History and specifics
On rare occasions, one of the binaries produces a dramatic solar flare in its corona, inducing the other to flare. The flares synchronize to form a brilliant, fiery arch that emits strong radiation bursts. This spectacle can be seen safely from the surface of the planet Santar.
In 2265, Neptor flared, triggering the arch phenomenon for the first time in a century. At that moment, the Federation starship USS Enterprise was in orbit of Santar and beaming James T. Kirk and Spock to the surface. Radiation misdirected the transporter beam to Muton in the adjacent star system. (TOS comic: "Slaves of the Frogmen")