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J.P. Halpern, D.J. Helfand, E.V. Gotthelf (Columbia U.), K.M. Leighly (U. Oklahoma)
\def\source{3EG~J2227+6122} \def\ro{ROSAT\/} \def\asca{ASCA\/}
The identity of the persistent EGRET sources in the Galactic plane is largely a mystery. For one of these, \source, our complete census of X-ray and radio sources in its error circle reveals a remarkable superposition of an incomplete radio shell with a flat radio spectrum, and a compact, power-law X-ray source with photon index \Gamma = 1.5 and with no obvious optical counterpart. The radio shell is polarized at a level of ~q 25%. The anomalous properties of the radio source prevent us from deriving a completely satisfactory theory as to its nature. Nevertheless, using data from \ro, \asca, the VLA, and optical imaging and spectroscopy, we argue that the X-ray source may be a young pulsar with an associated wind-blown bubble or bow shock nebula, and an example of the class of radio-quiet pulsars which are hypothesized to comprise the majority of EGRET sources in the Galaxy. The distance to this source can be estimated from its X-ray absorption as 3~kpc. At this distance, the X-ray and \gamma-ray luminosities would be \approx 1.7 \times 1033 and \approx 3.7 \times 1035 erg~s-1, respectively, which would require an energetic pulsar to power them.
If, on the contrary, this X-ray source is not the counterpart of \source, then by process of elimination the X-ray luminosity of the latter must be less than 10-4 of its \gamma-ray luminosity, a condition not satisfied by any established class of \gamma-ray source counterpart. This would require the existence of at least a quantitatively new type of EGRET source.