Due to the low fluxes from pulsars and the small collecting area of space-based telescopes above 50 GeV, the properties of the $\sim$150 pulsars newly discovered by Fermi are largely unexplored at these energies. In this regime, the uncertainties on the spectral data points and/or the constraints from upper-limits are not sufficient to provide robust tests of competing emission models in individual pulsars. The discovery of power-law-type emission from the Crab pulsar at energies exceeding 100 GeV provides a compelling justification for exploration of other pulsars at these energies. We introduce the method of “Aperture Photometry” to measure pulsar emission spectra from Fermi-LAT data and present a stacked analysis of 115 pulsars selected from the 2nd catalog of Fermi-LAT pulsars. This analysis, which uses an average of $\sim$4.2 years of data per pulsar, is sensitive to low-level emission which cannot be resolved in individual objects but can be detected from an ensemble. We find no significant stacked excess at energies above 50 GeV. An upper limit of $\sim$30% of the Crab pulsar level is found for the average flux from 115 pulsars in the 100-177 GeV energy range at the 95% confidence level. Stacked searches exclusive to the young pulsar sample, the millisecond pulsar sample, and several other promising sub-samples also return no significant excesses above 50 GeV.
Tue, 9 Dec 14
Comments: 10 pages, 8 Figures, 2 Tables. Submitted