What is the source of emergency power for a 727?
According this rather detailed page on the 727's engines:


  To restart in flight, an "air start" may be attempted. The start
  switch should be placed in the FLIGHT START position which will arm
  high energy ignition. The starter valve will not open. Ram air
  entering the engine is sufficient for start if the aircraft's speed is
  above 150 knots. When the start lever is moved to IDLE, fuel is
  introduced and the igniters are activated.