In alternating exposure mode (sometimes also referred to as interleaved mode), there is a long and a short frame time, and the observation alternates between them. This is sometimes done when observing a source the may be piled. The observer gets the long-frame "piled" information, as well as the short-frame unpiled data.

All the long frames are joined into one event file and the short frames are joined into another event file. The resulting event files have "e1" and "e2" in the filename to indicate the two exposure lengths. Check the EXPOSURE header keywords to see which the exposure time for each of the two modes is:

unix% dmkeypar acisf01612_000N003_e1_evt1.fits EXPTIME echo+
0.1

unix% dmkeypar acisf01612_000N003_e2_evt1.fits EXPTIME echo+
0.4

Do not merge the e1 and e2 files. They should be analyzed separately.