arXiv subjects

Jérôme Pétri

Publications and source records attributed to Jérôme Pétri.

29 records · Page 2Linked to original sources

arXiv2019

Radiative pulsar magnetospheres: aligned rotator

Force-free neutron star magnetospheres are nowadays well known and found through numerical simulations. Even extension to general relativity has recently been achieved. However, those solutions are by definition dissipationless, meaning that the star is unable to accelerate particles and let them radiate any photon. Interestingly, the force-free model has no free parameter however it must be superseded by a dissipative mechanism within the plasma. In this paper, we investigate the magnetosphere electrodynamics for particles moving in the radiation reaction regime, using the limit where acceleration is fully balanced by radiation, also called Aristotelian dynamics. An Ohm's law is derived, from which the dissipation rate is controlled by a one parameter family of solutions depending on the pair multiplicity~$κ$. The spatial extension of the dissipation zone is found self-consistently from the simulations. We show that the radiative magnetosphere of an aligned rotator tends to the force-free regime whenever the pair multiplicity becomes moderately large, $κ\gg 1$. However, for low multiplicity, a substantial fraction of the spindown energy goes into particle acceleration and radiation in addition to the Poynting flux, the latter remaining only dominant for large multiplicities. We show that the work done on the plasma occurs predominantly in the equatorial current sheet right outside the light-cylinder.

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arXiv2019

Joint radio and X-ray modelling of PSR~J1136+1551

Multi-wavelength observations of pulsar emission properties are powerful means to constrain their magnetospheric activity and magnetic topology. Usually a star centred magnetic dipole model is invoked to explain the main characteristics of this radiation. However in some particular pulsars where observational constraints exist, such simplified models are unable to predict salient features of their multi-wavelength emission. This paper aims to carefully model the radio and X-ray emission of PSR~J1136+1551 with an off-centred magnetic dipole to reconcile both wavelength measurements. We simultaneously fit the radio pulse profile with its polarization and the thermal X-ray emission from the polar cap hot spots of PSR~J1136+1551. We are able to pin down the parameters of the non-dipolar geometry (which we have assumed to be an offset dipole) and the viewing angle, meanwhile accounting for the time lag between X-rays and radio emission. Our model fits the data if the off-centred magnetic dipole lies about 20\% below the neutron star surface. We also expect very asymmetric polar cap shapes and sizes, implying non antipodal and non identical thermal emission from the hot spots. We conclude that a non-dipolar surface magnetic field is an essential feature to explain the multi-wavelength aspects of PSR~J1136+1551 and other similar pulsars.

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arXiv2019

General-relativistic pulsar radio and high-energy emission

According to current pulsar emission models, photons are produced within their magnetosphere or inside the current sheet outside the light-cylinder. Radio emission is favoured in the vicinity of the polar caps whereas the high-energy counterpart is presumably enhanced in regions around the light-cylinder, magnetosphere or/and wind. However, gravitational impacts on light-curves and their spectral properties have only been sparsely touched. In this paper, we present a new method to simulate the influence of the neutron star gravitational field on its emission according to general relativity. We numerically compute photon trajectories assuming a background Schwarzschild metric, applying our method to neutron star radiation mechanisms, like thermal emission from hot spots and non-thermal magnetospheric emission by curvature radiation. We detail the general-relativistic impacts onto observations made by a distant observer. Sky maps are computed using the vacuum electromagnetic field of a general-relativistic rotating dipole. We compare Newtonian results to their general-relativistic counterpart. For magnetospheric emission, we show that, more importantly than the aberration and the curvature of the trajectory of the photons, the Shapiro time delay significantly affected the phase delay between radio and high-energy light curves although the characteristic pulse profile that defines pulsar emission is kept.

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arXiv2019

Impact of an off-centred dipole on neutron star binaries

Neutron stars are strongly magnetized rotating compact objects. Therefore they also produce huge electric fields accelerating particles to ultra-relativistic energies. The simplest magnetic topology is a dipole traditionally located at the stellar centre. In this paper, we reinvestigate the consequences of an off-centred rotating magnetic dipole, showing accurate magnetic field line geometries, the associated spin-down luminosity as well as the corresponding electromagnetic kick and torque imprinted to the neutron star. Results are obtained by time dependent numerical simulations of Maxwell equations in vacuum using pseudo-spectral methods. We compare our results to known analytical expressions available 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. We found good agreement between our numerical computations and our analytical approximations even for well off-centred dipoles having large displacements with a sizeable fraction of the radius, i.e. $ε\lesssim 1$. An explanation for binary neutron star eccentricity distribution functions is given with an emphasize on highly eccentric systems as an alternative scenario to traditional binary formation.

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arXiv2019

The illusion of neutron star magnetic field estimates

Neutron stars radiate in a broad band spectrum from radio wavelengths up to very high energies. They have been sorted into several classes depending on their respective place in the $P-\dot{P}$ diagram and depending on spectral/temporal properties. Fundamental physical parameters such as their characteristic age and magnetic field strength are deduced from these primary observables. However this deduction relies mostly on interpretations based on simple vacuum or force-free rotating dipole models that are unrealistic. In this paper, we show that the computation of the stellar surface magnetic field is poorly estimated or even erroneous if multipolar components and particle loading are neglected. We show how quadrupolar magnetic field and monopolar winds alter field estimates and characteristic ages in the $P-\dot{P}$ diagram. Corrections brought by general relativity are also discussed. We derive some important parameters of pulsar physics such as the wind Lorentz factor~($γ$) times the pair multiplicity~($κ$) to be around $γ\,κ\approx 10^8-10^{10}$. Therefore, the standard magnetodipole radiation losses formula must be used with caution to reckon neutron star surface magnetic fields and related secular evolution parameters. Depending on models we found that all field strengths, both for magnetars and for pulsars lie below the quantum critical value of $B_{\rm c} \approx 4.4\cdot 10^9$~T.

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arXiv2019

Pulsar gamma-ray emission in the radiation reaction regime

Since the era of the Fermi/LAT and atmospheric Cerenkov telescopes, pulsars are known to emit high and very high-energy photons, in the MeV-GeV range and sometimes up to TeV. To date, it is still unclear where and how these photons are produced. Nevertheless gamma-ray photons require particle acceleration to ultra-relativistic speeds. In this paper, we compute single particle trajectories for leptons in an arbitrary strong electromagnetic field in the so-called radiation reaction limit. In this picture, particle velocity only depends on the local electromagnetic field which we assume to follow the vacuum dipole rotator. From this velocity field, we compute the curvature radiation spectrum and light-curves. Sky maps and phase-resolved spectra are then deduced accounting for realistic pulsar periods and magnetic field strengths. Emission sites within the pulsar magnetosphere where most of radiation emanates are then localized. For standard parameters of millisecond and normal pulsars, we show that a break in the spectrum occurs at several GeV in agreement with the Fermi/LAT second pulsar catalogue. A sample of representative phase-resolved spectra and sky-maps are shown. A pair multiplicity of several tenths to several thousands is required to account for the total gamma-ray luminosity. Moreover depending on the geometry, single or double-peaked light-curves are found. Our model shows that minimalist assumptions are already able to reproduce salient features of pulsar emission.

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arXiv2018

General-relativistic pulsar magnetospheric emission

Most current pulsar emission models assume photon production and emission within the magnetosphere. Low frequency radiation is preferentially produced in the vicinity of the polar caps whereas the high-energy tail is shifted to regions closer but still inside the light-cylinder. We conducted a systematic study of the merit of several popular radiation sites like the polar cap, the outer gap and the slot gap. We computed sky maps emanating from each emission site according to a prescribed distribution function for the emitting particles made of an electron/positron mixture. Calculations are performed using a three dimensional integration of the plasma emissivity in the vacuum electromagnetic field of a rotating centred general-relativistic dipole. We compare Newtonian electromagnetic fields to their general-relativistic counterpart. In the latter case, light bending is also taken into account. As a typical example, light-curves and sky maps are plotted for several power-law indices of the particle distribution function. The detailed pulse profiles strongly depend on the underlying assumption about the fluid motion subject to strong electromagnetic fields. This electromagnetic topology enforces the photon propagation direction directly, or indirectly, from aberration effects. We also discuss the implication of a net stellar electric charge on to the sky maps. Taking into account the electric field strongly affects the light-curves originating close to the light-cylinder where the electric field strength becomes comparable to the magnetic field strength.

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arXiv2017

Multipolar electromagnetic fields around neutron stars: general-relativistic vacuum solutions

Magnetic fields inside and around neutron stars are at the heart of pulsar magnetospheric activity. Strong magnetic fields are responsible for quantum effects, an essential ingredient to produce leptonic pairs and the subsequent broadband radiation. The variety of electromagnetic field topologies could lead to the observed diversity of neutron star classes. Thus it is important to include multipolar components to a presumably dominant dipolar magnetic field. Exact analytical solutions for these multipoles in Newtonian gravity have been computed in recent literature. However, flat spacetime is not adequate to describe physics in the immediate surrounding of neutron stars. We generalize the multipole expressions to the strong gravity regime by using a slowly rotating metric approximation such as the one expected around neutron stars. Approximate formulas for the electromagnetic field including frame dragging are computed from which we estimate the Poynting flux and the braking index. Corrections to leading order in compactness and spin parameter are presented. As far as spindown luminosity is concerned, it is shown that frame dragging remains irrelevant. For high order multipoles starting from the quadrupole, the electric part can radiate more efficiently than the magnetic part. Both analytical and numerical tools are employed.

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arXiv2015

On ultra-high energy cosmic ray acceleration at the termination shock of young pulsar winds

Pulsar wind nebulae (PWNe) are outstanding accelerators in Nature, in the sense that they accelerate electrons up to the radiation reaction limit. Motivated by this observation, this paper examines the possibility that young pulsar wind nebulae can accelerate ions to ultra-high energies at the termination shock of the pulsar wind. We consider here powerful PWNe, fed by pulsars born with $\sim\,$millisecond periods. Assuming that such pulsars exist, at least during a few years after the birth of the neutron star, and that they inject ions into the wind, we find that protons could be accelerated up to energies of the order of the Greisen-Zatsepin-Kuzmin cut-off, for a fiducial rotation period $P\,\sim\,1\,$msec and a pulsar magnetic field $B_{\star}\,\sim\,10^{13}\,$G, implying a fiducial wind luminosity $L_{\rm p}\,\sim\,10^{45}\,$erg/s and a spin-down time $t_{\rm sd}\,\sim 3\times 10^7\,$s. The main limiting factor is set by synchrotron losses in the nebula and by the size of the termination shock; ions with $Z\geq 1$ may therefore be accelerated to even higher energies. We derive an associated neutrino flux produced by interactions in the source region. For a proton-dominated composition, our maximum flux lies slightly below the 5-year sensitivity of IceCube-86 and above the 3-year sensitivity of the projected Askaryan Radio Array. It might thus become detectable in the next decade, depending on the exact level of contribution of these millisecond pulsar wind nebulae to the ultra-high energy cosmic ray flux.

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arXiv2011

Implication of the striped pulsar wind model for gamma-ray binaries

(abridged) Gamma-ray binaries are massive stars with compact object companions that are observed to emit most of their energy in the gamma-ray range. One of these binaries is known to contain a radio pulsar, PSR B1259-63. Synchrotron and inverse Compton emission from particles accelerated beyond the light cylinder in striped pulsar winds has been proposed to explain the X-ray to high energy (HE, $>$ 100 MeV) gamma-ray emission from isolated pulsars. This pulsar model extends naturally to binary environments, where seed photons for inverse Compton scattering are provided by the companion star. Here, we investigate the possibility of gamma-ray emission from PSR B1259-63 in the framework of the striped pulsar wind model. The orbital geometry of PSR B1259-63 is well constrained by observations and the double radio pulse suggests an almost orthogonal rotator so that the solid angle covered by the striped region is close to $4π$. We calculate the orbital and rotational phase-resolved spectral variability and light-curves to expect.

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arXiv2010

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.

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