This is the only known picture of the most mysterious and deadly unidentified flying object. The yellow arrow points to the only hostile alien craft on record in the Air Force's databases. The assumption is based on the degree of sophistication of the 'Group 3' type, and little else.
Nemesis actively transmits a unique signature which over time readily identifed the craft.
Of course, this always meant disaster to the aircraft capable of recognizing the signal. To hear it's identifier was literally followed by death. Once engaged, Nemesis' victim was totally destroyed, which has made any attempt at gathering detailed information virtually impossible. This image was taken from the cockpit of a B-52 while on exercise. Perhaps it was investigating the nature of the bomber, only to discover the B-52 in 'standown' state.