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 similar to 85% of the 95% confidence position contour for 28 of the 30 sources with >1 GeV fluxes above 4 x 10(-8) ph cm(-2) s(-1). We find an excess of hard X-ray sources with F2-10keV greater than or similar to 10(-12) ergs cm(-2) s(-1) positionally coincident with unidentified sources of GeV emission. We comment on radio-loud and radio-quiet pulsar candidates, as well as several SNRs 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.