A Systematic Search for MeV–GeV Pulsar Wind Nebulae without Gamma-Ray Detected Pulsars

An increasing number of pulsar wind nebulae (PWNe) are being identified in the TeV band by ground-based Imaging Air Cherenkov Telescopes such that they constitute the dominant source class of Galactic TeV emitters. However, MeV-GeV PWN counterparts are still largely lacking. To date, only a dozen PWNe are identified by the Fermi-Large Area Telescope (LAT) in the MeV-GeV band. Most PWNe are located along the Galactic plane embedded within the prominent, diffuse Galactic gamma-ray emission, which makes these sources difficult to disentangle from the bright diffuse background. We present a systematic search for gamma-ray counterparts to known PWNe in the 300 MeV-2 TeV energy band using the Fermi-LAT. We target the locations of previously identified PWNe that lack detected Fermi-LAT pulsars to minimize associated pulsar contamination. The sample includes six previously identified Fermi PWNe and eight Fermi-LAT sources associated with PWNe. We report the analysis of 58 regions of interest and classify Fermi-LAT detected sources as either a likely PWN or a candidate PWN counterpart based on their morphological and spectral characteristics across the broadband spectrum. There are nine unidentified Fermi-LAT sources that we consider as likely PWN counterparts, which, if confirmed to be PWNe, would greatly increase the PWN population detected by the Fermi-LAT from 12 to 21. The remaining Fermi-LAT detected sources are considered weaker PWN candidates. A second approach in the systematic search for gamma-ray emitting PWNe will involve studying the off-pulse phases of Fermi-LAT detected pulsars for the presence of an obscured PWN and will be reported in a subsequent paper.