Deep observation ( \sim 113 hrs ) of the Cygnus region at TeV energies using the HEGRA stereoscopic system of air Čerenkov telescopes has serendipitously revealed a signal positionally inside the core of the OB association Cygnus OB2 , at the edge of the 95 % error circle of the EGRET source 3EG J2033+4118 , and \sim 0.5 ^ { \circ } north of Cyg X-3 .
The source centre of gravity is RA \alpha _ { J 2000 } : 20 ^ { hr } 32 ^ { m } 07 ^ { s } \pm 9.2 ^ { s } _ { stat } \pm 2.2 ^ { s } _ { sys } , Dec \delta _ { J 2000 } : +41 ^ { \circ } 30 ^ { \prime } 30 ^ { \prime \prime } \pm 2.0 ^ { \prime } _ { stat } \pm 0.4 ^ { \prime } _ { sys } .
The source is steady , has a post-trial significance of +4.6 \sigma , indication for extension with radius 5.6 ^ { \prime } at the \sim 3 \sigma level , and has a differential power-law flux with hard photon index of -1.9 \pm 0.3 _ { stat } \pm 0.3 _ { sys } .
The integral flux above 1 TeV amounts \sim 3 % that of the Crab .
No counterpart for the TeV source at other wavelengths is presently identified , and its extension would disfavour an exclusive pulsar or AGN origin .
If associated with Cygnus OB2 , this dense concentration of young , massive stars provides an environment conducive to multi-TeV particle acceleration and likely subsequent interaction with a nearby gas cloud .
Alternatively , one could envisage \gamma -ray production via a jet-driven termination shock .