnitudes but the azimuthal mode number of the perturbations is fixed to m=1. In the linear approximation the critical magnetic field amplitudes and the growth rates of the instability are determined for both resting and rotating cylinders. Without rotation the critical Hartmann numbers do {\em not} depend on the magnetic Prandtl number but this is not true for the growth rates. For given product of viscosity and magnetic diffusivity the growth rates for small and large magnetic Prandtl number are much smaller than those for Pm=1. For gallium under the influence of a magnetic field at the outer cylinder of 1 kG the resulting growth time is 5 s. The minimum electric current through a container of 10 cm diameter to excite the kink-type instability is 3.20 kA. For a rotating container both the critical magnetic field and the related growth times are larger than for the resting column.Comment: 7 pages, 9 figures, submitted to Astron. Nach