The ASCA Catalog of Potential X-Ray Counterparts of GeV Sources
  We present a catalog of 2-10 keV ASCA Gas Imaging Spectrometer images of fields containing bright sources of GeV emission. The images cover $\sim 85%$ of the 95% confidence position contour for 28 of the 30 sources with $> 1$ GeV fluxes above $4\times 10^{-8}$ ph ${\rm cm}^{-2} {\rm s}^{-1}$. We find an excess of hard X-ray sources with $F_{2-10 keV} \ga 10^{-12}$ ergs ${\rm cm}^{-2} {\rm s}^{-1}$ positionally coincident with unidentified sources of GeV emission. We comment on radio-loud and radio-quiet pulsar candidates, as well as several SNR and massive binaries as possible sources of the GeV emission. We also find evidence for a class of variable $\gamma-$ray sources associated with extended regions of hard X-ray emission, and propose that for these sources, a significant percentage of the $\gamma-$ray emission is generated in synchrotron nebulae surrounding fast, young pulsars. 
Mallory S. E. Roberts, Roger W. Romani and Nobuyuki Kawai