Temporal flare

A temporal flare was a type of Time Lord technology, used as a distress signal across time and space. (AUDIO: One Life) They were similar in concept to flare guns found on Earth, except that they did not require oxygen, and burned cross-dimensionally. Temporal flares were ideal for signalling other timeships. Temporal flare guns had four shots each. (AUDIO: Ship in a Bottle)

In addition to signalling, a temporal flare could also set off an explosion, as they could burn in any environment, or otherwise serve as "mildly disappointing fireworks". The Eighth Doctor used a temporal flare aboard a scape capsule to start an explosion in its engine, hoping that this would provide enough of a shock wave to send him and his companions back through the Time Vortex, with temporal momentum, to safety. (AUDIO: Ship in a Bottle)

During the Last Great Time War, Ollistra used her last temporal flare shot to signal the Time Lords on Jedris. The Doctor warned her that the Daleks would see the flare too. This signalled a nearby Battle TARDIS to pick them up. (AUDIO: One Life)