arXiv subjects

Jérôme Pétri

Publications and source records attributed to Jérôme Pétri.

At least 19 recordsLinked to original sources

arXiv2026

Polar cap plasma loading and the morphology of pulsar $γ$-ray light curves

The discovery of more than 300 $γ$-ray pulsars by the Fermi Large Area Telescope (Fermi-LAT) has revealed a rich diversity of light-curve morphologies that remains challenging to reproduce within purely magnetospheric emission models, suggesting that particle acceleration and radiation may occur, at least in part, in the striped-wind current sheet. We compute a comprehensive atlas of pulsar $γ$-ray light curves based on the split-monopole current sheet geometry, with the goal of quantifying the role of pair plasma loading in shaping the observed pulse morphology. The current-sheet surface is described analytically and assumed to be the dominant site of high-energy photon emission. Spatially dependent emissivity prescriptions are introduced through a minimal set of parameters that explicitly break the north-south and azimuthal symmetries of the emitting plasma. With only a few physically motivated parameters, the model is able to reproduce a broad range of the observed $γ$-ray light-curve morphologies, including asymmetric, multi-peaked, and highly structured profiles. The results show that asymmetric emissivity, possibly related to pair-plasma loading, provides a natural and economical mechanism capable of reproducing a substantial fraction of the observed diversity of $γ$-ray pulse profiles. The current-sheet scenario therefore appears to provide a promising and physically motivated framework for high-energy pulsar emission.

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arXiv2026

The spin-orbit alignment hypothesis in millisecond pulsars

Millisecond pulsars (MSPs) are spun up during their accretion phase in a binary system. The exchange of angular momentum between the accretion disk and the star tends to align the spin and orbital angular momenta on a very short time scale compared to the accretion stage. In this work, we study a subset of $γ$-ray MSPs in binaries for which the orbital inclination angle $i$ has been accurately constrained thanks to the Shapiro delay measurements. Our goal is to constrain the observer viewing angle $ζ$ and to check whether it agrees with the orbital inclination angle $i$, in other words if $ζ\approx i$. We use a Bayesian inference technique to fit the MSP $γ$-ray light curves based on the third $γ$-ray pulsar catalogue (3PC). The emission model relies on the striped wind model deduced from force-free neutron star magnetosphere simulations. We found good agreement between the two angles $i$ and $ζ$ for a significant fraction of our sample, about four fifth, confirming the spin-orbit alignment scenario during the accretion stage. However about one fifth of our sample deviates significantly from this alignment. The reasons are manifold: either the $γ$-ray fit is not reliable or some precession and external torque avoid an almost perfect alignment.

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arXiv2026

Born to be recycled: a comprehensive population synthesis of the Galactic millisecond pulsars

Millisecond pulsars (MSPs) are the oldest but fastest pulsars known to date. In the 1980s, to explain how these pulsars could be formed, a new hypothesis was formulated: the recycling of pulsars, i.e the fact that a pulsar could accrete matter from a companion and been spun up. In this paper, we developed a population synthesis algorithm for pulsars which belong to a binary, in order to check whether most of the observed recycled pulsars were formed via an accretion mechanism and derive statistics about their properties, that are difficult to obtain through observations. We also make predictions for future surveys. Toward the presented objectives, we use the code Stellar EVolution for N-body (SEVN) to take into account all the binary processes and our own code to evolve each pulsar self-consistently by taking into account the secular evolution of a force-free magnetosphere, the magnetic field decay, gravitational braking and spatial evolution. Each pulsar is born in binary with a main sequence companion, and evolve to present time. The radio and $γ$-ray emission locations were modeled by the polar cap geometry and striped wind model, respectively. Our simulations seem to reproduce well the population of radio and $γ$-ray pulsars observed in the selected surveys. We also found that there should be less than $220$ unidentified pulsars in the Fourth Fermi-LAT catalogue of $γ$-ray sources (4FGL). High values of the viewing angle $ζ$ seem to be needed to be able to observe the recycled pulsars, and it also seems difficult to observe recycled pulsars with an aligned rotation axis and magnetic axis (i.e., $χ\leq 10$°). We find that only a small fraction, approximately $\sim 7.6\times10^{-3}$ %, of oxygen-neon white dwarfs (ONeWDs) in binary systems appear to contribute to the population of mildly recycled pulsars through accretion-induced collapse.

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arXiv2025

Multi-wavelength emission in resistive pulsar magnetospheres

In this paper, we compute a full set of neutron star magnetosphere structures from the basic vacuum regime to the dissipation-less force-free regime by implementing a resistive prescription for the plasma. A comparison to the radiation reaction limit is also discussed. We investigated the impact of these resistive magnetospheres onto the multi-wavelength emission properties based on the polar cap model for radio wavelengths, on the slot gap model for X-rays and on the striped wind model for $γ$-rays.} % methods heading (mandatory) {We performed time-dependent pseudo-spectral simulations of the full Maxwell equations including a resistive Ohm's law. We deduced the polar cap shape and size, the Poynting flux, the magnetic field structure and the current sheet surface, depending on the magnetic obliquity~$χ$ and on the conductivity~$σ$. We found that the geometry of the magnetosphere close to the stellar surface is not impacted by the amount of resistivity. Polar cap rims remain very similar in shape and size. However the Poynting flux varies significantly as well as the magnetic field sweep-back in the vicinity of the light-cylinder. This bending of field lines reflects into the $γ$-ray pulse profiles, changing the $γ$-ray peak separation~$Δ$ as well as the time lag~$δ$ between the radio pulse and $γ$-ray peaks. X-ray pulse profiles are also drastically affected by the resistivity. A full set of multi-wavelength light-curves can be compiled for future comparison with the third $γ$-ray pulsar catalogue. This systematic study will help to constrain the amount of magnetic energy flowing into particle kinetic energy and shared by radiation.

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arXiv2025

The Galactic population of magnetars : a simulation-based inference study

Population synthesis modeling of the observed dynamical and physical properties of a population is a highly effective method for constraining the underlying birth parameters and evolutionary tracks. In this work, we apply a population synthesis model to the canonical magnetar population to gain insight into the parent population. We utilize simulation-based inference to reproduce the observed magnetar population with a model which takes into account the secular evolution of the force-free magnetosphere and magnetic field decay simultaneously and self-consistently. Our observational constraints are such that no magnetar is detected through their persistent emission when convolving the simulated populations with the XMM-Newton EPIC-pn Galactic plane observations, and that all of the $\sim$30 known magnetars are discovered through their bursting activity in the last $\sim50$ years. Under these constraints, we find that, within 95 % credible intervals, the birth rate of magnetars to be $1.8^{+2.6}_{-0.6}$ kyr$^{-1}$, and lead to having $10.7^{+18.8}_{-4.4}$ % of neutron stars born as magnetars. We also find a mean magnetic field at birth ($μ_b$ is in T) $\log\left(μ_b\right) = 10.2^{+0.1}_{-0.2}$, a magnetic field decay slope $α_d = 1.9 ^{+0.9}_{-1.3}$, and timescale $τ_d = 17.9^{+24.1}_{-14.5}$ kyr, in broad agreement with previous estimates. We conclude this study by exploring detection prospects: an all-sky survey with XMM-Newton would potentially allow to get around 7 periodic detections of magnetars, with approximately 150 magnetars exceeding XMM-Newton's flux threshold, and the upcoming AXIS experiment should allow to double these detections.

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arXiv2024

The Galactic population of canonical pulsars II

Pulsars are highly magnetized rotating neutron stars, emitting in a broad electromagnetic energy range. Reproducing the observed pulsars population refines our understanding of their formation and evolution scenarios as well as their radiation processes and geometry. In this paper, we improve our previous population synthesis by focusing on both the radio and $γ$-ray pulsar populations, investigating the impact of the Galactic gravitational potential and of the radio emission death line. In order to elucidate the necessity of a death line, refined initial distributions of spin period and spacial position at birth were implemented, elevating the sophistication of our simulations to the most recent state-of-the-art. The motion of each individual pulsar is tracked in the Galactic potential by a fourth order symplectic integration scheme. Our pulsar population synthesis takes into account the secular evolution of the force-free magnetosphere and magnetic field decay simultaneously and self-consistently. Each pulsar is evolved from its birth up to the present time. The radio and $γ$-ray emission locations are modelled respectively by the polar cap geometry and the striped wind model. By simulating ten million pulsars we found that including a death line better reproduces the observational trend. However, when simulating one million pulsars, we obtain an even more realistic $P-\dot{P}$ diagram, whether or not a death line is included. This suggests that the ages of the detected pulsars might be overestimated, therefore questioning the real need for a death line in pulsar population studies. Kolmogorov-Smirnov tests confirm the statistical similarity between the observed and simulated $P-\dot{P}$ diagram. Additionally, simulations with increased $γ$-ray telescope sensitivities hint to a significant contribution of $γ$-ray pulsars to the GeV excess in the Galactic centre.

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arXiv2024

Multi-wavelength pulse profiles from the force-free neutron star magnetosphere

In this paper, we compute a full atlas of radio, X-ray and $γ$-ray pulse profiles relying on the force-free magnetosphere model. Our goal is to use such data bank of multi-wavelength profiles to fit a substantial number of radio-loud $γ$-ray pulsars also detected in non-thermal X-rays to decipher the X-ray radiation mechanism and sites. Using results from the third $γ$-ray pulsar catalogue (3PC), we investigate the statistical properties of this population. We assume that radio emission emanates from field lines rooted to the polar caps, at varying height above the surface, close to the surface, at an altitude about 5-10% of the light-cylinder radius $r_{\rm L}$. The X-ray photons are produced in the separatrix region within the magnetosphere, i.e. the current sheet formed by the jump from closed to open magnetic field lines. We allow for substantial variations in emission height. The $γ$-ray are produced within the current sheet of the striped wind, outside the light-cylinder. A comprehensive set of light-curves in radio, X-ray and $γ$-ray has been computed. Based on only geometric considerations about magnetic obliquity, line of sight inclination and radio beam cone opening angle, pulsars can be classified as radio-loud or quiet and $γ$-ray loud or quiet. We found that the 3PC sample is compatible with an isotropic distribution of obliquity and line of sight. The atlases constructed in this work are the fundamental tools to explore individual pulsars and fit their multi-wavelength pulse profiles in order to constrain their magnetic topology, the emission sites as well as the observer line of sight.

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arXiv2023

A new radiation reaction approximation for particle dynamics in the strong field regime

(Abridged) Following particle trajectories in the intense electromagnetic field of a neutron star is prohibited by the large ratio between the cyclotron frequency $ω_{\rm B}$ and the stellar rotation frequency $Ω$. No fully kinetic simulations on a macroscopic scale and with realistic field strengths have been performed so far due to the huge computational cost implied by this enormous scale of separation. In this paper, we derive new expressions for the particle velocity subject to strong radiation reaction that are intended to be more accurate than the current state-of-the-art expression in the radiation reaction limit regime, the so-called Aristotelian regime. We shortened the timescale hierarchy by solving the particle equation of motion in the radiation reaction regime, where the Lorentz force is always and immediately balanced by the radiative drag, and including a friction not necessarily opposite to the velocity vector, as derived in the Landau-Lifshitz approximation. Starting from the reduced Landau-Lifshitz equation (i.e. neglecting the field time derivatives), we found expressions for the velocity depending only on the local electromagnetic field configuration and on a new parameter related to the field strength that controls the strength of the radiative damping. The new velocity expressions produce results similar in accuracy to the radiation reaction limit approximation. We therefore do not expect this new method to improve the accuracy of neutron star magnetosphere simulations. The radiation reaction limit is a simple but accurate, robust, and efficient way to follow ultra-relativistic particles in a strong electromagnetic field.

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arXiv2023

Particle motion in neutron stars ultra-strong electromagnetic field: the influence of radiation reaction

As a follow-up to our previous work on particle acceleration simulation near neutron stars, in this paper, we discuss the impact of radiation reaction on test particles injected into their magnetosphere. We therefore neglect the interaction between particles through the electromagnetic field as well as gravitation. We integrate numerically the reduced Landau-Lifshitz equation for electrons and protons in the vacuum field of a rotating magnetic dipole based on analytical solutions in a constant electromagnetic field. These expressions are simple in a frame where the electric and magnetic field are parallel. Lorentz transforms are used to switch back and forth between this frame and the observer frame. We found that, though due solely to the Lorentz force, electrons reach Lorentz factors up to $γ=10^{14}$ and protons reach them up to $γ=10^{10.7}$. When radiation reaction is enabled, electrons reach energies up to $γ=10^{10.5}$ and protons reach energies up to $γ=10^{8.3}$. The second set of values are more realistic since the radiation reaction feedback is predominant within the magnetosphere. Moreover, as expected, symmetrical behaviours between the north and south hemispheres are highlighted, either with respect to the location around the neutron star or with respect to particles of opposite charge to mass ratio~$(q/m)$. The study of the influence of the magnetic dipolar moment inclination shows similar behaviours regardless of whether radiation reaction is enabled. Protons (respectively electrons) impact the surface of the neutron star less as the inclination angle increases (decreases for electrons), while if the rotation and magnetic axes are aligned, all the protons impact the neutron star, and all the electrons impact the surface if the rotation and magnetic axes are anti-aligned.

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arXiv2022

The Galactic population of canonical pulsars

Current wisdom accounts to the diversity of neutron star observational manifestations to their birth scenarios, influencing their thermal and magnetic field evolution. Among the kind of observed neutron stars, radio pulsars represent by far the largest population of neutron stars. In this paper, we aim at constraining the observed population of the canonical neutron star period, magnetic field and spatial distribution at birth in order to understand the radio and high-energy emission processes in a pulsar magnetosphere. For this purpose we design a population synthesis method self-consistently taking into account the secular evolution of a force-free magnetosphere and the magnetic field decay. We generate a population of pulsars and evolve them from their birth to the present time, working in the force-free approximation. We assume a given initial distribution for the spin period, surface magnetic field and spatial galactic location. Radio emission properties are accounted by the polar cap geometry, whereas the gamma-ray emission is assumed to be produced within the striped wind model. We found that a decaying magnetic field gave better agreement with observations compared to a constant magnetic field model. Starting from an initial mean magnetic field strength of $B=2.5\times 10^8$~T with a characteristic decay timescale of $4.6 \times 10^5~$yr, a neutron star birth rate of $1/70~$yr and a mean initial spin period of 60~ms, we found that the force-free model satisfactorily reproduces the distribution of pulsars in the $P-\dot{P}$ diagram with simulated populations of radio-loud, radio-only and radio quiet gamma-ray pulsars similar to the observed populations.

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arXiv2022

Radiative pulsar magnetospheres: oblique rotators

Pulsar magnetospheres are filled with relativistic pairs copiously emitting photons detected from the radio wavelengths up to high and very high energies, in the GeV and sometimes in the TeV range. Efficient particle acceleration converts the stellar rotational kinetic energy into radio, X-ray and gamma-ray photons. Force-free magnetospheres, being dissipationless, cannot operate this conversion. Some non ideal plasma effects must set in within the magnetosphere. In this paper, we compute numerical solutions of pulsar radiative magnetospheres in the radiation reaction limit, where radiation fully balances single particle acceleration. Using an appropriate Ohm's law, the dissipation is only controlled by the pair multiplicity factor~$κ$. Moreover we allow for either a minimal radiative region where dissipation is added only where required or for a force-free inside radiative outside model. This approach naturally and self-consistently connects the particle dynamics to its radiation field in the ultra-relativistic regime. Our solutions tend to the force-free limit for moderately large multiplicities, $κ\gg 1$, decreasing the spin-down energy conversion into radiation. Nevertheless, for sufficiently low multiplicity $κ\lesssim1$, a significant fraction of the spin-down energy flows into radiation via particle acceleration. The work done by the electromagnetic field on the plasma mainly occurs in the current sheet of the striped wind, right outside the light-cylinder. Nevertheless the impact on the magnetic topology is negligible whatever the model. Therefore the associated sky maps and light-curves are only weakly impacted as shown.

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arXiv2021

Spheroidal force-free neutron star magnetospheres

Abridged. Fast rotating and self-gravitating astrophysical objects suffer strong deformations from centrifugal forces. If moreover they are magnetized, they generate an electromagnetic wave that is perturbed accordingly. When stellar objects are also surrounded by an ideal plasma, a magnetosphere is formed. We study the electromagnetic configuration of a force-free magnetosphere encompassing an ideal spheroidal rotating conductor as an inner boundary. We put special emphasize to millisecond period neutron star magnetospheres, those showing a significant oblate shape. Force-free solutions are computed by numerical integration of the time-dependent Maxwell equations in spheroidal coordinates. Relevant quantities such as the magnetic field structure, the spin down luminosity, the polar cap rims and the current density are shown. We find that the force-free magnetic field produced by spheroidal stars remains very similar to their spherical counterpart. However, the spin down luminosity slightly decreases with increasing oblateness or prolateness. Moreover the polar cap area increases and always mostly encompasses the equivalent spherical star polar cap rims. The polar cap current density is also drastically affected.

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arXiv2021

Spheroidal magnetic stars rotating in vacuum

Gravity shapes stars to become almost spherical because of the isotropic nature of gravitational attraction in Newton's theory. However, several mechanisms break this isotropy like for instance their rotation generating a centrifugal force, magnetic pressure or anisotropic equations of state. The stellar surface therefore deviates slightly or significantly from a sphere depending on the strength of these anisotropic perturbations. In this paper, we compute analytical and numerical solutions of the electromagnetic field produced by a rotating spheroidal star of oblate or prolate nature. This study is particularly relevant for millisecond pulsars for which strong deformations are produced by the rotation or a strong magnetic field, leading to indirect observational signatures of the polar cap thermal X-ray emission. First we solve the time harmonic Maxwell equations in vacuum by using oblate and prolate spheroidal coordinates adapted to the stellar boundary conditions. The solutions are expanded in series of radial and angular spheroidal wave functions. Particular emphasize is put on the magnetic dipole radiation. Second, we compute approximate solutions by integrating numerically the time-dependent Maxwell equations in spheroidal coordinates. We show that the spin down luminosity corrections compared to a perfect sphere are to leading order given by terms involving $(a/r_L)^2$ and $(a/R)^2$ where $a$ is the stellar oblateness or prolateness, $R$ the smallest star radius and $r_L$ the light-cylinder radius. The corresponding perturbations in the electromagnetic field are only perceptible close to the surface, deforming the polar cap rims. At large distances $r\gg a$, the solution tends asymptotically to the perfect spherical case of a rotating dipole.

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arXiv2021

Particle acceleration and radiation reaction in strong spherical electromagnetic waves

Strongly magnetized and fast rotating neutron stars are known to be efficient particle accelerators within their magnetosphere and wind. They are suspected to accelerate leptons, protons and maybe ions to extreme relativistic regimes where the radiation reaction significantly feeds back to their motion. In the vicinity of neutron stars, magnetic field strengths are close to the critical value of $B_{\rm c} \sim 4.4\times10^9$T and particle Lorentz factors of the order $γ\sim 10^9$ are expected. In this paper, we investigate the acceleration and radiation reaction feedback in the pulsar wind zone where a large amplitude low frequency electromagnetic wave is launched starting from the light-cylinder. We design a semi-analytical code solving exactly the particle equation of motion including radiation reaction in the Landau-Lifshits approximation for a null-like electromagnetic wave of arbitrary strength parameter and elliptical polarization. Under conventional pulsar conditions, asymptotic Lorentz factor as high as $10^8-10^9$ are reached at large distances from the neutron star. However, we demonstrate that in the wind zone, within the spherical wave approximation, radiation reaction feedback remains negligible.

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arXiv2020

Off-centred force-free neutron star magnetospheres

Neutron star electromagnetic activity produces pairs that fill their magnetosphere represented to the zeroth order by the force-free approximation. Neither dissipation nor acceleration nor radiation from charged particles is expected from this simplified model. So far, only centred dipole magnetic fields have been studied in this limit. In this paper, we explore the consequences of a rotating off-centred dipole on the force-free magnetosphere, showing the new magnetic field geometry, its spin-down luminosity as well as the electromagnetic kick and torque felt by the neutron star. Solutions are obtained by time-dependent numerical simulations of the force-free regime using our pseudo-spectral code written in spherical coordinates. Our results are also compared to known analytical expressions found for the off-centred vacuum dipole by an expansion to lowest order in the parameter $ε= d/R$, where $d$ is the displacement of the dipole from the stellar centre and $R$ the neutron star radius. The presence of a force-free plasma enhances the spin-down luminosity as well as the electromagnetic kick and torque with respect to a centred force-free dipole. The impact on isolated and binary neutron stars is revised in light of these new results.

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arXiv2020

Frequency resolved radio and high-energy emission of pulsars

Context. Pulsars are detected as broadband electromagnetic emitters from the radio wavelength up to high and very high energy in the MeV/GeV and sometimes even in the TeV range. Multi-wavelength phase-resolved spectra and light curves offer an unrivalled opportunity to understand their underlying radiation mechanisms and to localize their emission sites and therefore the particle acceleration regions. Aims. In this paper, we compute pulsars multi-wavelength phase-resolved light-curves and spectra assuming that curvature radiation operates from inside the magnetosphere of a rotating vacuum magnet. Radio emission arises from the polar caps whereas gamma-ray energy emanates from the slot gaps in the vicinity of the separatrix between closed and open field lines. Methods. By integrating particle trajectories within the slot gaps, we compute energy dependent photon sky maps in the radio band (MHz-GHz) and in the gamma-ray band (MeV/GeV) for mono-energetic distribution functions of leptons. Results. We obtained many details of the energy dependent light curves and phase-resolved spectra from the radio wavelength up to the gamma-ray energies. Choosing Lorentz factors of γ = 30 for radio emitting particles and γ = 10^7 for gamma-ray emitting particles, we found realistic spectra accounting for the wealth of multi-wavelength pulsar observations.

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arXiv2020

Particle acceleration in neutron star ultra-strong electromagnetic fields

In this paper, we discuss the results of a new particle pusher in realistic ultra-strong electromagnetic fields as those encountered around rotating neutron stars. After presenting results of this algorithm in simple fields and comparing them to expected exact analytical solutions, we present new simulations for a rotating magnetic dipole in vacuum for a millisecond pulsar by using Deutsch solution. Particles are injected within the magnetosphere, neglecting radiation reaction, interaction among them and their feedback on the fields. Our simulations are therefore not yet fully self-consistent because Maxwell equations are not solved according to the current produced by these particles. The code highlights the symmetrical behaviour of particles of opposite charge to mass ratio $q/m$ with respect to the north and south hemispheres. The relativistic Lorentz factor of the accelerated particles is proportional to this ratio $q/m$: protons reach up to $γ_p \simeq 10^{10.7}$, whereas electrons reach up to $γ_e \simeq 10^{14}$. Our simulations show that particles could be either captured by the neutron star, trapped around it, or ejected far from it, well outside the light-cylinder. Actually, for a given charge to mass ratio, particles follow similar trajectories. These particle orbits show some depleted directions, especially at high magnetic inclination with respect to the rotation axis for positive charges and at low inclination for negative charges because of symmetry. Other directions are preferred and loaded with a high density of particles, some directions concentrating the highest or lowest acceleration efficiencies.

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arXiv2020

Electrodynamics and radiation from rotating neutron star magnetospheres

Neutron stars are compact objects rotating at high speed, up to a substantial fraction of the speed of light (up to 20\% for millisecond pulsars) and possessing ultra-strong electromagnetic fields (close to and sometimes above the quantum critical field of \numprint{4.4e9}~\SIunits{\tesla}). Moreover, due to copious $e^\pm$ pair creation within the magnetosphere, the relativistic plasma surrounding the star is forced into corotation up to the light cylinder where the corotation speed reaches the speed of light. The neutron star electromagnetic activity is powered by its rotation which becomes relativistic in the neighbourhood of this light cylinder. These objects naturally induce relativistic rotation on macroscopic scales about several thousands of kilometers, a crucial ingredient to trigger the central engine as observed on Earth. In this paper, we elucidate some of the salient features of this corotating plasma subject to efficient particle acceleration and radiation, emphasizing several problems and limitations concerning current theories of neutron star magnetospheres. Relativistic rotation in these systems is indirectly probed by the radiation produced within the magnetosphere. Depending on the underlying assumptions about particle motion and radiation mechanisms, different signatures on their light-curves, spectra, pulse profiles and polarisation angles are expected in their broadband electromagnetic emission. We show that these measurements put stringent constraints on the way to describe particle electrodynamics in a rotating neutron star magnetosphere.

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