Abstract
Context. Since the launch of the Fermi gamma-ray telescope, several hundred radio-loud gamma-ray pulsars have been detected, many belonging to millisecond pulsars but some belonging to the young pulsar population with spin periods longer than 30 ms. Aims. Observing simultaneously pulsed radio and gamma-ray emission from these stars helps to constrain the geometry and radiation mechanisms within their magnetosphere and to localize the multiple photon production sites. In this paper we fit the time-aligned gamma-ray light curves of young radio-loud gamma-ray pulsars. We assume a dipole force-free magnetosphere where radio photons emanate from high altitudes above the polar caps and gamma rays originate from outside the light cylinder, within the striped wind current sheet. Methods. We computed a full atlas of radio and gamma-ray pulse profiles depending on the magnetic axis obliquity and line-of-sight inclination with respect to the neutron star rotation axis. By applying a χ2 fitting technique, we were able to pin down accurately the magnetosphere geometry. Further constraints were obtained from radio polarization measurement following the rotating vector model, including aberration and retardation effects. Results. We find a good agreement between our model and the time-aligned single- or double-peaked gamma-ray pulsar observations. We deduce the magnetic inclination angle and the observer line of sight with respect to the rotation axis within a small error bar. The distinction between radio-loud or radio-quiet gamma-ray pulsars or only radio pulsars can entirely be related to the geometry of the associated emitting regions. Conclusions. The high-altitude polar cap model combined with the striped wind represents a minimalistic approach able to reproduce a wealth of gamma-ray pulse profiles for young radio pulsars. Based on self-consistent force-free simulations, it gives a full geometrical picture of the emission properties without resorting to detailed knowledge of the individual particle dynamics and energetics.
Highlights
The observed pulsed emission properties of pulsars in the radio and high-energy bands, like their light curves and spectra, are very sensitive to their global geometry. This geometry is defined by their electromagnetic field topology and the angles, on the one hand between the rotation axis and the magnetic dipole axis, and on the other hand between the rotation axis and the line of sight
We show in the same figure first the radio pulse profile with the best RVM fit, the radio and gamma-ray χ2 fit, and the best radio and gamma-ray light curves predictions compared to observations
We showed that simultaneously fitting the radio and gamma-ray pulse profile of radioloud gamma-ray pulsars severely constrains the geometry of the dipole magnetic field and observer line of sight with respect to the rotation axis
Summary
The observed pulsed emission properties of pulsars in the radio and high-energy bands, like their light curves and spectra, are very sensitive to their global geometry. Pierbattista et al (2015) performed a comprehensive analysis of light curve modelling of young gammaray pulsars assuming different geometries like polar cap, slot gap, outer gap, and one-pole caustic but did not include the striped wind They pointed out the importance of joint radio–γ-ray fit to constrain the geometry. Their model is based on accurate dipole force-free magnetosphere simulations. Time-aligned radio and gamma-ray light curves are computed for geometric configurations and summarized in several sky maps We generalize this approach to the more realistic dipole field, smoothly joining the stellar surface to the striped wind and referred to as the dipole force-free magnetosphere.
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