260 research outputs found
Quantum electrodynamical corrections to a magnetic dipole in general relativity
Magnetized neutron stars are privileged places where strong electromagnetic
fields as high as \BQ=4.4\times10^9~T exist, giving rise to non-linear
corrections to Maxwell equations described by quantum electrodynamics (QED).
These corrections need to be included to the general relativistic (GR)
description of a magnetic dipole supposed to be anchored in the neutron star.
In this paper, these QED and GR perturbations to the standard flat space-time
dipole are calculated to the lowest order in the fine structure
constant~ and to any order in the ratio \Rs/R where is
the neutron star radius and \Rs its Schwarzschild radius. Following our new
3+1~formalism developed in a previous work, we compute the multipolar
non-linear corrections to this dipole and demonstrate the presence of a small
dipolar~ and hexapolar~ component.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society Main Journa
High-energy emission from the pulsar striped wind: a synchrotron model for gamma-ray pulsars
(abridged) Gamma-ray pulsars constitute a class of high and very high-energy
emitters for which the known population is steadily increasing thanks to the
Fermi/Large Area Telescope. In this paper, their gamma-ray luminosity and
spectral features are explained in the framework of synchrotron radiation from
particles located in the stripe of the pulsar wind. Apart from radiative
losses, particles are also subject to a constant re-acceleration and reheating
for instance by a magnetic reconnection induced electric field. The high-energy
luminosity scales as where is
the pulsar spindown luminosity and its period. From this relation, we
derive important parameters of pulsar magnetosphere and wind theories. Indeed,
we find bulk Lorentz factor of the wind scaling as , pair
multiplicity related to the magnetization parameter by
, and efficiency of
spin-down luminosity conversion into particle kinetic energy according to the
relation . A good guess for the associated reconnection
rate is then . Finally, pulses in gamma-rays are visible only if . This model differs from other high-energy
emission mechanisms because it makes allowance not only for rotational kinetic
energy release but also for an additional reservoir of energy anchored to the
magnetic field of the stripe and released for instance by some magnetic
reconnection processes.Comment: 5 pages; 2 figures; accepted by MNRA
Pulsed high energy gamma-rays from thermal populations in the current sheets of pulsar winds
Context. More than one hundred GeV pulsars have been detected up to now by
the LAT telescope on the Fermi gamma-ray observatory, showing peak energies
around a few GeV. Current modelling proposes that the high energy emission
comes from outer magnetospheric gaps, however radiation from the equatorial
current sheet which separates the two magnetic hemispheres outside the light
cylinder has also been investigated. Aims. In this paper we discuss the region
right outside the light cylinder, or "near wind" zone. We investigate the
possibility that synchrotron radiation emitted by thermal populations in the
equatorial current sheet of the pulsar wind in this region can explain the
lightcurves and spectra observed by Fermi/LAT. Methods. We use analytical
estimates as well as detailed numerical computation to calculate the gamma-ray
luminosities, lightcurves and spectra of gamma-ray pulsars. Results. Many of
the characteristics of the gamma-ray pulsars observed by Fermi/LAT can be
reproduced by our model, most notably the position of these objects in the P -
Pdot diagram, and the range of gamma-ray luminosities. A testable result is a
sub-exponential cutoff with an index b = 0.35. We also predict the existence of
a population of pulsars with cutoff energies in the MeV range. These have
systematically lower spindown luminosities than the Fermi/LAT detected pulsars.
Conclusions. It is possible for relativistic populations of electrons and
positrons in the current sheet of a pulsar's wind right outside the light
cylinder to emit synchrotron radiation that peaks in the sub-GeV to GeV regime,
with gamma-ray efficiencies similar to those observed for the Fermi/LAT
pulsars.Comment: 13 pages, submitted to A&
A new model for QPOs in accreting black holes: application to the microquasar GRS 1915+105
(abridged) In this paper we extend the idea suggested previously by Petri
(2005a,b) that the high frequency quasi-periodic oscillations observed in
low-mass X-ray binaries may be explained as a resonant oscillation of the
accretion disk with a rotating asymmetric background (gravitational or
magnetic) field imposed by the compact object. Here, we apply this general idea
to black hole binaries. It is assumed that a test particle experiences a
similar parametric resonance mechanism such as the one described in paper I and
II but now the resonance is induced by the interaction between a spiral density
wave in the accretion disk, excited close to the innermost stable circular
orbit, and vertical epicyclic oscillations. We use the Kerr spacetime geometry
to deduce the characteristic frequencies of this test particle. The response of
the test particle is maximal when the frequency ratio of the two strongest
resonances is equal to 3:2 as observed in black hole candidates. Finally,
applying our model to the microquasar GRS 1915+105, we reproduce the correct
value of several HF-QPOs. Indeed the presence of the 168/113/56/42/28 Hz
features in the power spectrum time analysis is predicted. Moreover, based only
on the two HF-QPO frequencies, our model is able to constrain the mass and angular momentum of the accreting black hole.Comment: Accepted for publication in Astrophysics & Space Scienc
An unified polar cap/striped wind model for pulsed radio and gamma-ray emission in pulsars
(abridged) Thanks to the recent discovery by Fermi of about fifty new
gamma-ray pulsars, it becomes possible to look for statistical properties of
their pulsed high-energy emission, especially their light-curves and
phase-resolved spectra. These pulsars emit by definition mostly gamma-ray
photons but some of them are also detected in the radio band. For those seen in
these two extreme energies, the relation between time lag of radio/gamma-ray
pulses and gamma-ray peak separation, in case both high-energy pulses are seen,
helps to put some constrain on the magnetospheric emission mechanisms and
location. This idea is analyzed in detail in this paper, assuming a polar cap
model for the radio pulses and the striped wind geometry for the pulsed
high-energy counterpart.
Combining the time-dependent emissivity in the wind, supposed to be inverse
Compton radiation, with a simple polar cap emission model along and around the
magnetic axis, we compute the radio and gamma-ray light-curves, summarizing the
results in several phase plots. The phase lag as well as the gamma-ray peak
separation dependence on the pulsar inclination angle and on the viewing angle
are studied. Using the gamma-ray pulsar catalog compiled from the Fermi data,
we are able to predict the radio lag/peak separation relation and compare it
with available observations taken from this catalog.Comment: Accepted by MNRA
The magnetron instability in a pulsar's cylindrical electrosphere
(abridged) The physics of the pulsar magnetosphere remains poorly constrained
by observations. Little is known about their emission mechanism. Large vacuum
gaps probably exist, and a non-neutral plasma partially fills the neutron star
surroundings to form an electrosphere. We showed that the differentially
rotating equatorial disk in the pulsar's electrosphere is diocotron unstable
and that it tends to stabilise when relativistic effects are included. However,
when approaching the light cylinder, particle inertia becomes significant and
the electric drift approximation is violated. In this paper, we study the most
general instability, i.e. by including particle inertia effects, as well as
relativistic motions. This general non-neutral plasma instability is called the
magnetron instability. We linearise the coupled relativistic cold-fluid and
Maxwell equations. The non-linear eigenvalue problem for the perturbed
azimuthal electric field component is solved numerically. The spectrum of the
magnetron instability in a non-neutral plasma column confined between two
cylindrically conducting walls is computed for several cylindrical
configurations. For a pulsar electrosphere, no outer wall exists. In this case,
we allow for electromagnetic wave emission propagating to infinity. When the
self-field induced by the plasma becomes significant, it can first increase the
growth rate of the magnetron instability. However, equilibrium solutions are
only possible when the self-electric field, measured by the parameter and tending to disrupt the plasma configuration, is bounded to an upper
limit, . For close to but smaller than this value
, the instability becomes weaker or can be suppressed as was the
case in the diocotron regime.Comment: Accepted by A&
High-energy pulses and phase-resolved spectra by inverse Compton emission in the pulsar striped wind - Application to Geminga
(abridged) Although discovered 40 years ago, the emission mechanism
responsible for the observed pulsar radiation remains unclear. However, the
high-energy pulsed emission is usually explained in the framework of either the
polar cap or the outer gap model. The purpose of this work is to study the
pulsed component, that is the light-curves as well as the spectra of the
high-energy emission, above 10 MeV, emanating from the striped wind model.
Gamma rays are produced by scattering off the soft cosmic microwave background
photons on the ultrarelativistic leptons flowing in the current sheets. We
compute the time-dependent inverse Compton emissivity of the wind, in the
Thomson regime, by performing three-dimensional numerical integration in space
over the whole striped wind. The phase-dependent spectral variability is then
calculated as well as the change in pulse shape when going from the lowest to
the highest energies. Several light curves and spectra of inverse Compton
radiation with phase resolved dependence are presented. We apply our model to
the well-known gamma-ray pulsar Geminga. We are able to fit the EGRET spectra
between 10 MeV and 10 GeV as well as the light curve above 100 MeV with good
accuracy.Comment: Accepted by A&
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