These three files list the times at which individual photons
were detected by the Proportional Counter Array (PCA) on-board
the Rossi X-ray Timing Explorer (RXTE). 

The first column is the cumulative count of photons during the
observation (ie equivalent to the row number of the file). The second
column is the exact moment at which each photon was detected, in units
of "Mission Elapsed Time" (MET). This is defined to be the number of
seconds since midnight UTC on 1 Jan 1994.