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~$\alpha_{\rm sf}$ and to any order in the ratio \Rs/R where $R$ 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~$\ell=1$ and hexapolar~$\ell=3$ 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 $L_\gamma \approx 2\times10^{26} \textrm{W} \, (L_{\rm sd}/10^{28} \textrm{W})^{1/2} \, (P/1 \textrm{s})^{-1/2}$ where $L_{\rm sd}$ is the pulsar spindown luminosity and $P$ 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 $\Gamma_{\rm v} \approx 10 \, \tau_{\rm rec}^{1/5} \, (L_{\rm sd}/10^{28} \textrm{W})^{1/2}$, pair multiplicity $\kappa$ related to the magnetization parameter $\sigma$ by $\kappa\,\sigma \, \tau_{\rm rec}^{1/5} \approx 10^8$, and efficiency $\eta$ of spin-down luminosity conversion into particle kinetic energy according to the relation $\eta\,\sigma\approx1$. A good guess for the associated reconnection rate is then $\tau_{\rm rec} \approx 0.5 \, (L_{\rm sd}/10^{28} \textrm{W})^{-5/12}$. Finally, pulses in gamma-rays are visible only if $L_{\rm sd}/P\gtrsim 10^{27} \textrm{W/s}$. 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 $M_{\rm BH}$ and angular momentum $a_{\rm BH}$ 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 $s_{\rm e}$ and tending to disrupt the plasma configuration, is bounded to an upper limit, $s_{\rm e,max}$. For $s_{\rm e}$ close to but smaller than this value $s_{\rm e,max}$, 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&