• binary pulsars
• cosmic rays

The main purpose of my thesis is to examine some electrodynamic
properties of binary pulsars, trying to understand the peculiar
physical processes that can happen in their magnetospheres; the
ultimate aim is to discuss if such systems can be the source of the
observed flux of cosmic rays between the knee and the ankle, since the
mechanisms of acceleration for the cosmic rays in this range of
energies are still unknown.

Attention around binary pulsars has arisen after the recent
discovery (December 2003) of the first double neutron star system in
which both the stars are visible as pulsars (PSR J0737-3039); the
inspection of the physical features of this binary pulsar has led to
some intriguing possibilities up to now unexplored.

In this thesis I will first of all review what is already known
about the main properties of this binary system. I will
describe in particular the possibility to go further in the
verification of the predictions of general relativity with the
so-called post-Keplerian parameters; I will discuss the possibility
of studying the optical properties of the magnetospheres, since the
inclination angle of the orbit is nearly 90° and some orbital phases
show an eclipse of the light from one pulsar due to absorption by
the magnetosphere of the companion; I will rapidly summarize how the
discovery of that binary pulsar can enlarge our knowledge about the
origin and evolution of double neutron star systems; lastly, I will
examine the increase in the estimate of the Galactic double neutron
star merger rate due to the discovery of PSR J0737-3039.

I will then summarize the current knowledge about the magnetosphere
of a single pulsar. After describing the Gold-Pacini
model for the energy loss of the oblique rotator (in which the
magnetic and rotational axes are not parallel), I will discuss the
Goldreich-Julian model for the aligned axisymmetric rotator in the
force-free approximation in which the inertial and
gravitational forces are neglected with respect to the
electromagnetic ones and the Lorentz force per unit volume is
assumed to be zero outside the pulsar; after showing the main
unsolved problems about this model, I will try to examine the origin
of the leptons (positrons and electrons) which are expected to fill
the pulsar magnetosphere and to continuously stream away from the
star through the light cylinder (where co-rotation with the
pulsar would mean traveling at the speed of light).

Since even the magnetosphere of a single isolated pulsar is not well
understood, my approach in considering some hitherto unexplored
properties of the joint magnetosphere of a binary pulsar will mainly
be qualitative, trying to understand through order-of-magnitude
estimates the physical processes involved.

First of all I will describe the possibility that, for
binary pulsars in which the orbital separation is less than the sum
of the light cylinder radii of the stars, the region at the center
of the system could show a time-dependent distortion of the two
co-rotating magnetospheres which could give origin to an induced
electric field. I will then examine the possibility that such a
field is quenched by a local production of pairs caused by the
electric field itself. After showing that the electric field can not
be switched-off by the pairs, I will discuss the possible
observational consequences of the production of such a large number
of leptons, which will be accelerated by the electric field along
the magnetic field lines toward the pulsars and will then radiate
their energy via curvature radiation; unfortunately, the small
energy flux emitted, together with the rarity of double neutron star
systems, will not likely allow us to detect the radiation emitted.

Lastly, I will discuss the original idea that the
strong induced electric fields could be responsible for the
acceleration of cosmic rays whose energy lies between the
knee and the ankle of the cosmic ray spectrum. In this
case the unsolved problem is the origin for those ions, and I will
examine three possibilities, comparing their respective predictions
with the flux and power-law of the observed spectrum: they could
come from the mass loss induced by the tidal heating caused by the
strong gravitational fields of the two orbiting stars, or maybe they
could fill the Goldreich-Julian magnetosphere as well as positrons
and electrons, or they could even be extracted from the pulsar
atmosphere which is continuously replenished by the evaporation of
nuclei from the stellar surface, since the star is being heated by
the flux of high energy leptons discussed above.

The last hypothesis seems to be well confirmed by the observational
constraints, even if we are not able to fully explain how to free
the accelerated cosmic rays from the binary system and inject them
in the interstellar medium; anyway, our qualitative approach could
pave the way for further and more quantitative work on the
electrodynamics of binary pulsars, in order to explain the details
of the acceleration and escape of ions from such fascinating
systems.