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Pulsars are mysterious objects that emit electromagnetic radiation in a
clockwork-like fashion. The period of this pulsing can be extremely short.
Back in 1967, Jocelyn Bell Burnell, using a newly commissioned radio telescope,
discovered a very strange signal. It was initially dismissed as radio
interference by her supervisor. But continued observation showed that it
appeared at the same declination and right ascension, which ruled out a
terrestrial source of interference. They eventually were able to resolve the signal
as a series of pulses, evenly spaced every 1.32 seconds.
At the time, no astronomical object of that nature had ever been observed.
Speculation as to the origin of the signal started to grow, with some even
suggesting that the signal might be beamed to Earth from extraterrestrial intelligence.
In December of the same year she managed to discover a second pulsar and ruled out
the idea of it being a signal from ETs. In 1968 Thomas Gould suggested that
pulsars were highly magnetized, rapidly rotating neutron stars.
This model required a high energy plasma beam to be emitted from the poles of a
spinning neutron star, and accelerated along the star's dipole magnetic field,
emitting radiation as pulses towards the observer. Like the sweeping lights from a lighthouse.
Since then many more pulsars have been observed, and the shortest pulse ever
detected was a staggering 1.4 milliseconds. Where does the neutron star come from
and why does it rotate so fast? The mainstream idea is that these
objects are the remnants of a supernova explosion. The central region of a star
collapses, crushing together every proton and electron into a neutron.
The dense core of the star is all that is left. The angular momentum of the
original star is conserved, meaning the much smaller object has to rotate at an incredible speed.
The only way for those beams to be produced is in the presence of a very strong magnetic field.
But all we have are neutrons. The origin of the critically strong
magnetic field required for their model is therefore unclear. To add to these problems, a new pulsar
has been discovered, which seems to rewrite a lot of these ideas. The pulsar had one of the slowest pulse
rates ever seen, at 76 seconds. Most are between a few seconds down to milliseconds.
Apart from the unexpected pulse rate, it also seems to reside in what scientists
call the “neutron star graveyard.” This is a region of space where they
don't expect to detect any radio emissions, at all. This is because they theorize that
neutron stars here, are at the end of their life cycle and therefore not active.
It is also unique, as it appears to produce at least seven distinctly different pulse shapes,
whereas most neutron stars don't exhibit much variety. This makes it hard for them to explain,
using their model of a rotating beam. One particular type of pulse shows a
strong quasi-periodic structure. This suggests that a type of oscillation is driving the radio emissions.
This is a large clue to understanding what causes the pulses in the pulsar.
Anthony Peratt and Kevin Healy have proposed a completely different mechanism,
which would not require a fast spinning plasma beam, but instead, utilize the
known properties of transmission lines. This is similar to the way planetary
magnetospheres are described. If the star is surrounded by a disk of plasma,
then it is likely that it is electro- magnetically coupled to the
magnetosphere via field-aligned currents. In a lossless line with perfect end-conductors,
the pulses propagate back and forth forever. In reality, the kinetic
energy of the relativistic current flowing along the guide’s magnetic field
is lost in synchrotron radiation. Any disturbance on a transmission line,
such as an arc discharge or an interruption of steady-state conditions,
results in initiation of travelling waves, which propagate towards the ends of the line,
where they may be reflected and modified, attenuated and distorted by any losses
until they die out. Field-aligned current pulses propagate at high latitudes along the dipole
magnetic field. Two regions of high density plasma terminate this magnetospheric
transmission line: plasma near the pulsar surface, and plasma at the disk.
The transmission line may also contain a double layer. This structure forms a
toroid and the relativistic current pulses propagate within the toroid waveguide.
The pulsar's magnetospheric transmission line can be modeled by two current-
conducting boundaries that represent the inward and outward field-aligned currents.
The line is terminated by two conductors - the pulsar surface and the disc. A voltage
pulse is launched from the pulsar surface. This ends up being reflected back, which
changes its polarity. In both simulations and experiments carried out by Peratt and Healy,
they saw both single and double pulse profiles. When they included the effects of plasma
flowing between the current boundaries, some important modifications were observed.
As the electric field propagates along the line, electrons accelerate away from the boundary.
Due to the intense magnetic field associated with the current pulse, they
are constrained to flow along the ExB drift, rather than across the line.
This means that the entire line acts as a continuous source of low level radiation and noise.
All pulsars show this background radiation. Whether the radiated pulse has a single
or double pulse profile, will depend on the extent of the radiation region at the pulsar surface.
If the radiating current is confined to a shallow region instead of an extended
region, a single pulse profile will replace the double profile.
This effect is actually frequency dependent. Lower frequencies are associated with
fields that are extended in the emission region and are more likely to show pulse structure.
The source of the radiation energy is not contained within the pulsar, but may
instead derive from either the pulsar's interaction with its environment, or by
energy delivered by an external circuit. This means that pulsars do not have to
be composed of neutrons. They are simply no different than a normal star.
What has changed is its electrical environment causing a resonance in
the circuit, which creates the pulses. In the distant past, this star might have
suffered a catastrophic short circuit. So violent is the discharge, that it is
not only creating heavy elements in the process, but also explodes the entire surface double layer.
With nothing to hold back the charges, the majority of the material that made
up the star races outwards in an explosion. In some cases, a remnant remains after this process.
The electrical power input continues to drive the magnetospheric circuit to
produce the characteristic pulsar emissions. No neutrons, no spinning beams, simply a star that has
changed its electrical environment, causing a resonance in its circuit which creates the pulses.
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