Spot-like Structures of Neutron Star Surface Magnetic Fields

There is growing evidence, based on both X-ray and radio observations of isolated neutron stars, that besides the large--scale (dipolar) magnetic field, which determines the pulsar spin--down behaviour, small--scale poloidal field components are present, which have surface strengths one to two orders of magnitude larger than the dipolar component. We argue in this paper that the Hall--effect can be an efficient process in producing such small--scale field structures just above the neutron star surface. It is shown that due to a Hall--drift induced instability, poloidal magnetic field structures can be generated from strong subsurface toroidal fields, which are the result of either a dynamo or a thermoelectric instability acting at early times of a neutron star's life. The geometrical structure of these small--scale surface anomalies of the magnetic field resembles that of some types of ``star--spots''. The magnetic field strength and the length--scales are comparable with values that can be derived from various observations.Comment: 4 pages, 2 figures, accepted by Astronomy & Astrophysics Letters; language improved, 2nd para of Sect. 3 change

Spark Model for Pulsar Radiation Modulation Patterns

A non-stationary polar gap model first proposed by Ruderman & Sutherland (1975) is modified and applied to spark-associated pulsar emission at radio wave-lengths. It is argued that under physical and geometrical conditions prevailing above pulsar polar cap, highly non-stationary spark discharges do not occur at random positions. Instead, sparks should tend to operate in well determined preferred regions. At any instant the polar cap is populated as densely as possible with a number of two-dimensional sparks with a characteristic dimension as well as a typical distance between adjacent sparks being about the polar gap height. Our model differs, however, markedly from its original 'hollow cone' version. The key feature is the quasi-central spark driven by pair production process and anchored to the local pole of a sunspot-like surface magnetic field. This fixed spark prevents the motion of other sparks towards the pole, restricting it to slow circumferential drift across the planes of field lines converging at the local pole. We argue that the polar spark constitutes the core pulsar emission, and that the annular rings of drifting sparks contribute to conal components of the pulsar beam. We found that the number of nested cones in the beam of typical pulsar should not excced three; a number also found by Mitra & Deshpande (1999) using a completely different analysis.Comment: 31 pages, 8 figures, accepted by Ap

A Next-to-Minimal Supersymmetric Model of Hybrid Inflation

We propose a model of inflation based on a simple variant of the NMSSM, called $\phi$NMSSM, where the additional singlet $\phi$ plays the role of the inflaton in hybrid (or inverted hybrid) type models. As in the original NMSSM, the $\phi$NMSSM solves the $\mu$ problem of the MSSM via the VEV of a gauge singlet $N$, but unlike the NMSSM does not suffer from domain wall problems since the offending $Z_3$ symmetry is replaced by an approximate Peccei-Quinn symmetry which also solves the strong CP problem, and leads to an invisible axion with interesting cosmological consequences. The PQ symmetry may arise from a superstring model with an exact discrete $Z_3 \times Z_5$ symmetry after compactification. The model predicts a spectral index $n=1$ to one part in $10^{12}$.Comment: 17 pages, Latex; note added, accepted for Phys. Lett.

Time-scales of Radio Emission in PSR J0437-4715 at 327 MHz

Time-scales of radio emission are studied in PSR J0437-4715 at 327 MHz using almost half a million periods of high quality data from Ooty Radio Telescope. The radio emission in this milli second pulsar occurs on a short (s) time-scale of approximately 0.026 +- 0.001 periods, and on a (l) time-scale that is much longer than the widths of the components of the integrated profile (approximately 0.05 periods). The width of the s emission increases with its increasing relative contribution to the total radio emission. This may provide constraints for the details of discharge of vacuum gaps above pulsar polar caps. The s emission occasionally takes place in the form of intense spikes, which are confined to the main component of the integrated profile for 90 per cent of the time. The positions of spikes within a component of the integrated profile have no simple relation to the shape of that component. This may have impact on the interpretation of the integrated profile components in terms of independent regions of emission on the polar cap.Comment: Accepted for publication in Vol 543 (1 Nov 2000) of The Astrophysical Journa