339 research outputs found
On the possibility of radio emission of planets around pulsars
A planet orbiting around a pulsar would be immersed in an ultra-relativistic
under-dense plasma flow. It would behave as a unipolar inductor, with a
significant potential drop along the planet. As for Io in Jupiter's
magnetosphere, there would be two stationary Alfv\'en waves, the Alfv\'en wings
(AW), attached to the planet. The AW would be supported by strong electric
currents, in some circumstances comparable to those of a pulsar. It would be a
cause of powerful radio waves emitted all along the AW, and highly collimated
through relativistic aberration. There would be a chance to detect these
radio-emissions from Earth. The emission would be pulses as for ordinary
pulsars; their occurrence would depend on the planet-star-observer angle. These
results are still preliminary, further work needs to be done.Comment: Accepted for pulbication in the (reviewed) proceedings of the
International Workshop on Planetary, Solar and Heliospheric Radio Emissions
(PRE VII) held in Graz, Austria, sep 15-17 201
The magnetic coupling of planets and small bodies with a pulsar's wind
We investigate the electromagnetic interaction of a relativistic stellar wind
with a planet or a smaller body in orbit around a pulsar. This may be relevant
to objects such as PSR B1257+12 and PSR B1620-26 that are expected to hold a
planetary system, or to pulsars with suspected asteroids or comets. Most models
of pulsar winds predict that, albeit highly relativistic, they are slower than
Alfv\'en waves. In that case, a pair of stationary Alfv\'en waves, called
Alfv\'en wings (AW), is expected to form on the sides of the planet. The wings
expand far into the pulsar's wind and they could be strong sources of radio
emissions. The Alfv\'en wings would cause a significant drift over small bodies
such as asteroids and comets.Comment: proceeding of the SF2A conference in Nice, 201
A magnetic thrust action on small bodies orbiting a pulsar
We investigate the electromagnetic interaction of a relativistic stellar wind
with small bodies in orbit around the star. Based on our work on the theory of
Alfv\'en wings to relativistic winds presented in a companion paper, we
estimate the force exerted by the associated current system on orbiting bodies
and evaluate the resulting orbital drift. This Alfv\'enic structure is found to
have no significant influence on planets or smaller bodies orbiting a
millisecond pulsar. %influence on the orbit of bodies around a millisecond
pulsar. On the timescale of millions of years, it can however affect the orbit
of bodies with a diameter of 100 kilometres around standard pulsars with a
period 1 s and a magnetic field T. Kilometer-sized
bodies experience drastic orbital changes on a timescale of years.Comment: accepted for publication in "Astronomy and Astrophysics
Electron-positron pair production by gamma rays in an anisotropic flux of soft photons, and application to pulsar polar caps
Electron-positron pair production by collision of photons is investigated in
view of application to pulsar physics. We compute the absorption rate of
individual gamma-ray photons by an arbitrary anisotropic distribution of softer
photons, and the energy and angular spectrum of the outgoing leptons. We work
analytically within the approximation that 1 mc 2 /E > /E, with E and the
gamma-ray and soft-photon maximum energy and mc 2 the electron mass energy. We
give results at leading order in these small parameters. For practical
purposes, we provide expressions in the form of Laurent series which give
correct reaction rates in the isotropic case within an average error of
7%. We apply this formalism to gamma rays flying downward or upward from a hot
neutron star thermally radiating at a uniform temperature of 10 6 K. Other
temperatures can be easily deduced using the relevant scaling laws. We find
differences in absorption between these two extreme directions of almost two
orders of magnitude, much larger than our error estimate. The magnetosphere
appears completely opaque to downward gamma rays while there are up to
10% chances of absorbing an upward gamma ray. We provide energy and angular
spectra for both upward and downward gamma rays. Energy spectra show a typical
double peak, with larger separation at larger gamma-ray energies. Angular
spectra are very narrow, with an opening angle ranging from 10 --3 to 10 --7
radians with increasing gamma-ray energies
Quantum theory of curvature and synchro-curvature radiation in a strong and curved magnetic field, and applications to neutron star magnetospheres
In a previous paper, we derived the quantum states of a Dirac particle in a
circular, intense magnetic field in the limit of low momentum perpendicular to
the field with the purpose of giving a quantum description of the trajectory of
an electron, or a positron, in a typical pulsar or magnetar magnetosphere. Here
we continue this work by computing the radiation resulting from transitions
between these states. This leads to derive from first principles a quantum
theory of the so-called curvature and synchro-curvature radiations relevant for
rotating neutron-star magnetospheres. We find that, within the approximation of
an infinitely confined wave-function around the magnetic field and in the
continuous energy-level limit, classical curvature radiation can be recovered
in a fully consistent way. Further we introduce discrete transitions to account
for the change of momentum perpendicular to the field and derive expressions
for what we call quantum synchro-curvature radiation. Additionally, we express
deconfinement and quantum recoil corrections
Towards a theory of extremely intermittent pulsars I: Does something orbits PSR B1931 + 24 ?
We investigate whether one or many companions are orbiting the extremely
intermittent pulsar PSR B1931+24. We constrained our analysis on previous
observations of eight fundamental properties of PSR B1931+24. The most puzzling
properties are the intermittent nature of the pulsar's activity, with active
and quiet phases that alternate quasi-periodically; the variation of the
slowing-down rate of its period between active and quiet phases; and because
there are no timing residuals, it is highly unlikely that the pulsar has a
massive companion. Here, we examine the effects that one putative companion
immersed in the magnetospheric plasma or the wind of the pulsar might have, as
well as the associated electric current distribution. We analysed several
possibilities for the distance and orbit of this hypothetical companion and the
nature of its interaction with the neutron star. We show that if the
quasi-periodic behaviour of PSR B1931+24 was caused by a companion orbiting the
star with a period of 35 or 70 days, the radio emissions, usually considered to
be those of the pulsar would in that specific case be emitted in the
companion's environment. We analysed four possible configurations and conclude
that none of them would explain the whole set of peculiar properties of PSR
1931+24. We furthermore considered a period 70 days for the precession of the
periastron associated to an orbit very close to the neutron star. This
hypothesis is analysed in a companion paper.Comment: Accepted for publication in Astrnomy and Astrophysic
Towards a theory of extremely intermittent pulsars II: Asteroids at a close distance
We investigate whether there may be one or many companions orbiting at close
distance to the light cylinder around the extremely intermittent pulsars PSR
B1931+24 and PSR J1841-0500. These pulsars, behaving in a standard way when
they are active, also "switch off" for durations of several days, during which
their magnetospheric activity is interrupted or reduced. We constrained our
analysis on eight fundamental properties of PSR B1931+24 that summarise the
observations. We considered that the disruption/activation of the
magnetospheric activity would be caused by the direct interaction of the star
with the Alfv\'en wings emanating from the companions. We also considered the
recurrence period of 70 days to be the period of precession of the periastron
of the companions orbit. We analysed in which way the time scale of the
"on/off" pseudo-cycle would be conditioned by the precession of the periastron
and not by the orbital time scale, and we derived a set of orbital constraints
that we solved. We then compared the model, based on PSR 1931+24, with the
known properties of PSR 1841+0500. We conclude that PSR B1931+24 may be
surrounded at a close distance to the star by a stream of small bodies of
kilometric or sub-kilometric sizes that could originate from the tidal
disruption of a body of moderate size that fell at a close distance to the
neutron star on an initially very eccentric orbit. This scenario is also
compatible with the properties of PSR J1841-0500, although the properties of
PSR J1841-0500 are, by now, less constrained. These results raise new
questions. Why are the asteroids not yet evaporated ? What kind of interaction
can explain the disruption of the magnetospheric activity ? These questions are
the object of two papers in preparation that will complete the present
analysis.Comment: Accepted for publication in Astronomy and Astrophysic
On the role of Alfvén waves as precursors of quasi-static acceleration processes in the Earth auroral zone
Proceeding of the SF2A conference in Nice, 2012.In the Earth auroral zone, the electron acceleration by Alfvén waves is sometimes a precursor of the non-propagating acceleration structures. In order to investigate how Alfvén waves could generate non-propagating electric fields, a series of simulations of counter-propagating waves in a uniform plasma is presented. The waves (initially not configured to accelerate particles) propagate along the ambient magnetic field direction. It is shown that non propagating electric fields are generated at the locus of the Alfvén waves crossing. These electric fields have a component orientated along the direction of the ambient magnetic field, and they generate acceleration and a significant perturbation of the plasma density. The non-linear interaction of down and up-going Alfvén waves might be a cause of plasma density fluctuations (with gradients along the magnetic field) on a scale comparable to those of the Alfvén wavelengths
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