Pulsars
are a unique laboratory for studying physics under extreme
conditions which cannot be duplicated on earth. Their magnetospheres
contain highly relativistic electron-positron plasmas which are
confined and channeled by teragauss magnetic fields. The extremely
high precision which which their rotation rate can be measured has
provided a new, and successful, test of Einstein's theory of general
relativity.
However, despite nearly thirty years of study, in which a
wealth of high-precision radio data has been gathered on these
objects, we still do not understand the basic mechanism by which these
dense stars radiate.
We have, therefore, undertaken a program to combine new radio data
with new theoretical work to attempt to determine how pulsars shine.
On the observational side, we are using new technologies to develop
new radio instrumentation. When used with the VLA, or other telescopes such
as Arecibo or Parkes, our experiments
sample the signal from several bright pulsars at rates up to several tens
of million samples per second.
Thus, we can measure fluctuations
in the emission at a time resolution down to a few nanoseconds.
This allows us to detect structures as smaller than 1 m
in the pulsar's atmosphere, and to follow their evolution in time. We
believe these small structures are intimately connected to the
still-unknown radio emission mechanism.
On the theoretical side, we are studying the basic physics of the
region which emits the radio signal. This region is believed to
contain a relativistic plamsa streaming out from the star in two
``lighthouse beams'', one located over each magnetic pole of the
starr.
This is the region where the high particle energies, high
plasma densities and strong magnetic fields make the physics
interestingly complex. These plasma streams are thought to be very
unstable, and to break up into turbulent clumps on their way out of
the magnetosphere.
These clumps are probably the source of the radio
emission; we believe they are the tiny structures we are resolving in
time. We are applying theoretical techniques developed in the study
of terrestrial, laboratory plasmas with numerical methods, in order to
study the growth and development of this streaming plasma. Comparison
of our theoretical predictions with the high time resolution data will
allow us to specify the physical conditions, and the emission
mechanism, from these stars.
--- Tim Hankins, Jean Eilek; with J. Kern (NMT/NRAO),
J. Weatherall (FAA), J. Rankin (Vermont), A. Jessner (MPIfR)
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