A Possible X‐Ray and Radio Counterpart of the High‐Energy Gamma‐Ray Source 3EG J2227+6122

Abstract

The identity of the persistentEGRET sources in the Galactic plane is largely a mystery. For oneof these, 3EG J2227+6122, our complete census of X-ray and radiosources in its error circle reveals a remarkable superposition of anincomplete radio shell with a flat radio spectrum and a compact, power-lawX-ray source with photon index Gamma=1.5 and with no obvious opticalcounterpart. The radio shell is polarized at a level of ~=25%. Theanomalous properties of the radio source prevent us from deriving acompletely satisfactory theory as to its nature. Nevertheless, using datafrom ROSAT, ASCA, the VLA, and optical imaging and spectroscopy, weargue that the X-ray source may be a young pulsar with an associatedwind-blown bubble or bow-shock nebula and an example of the class ofradio-quiet pulsars that are hypothesized to comprise the majorityof EGRET sources in the Galaxy. The distance to this source can beestimated from its X-ray absorption as 3 kpc. At this distance, theX-ray and gamma-ray luminosities would be ~1.7�10 33 and~3.7�10 35 ergs s -1, respectively, which would requirean energetic pulsar to power them. If, on the contrary, this X-raysource is not the counterpart of 3EG J2227+6122, then by process ofelimination the X-ray luminosity of the latter must be less than10 -4 of its gamma-ray luminosity, a condition not satisfied byany established class of gamma-ray source counterpart. This wouldrequire the existence of at least a quantitatively new type of EGRETsource, as has been suggested in studies of other EGRET fields.