Discovery of a hard X-ray source SAX J0635+0533, in the error box of the gamma-ray source 2EG 0635+0521

We have discovered an X-ray source, SAX J0635 + 0533, with a hard spectrum within the error box of the GeV gamma-ray source in Monoceros, 2EG J0635 + 0521. The unabsorbed flux from the source is 1.2 x 10(-11) ergs cm(-2) s(-1) in the 2-10 keV band. The X-ray spectrum is consistent with a simple power-law model with absorption. The photon index is 1.50 +/- 0.08, and we detect emission out to 40 keV. Optical observations identify a counterpart with a V magnitude of 12.8. The counterpart has broad emission lines and the colors of an early B-type star. If the identification of the X-ray/optical source with the gamma-ray source is correct, then the source would be a gamma-ray-emitting X-ray binary.