Some pulsars are strong-ray sources; Vladimir Usov and Don Melrose have suggested that this may be explained by the presence of superstrong magnetic fields.
As pulsars radiate energy their spinning motion slows down,
and the power they radiate can be inferred from observations
of this slowing down.
For nearly all pulsars the observed electromagnetic emission
- that seen at optical, radio, X-ray and -ray energies -
is an insignificant fraction of the spin-down power,
and it is assumed that most of the power is carried away in
a wind of relativistic particles
(particles moving very close to the speed of light).
However, for a handful of pulsars a sizable
fraction of the spin-down power appears
in the form of -rays.
It is unclear why these particular `-ray pulsars' are efficient
emitters of -rays, while most pulsars are not.
The -rays are thought to be produced in the following fashion.
In the `polar cap' regions above the neutron star's
magnetic poles,
the rotating magnetic field produces a very strong electric field.
This `vacuum electric field' is partially aligned
with the magnetic field and it pulls so-called primary particles,
mostly electrons,
from the surface of the neutron star, accelerating
them to very high energies.
These electrons travel along the magnetic field lines and,
like all charged particles traveling on a curved path,
emit electromagnetic radiation of energy which depends on the electron energy.
The primary particles accelerated in the vacuum electric field
are energetic enough to emit -rays.
Figure: Sketches of the polar cap region of a pulsar.
The left-hand sketch shows the standard picture in which
the vacuum electric field is shielded by an electron-positron plasma.
The right-hand sketch shows the much larger vacuum gap that results
if the -rays decay to form positronium rather than free
electron-positron pairs.
In typical pulsars, it is argued, these -rays travel only a short
distance
before they decay into an electron and its antiparticle, a positron.
These free electron-positron pairs orient themselves so as to reduce
the electric field in their immediate vicinity.
One pair alone would only produce a tiny effect,
but enough particle pairs are created that together they screen
out the vacuum electric field beyond a certain height above
the pulsar's surface.
The region below this is dominated by primary particles and -rays,
and is called the `vacuum-gap'.
The height at which screening of the electric field
occurs determines the maximum energy of the primary particles,
and the power in escaping -rays.
Evidently, in -ray pulsars the screening is far less effective
than in typical pulsars, and the vacuum gap extends to a much greater height.
Don Melrose and Vladimir Usov (WI)
have developed a model for -ray pulsars.
In a superstrong magnetic field (greater than about 400 million Tesla)
the -rays tend to decay into bound electron-positron pairs
- positronium - rather than free pairs.
Positronium is ineffective in screening the vacuum field,
allowing a much greater power to go into escaping -rays.
The difference between the conventional model and this new picture is
illustrated in Figure 6.
For the model to be effective the positronium must not only form,
it must also persist and not be quickly ionized
- reduced to a free electron-positron pair.
This requires that -ray pulsars have
the required strong magnetic fields,
be sufficiently cool (
) to prevent free
escape of particles from the surface,
and to have rotation periods in a relatively
narrow range (0.1 to 0.5 seconds).
Don and Vladimir have argued that most of the known -ray
pulsars satisfy these criteria.
There are also a comparable number of pulsars that satisfy these criteria,
but are not observed as -ray pulsars.
The simplest explanation for this is that the -ray
emission is anisotropic,
so that for us to see the -ray pulses the angle between the
pulsar's magnetic axis and our line of sight must be no more than
about 45
, and the probability of this happening is about 50%.
Two pulsars, the one in the Crab Nebula and another very similar to it,
are exceptional, in that their rotation periods are too short to be
consistent with the model, and an alternative explanation for the
-ray emission from these pulsars is required.
Apart from the two Crab-like pulsars, the model based on a cool
stellar surface, to allow the vacuum gap to form, and a strong
magnetic field, so that bound rather than free pairs form, seems
capable of accounting for the observed properties of -ray
pulsars.