It is a function of the PSPC live time,
PLIVE = SQRT ( 1.0 - 2.0E-06 * A1LL * DEADTP )(where A1LL is the event rate seen by the instrument and DEADTP is the dead time parameter in microseconds. This is in reality a function of photon energy but for now the program uses a value of DEADTP=250 microseconds for all events.) and the telemetry livetime, TLIVE = AEXE / AXE (where AEXE is the accepted and evaluated event rate and AXE is the accepted event rate) Making the dead time correction factor DEADFACT = 1.0 / (PLIVE * TLIVE)