Gamma-ray and Radio Properties of Six Pulsars Detected by the Fermi Large Area Telescope

We report the detection of pulsed gamma-rays for PSRs J0631+1036, J0659+1414, J0742-2822, J1420-6048, J1509-5850 and J1718-3825 using the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (formerly known as GLAST). Although these six pulsars are diverse in terms of their spin parameters, they share an important feature: their gamma-ray light curves are (at least given the current count statistics) single peaked. For two pulsars there are hints for a double-peaked structure in the light curves. The shapes of the observed light curves of this group of pulsars are discussed in the light of models for which the emission originates from high up in the magnetosphere. The observed phases of the gamma-ray light curves are, in general, consistent with those predicted by high-altitude models, although we speculate that the gamma-ray emission of PSR J0659+1414, possibly featuring the softest spectrum of all Fermi pulsars coupled with a very low efficiency, arises from relatively low down in the magnetosphere. High-quality radio polarization data are available showing that all but one have a high degree of linear polarization. This allows us to place some constraints on the viewing geometry and aids the comparison of the gamma-ray light curves with high-energy beam models.

💡 Research Summary

The paper reports the first detection of pulsed gamma‑ray emission from six radio pulsars—PSR J0631+1036, J0659+1414, J0742‑2822, J1420‑6048, J1509‑5850, and J1718‑3825—using the Large Area Telescope (LAT) aboard the Fermi Gamma‑ray Space Telescope. Although these objects span a wide range of spin periods, period derivatives, spin‑down powers, and surface magnetic fields, their gamma‑ray light curves share a striking similarity: with the present photon statistics each light curve appears single‑peaked, and only two sources show tentative hints of a second peak. The authors compare these observed profiles with predictions from high‑altitude emission models (Outer Gap and Slot Gap) that locate the gamma‑ray production region far from the neutron‑star surface, near the light cylinder. In most cases the phase offsets between the radio and gamma‑ray peaks, as well as the overall shape of the gamma‑ray profiles, are consistent with the expectations of such models, supporting the view that the bulk of the emission originates at high altitude.

A notable exception is PSR J0659+1414, which exhibits the softest gamma‑ray spectrum among the Fermi pulsar population and an unusually low gamma‑ray efficiency (≈ 1 % of its spin‑down power). The authors argue that this source may be an outlier in which the gamma‑ray photons are produced at relatively low altitude, perhaps within a traditional polar‑cap region, rather than in the outer magnetosphere. This hypothesis is reinforced by its comparatively low linear polarization in the radio band and a different radio‑gamma phase relationship.

High‑quality radio polarization measurements are available for five of the six pulsars. All but one display a high degree of linear polarization, allowing the authors to constrain the magnetic inclination angle (α) and the observer’s line‑of‑sight angle (ζ) through the rotating‑vector model. These geometric constraints are then used to test the high‑altitude beam models: the derived α and ζ values generally place the line of sight through the predicted caustic emission zones, reproducing the observed gamma‑ray peak phases. For PSR J0742‑2822 and PSR J1509‑5850, the weak evidence for a second gamma‑ray peak aligns with the expectation that, for certain viewing geometries, the line of sight cuts through both leading and trailing caustics.

The paper emphasizes the power of a multi‑wavelength approach. By combining gamma‑ray timing, spectral analysis, and radio polarization data, the study provides a more complete picture of the emission geometry than could be achieved with any single band alone. The authors acknowledge that the current LAT photon statistics limit the ability to resolve fine structure in the light curves, and they call for continued observations to improve signal‑to‑noise ratios. Future work, integrating longer LAT exposures with even higher‑resolution radio polarimetry, should enable tighter constraints on the location of particle acceleration zones, the efficiency of conversion from rotational energy to high‑energy photons, and the possible coexistence of both high‑altitude and low‑altitude emission components in individual pulsars.