Fermi LAT Observations of the Vela Pulsar

TL;DR

The paper reports initial Fermi LAT observations of the Vela pulsar, revealing detailed pulse structures, spectral characteristics, and supporting outer magnetosphere emission models, while setting limits on unpulsed emission.

Contribution

First detailed gamma-ray observations of the Vela pulsar with Fermi LAT, including high-resolution pulse profiles and spectral analysis supporting outer magnetosphere emission models.

Findings

High-quality pulse profile with 32,400 photons at E>0.03 GeV.

Detection of a third pulse peak shifting with energy.

Spectral fit indicating a power law with exponential cutoff at 2.9 GeV.

Abstract

The Vela pulsar is the brightest persistent source in the GeV sky and thus is the traditional first target for new gamma-ray observatories. We report here on initial Fermi Large Area Telescope observations during verification phase pointed exposure and early sky survey scanning. We have used the Vela signal to verify Fermi timing and angular resolution. The high quality pulse profile, with some 32,400 pulsed photons at E>0.03 GeV, shows new features, including pulse structure as fine as 0.3ms and a distinct third peak, which shifts in phase with energy. We examine the high energy behavior of the pulsed emission; initial spectra suggest a phase-averaged power law index of Gamma=1.51{+0.05/-0.04} with an exponential cut-off at E_c=2.9+/-0.1 GeV. Spectral fits with generalized cut-offs of the form e^{-(E/E_c)^b} require b<1, which is inconsistent with magnetic pair attenuation, and thus favor outer magnetosphere emission models. Finally, we report on upper limits to any unpulsed component, as might be associated with a surrounding synchrotron wind nebula (PWN).