The Vela Pulsar: Results from the First Year of Fermi LAT Observations

We report on analysis of timing and spectroscopy of the Vela pulsar using eleven months of observations with the Large Area Telescope on the Fermi Gamma-Ray Space Telescope. The intrinsic brightness of Vela at GeV energies combined with the angular resolution and sensitivity of the LAT allow us to make the most detailed study to date of the energy-dependent light curves and phase-resolved spectra, using a LAT-derived timing model. The light curve consists of two peaks (P1 and P2) connected by bridge emission containing a third peak (P3). We have confirmed the strong decrease of the P1/P2 ratio with increasing energy seen with EGRET and previous Fermi LAT data, and observe that P1 disappears above 20 GeV. The increase with energy of the mean phase of the P3 component can be followed with much greater detail, showing that P3 and P2 are present up to the highest energies of pulsation. We find significant pulsed emission at phases outside the main profile, indicating that magnetospheric emission exists over 80% of the pulsar period. With increased high-energy counts the phase-averaged spectrum is seen to depart from a power- law with simple exponential cutoff, and is better fit with a more gradual cutoff. The spectra in fixed-count phase bins are well fit with power-laws with exponential cutoffs, revealing a strong and complex phase dependence of the cutoff energy, especially in the peaks. By combining these results with predictions of the outer magnetosphere models that map emission characteristics to phase, it will be possible to probe the particle acceleration and the structure of the pulsar magnetosphere with unprecedented detail.

💡 Research Summary

This paper presents a comprehensive analysis of the Vela pulsar using eleven months of data from the Large Area Telescope (LAT) aboard the Fermi Gamma‑Ray Space Telescope. By constructing a timing solution directly from LAT photons, the authors achieve sub‑microsecond phase alignment, surpassing the precision of traditional radio or X‑ray ephemerides. The high photon statistics and improved angular resolution of LAT enable an unprecedented study of the pulsar’s energy‑dependent light curve and phase‑resolved spectra.

The light curve exhibits the classic double‑peak structure (P1 and P2) linked by bridge emission that contains a third, distinct peak (P3). The authors confirm the long‑known decrease of the P1/P2 intensity ratio with increasing energy, and they demonstrate that P1 vanishes completely above ~20 GeV, leaving P2 and P3 as the sole contributors at the highest energies. Moreover, the phase of P3 shifts systematically to later phases as the photon energy rises, indicating that the emission region for the highest‑energy particles moves outward along the magnetic field lines. Off‑pulse emission is detected over roughly 80 % of the rotation, implying that magnetospheric radiation is not confined to narrow zones but is instead widespread.

Spectrally, the phase‑averaged emission departs from a simple power‑law with an exponential cutoff (∝ E^−Γ exp