It had a unique anomaly: a superstring twisted into a Moebius configuration, rotating and spinning in place above the south pole, at a distance of 100 million miles. Major sunspot activity on Rigel B caused the anomaly to "flare up", opening a temporal anomaly at its center, likely a rift between two points in space-time. Fortunately, it was far enough away from the heavily used space-routes around the Rigel system for safety.
Federation theories maintained the anomaly was artificial, created by some ancient and unknown race, labeled the Architects. (Last Unicorn RPG module: All Our Yesterdays: The Time Travel Sourcebook, Decipher RPG module: Worlds)
- The relationship to Rigel B's sunspots implies that Rigel XIII orbits that star rather than the primary of the system, Rigel A.