Our pulsar work addresses the question, "why do pulsars shine?".
This question is easy to formulate but hard to answer. Our approach
is to combine theoretical and observational work in order to determine
the physical conditions in the pulsar emission region.
This page is under construction, please be patient . . .
See also our overview page
THE NATURE OF THE PULSAR MAGNETIC FIELD
The simplest model of a pulsar assumes the magnetic field is a pure
dipole, centered on the star. This has been a very useful picture,
but it is now clear that it cannot account for everything seen in new,
high-qulaity data. We are exploring the nature, and consequences, of
other field configurations. At left we show two views of a field
which mixes quadrupole and dipole components. At right we show the
effects of relativity on an oblique, rotating dipole.
Close-up view of mixed quadropolar field, representative
field lines.
More distant view of mixed quadropolar field,
representative field lines.
Oblique rotating dipole with relativistic
effects. The last field lines which close inside the light cylinder
are clown.
HIGH TIME RESOLUTION OBSERVATIONS OF PULSAR EMISSION
Theoretical models of the pulsar emission process predict that the
fundamental dynamical timescales of the underlying plasma should be on
the order of nanoseconds. Different models predict different detailed
temporal structure on this timescale. We are carrying out very high
time resolution observations in order to gather data which can be
directly compared with predictions of the theory. We have
multi-frequency data on giant pulses from the Crab pulsar at 10 nsec
resolution which reveal in detail the time and frequency structure of
the emission. These data are mapping the tiny micro-storms (about
100 m in size) which constantly appear and disappear within the
pulsar emission region.
STRONG PLASMA TURBULENCE IN THE RADIO-LOUD PLASMA
Plasma wave turbulence in a pulsar magnetosphere organizes into
localized regions of intense electric field.
Pulsar electromagnetics seems to require electrons and positrons in the
pair plasma above the neutron star to stream through each other.
According to theory, the streaming will produce electrostatic waves,
or plasma turbulence. One of several prominent candidates for the
pulsar emission mechanism is based on the conversion of the plasma
turbulence into electromagnetic waves. The conversion could occur through
a nonlinear state (so-called "strong" turbulence) in which
wavepackets form and "collapse" spatially. Our hypothesis is that these
transient wavepackets produce the bursts of radiation which constitute the
pulsar radio emission. We are conducting computer
simulations of the turbulence in order to characterize the temporal
behavior of the bursts. By comparison of observational and the
theoretical data sets, we hope to determine if these processes are
underlying the emission physics.
THE PAIR CASCADE IN STRONG AND WEAK MAGNETIC FIELDS
What is the origin of the radio-loud plasma in a pulsar
atmosphere? Most folks believe that it is an electron-positron pair
plasma, generated in a pair-creation cascade which is initiated by a
stray gamma ray photon which encounter's the stars strong magnetic
field. This cascade also leads to the pulsed X-ray and gamma-ray
emission from these stars. We have carried out numerical modelling of
the cascade, and find that the cascade development depends strongly on
the strength of the local magnetic field. The pair plasmas in
strong-field and weak-field pulsars will have very different momentum
distribution functions; these differences should be reflected in
their radio signatures. Strong-field and weak-field pulsars also have
quite different X-ray signatures, which may allow a direct measurement
of the magnetic field in the emission region.
Last Update: July 28, 1999