What is the CYCLE keyword?
Should I be concerned if it's missing from my data?
The CYCLE keyword is needed in alternating mode observations (also referred to as interleaved mode) to indicate which frame time was used in the file. There are two possible values: P = primary exposure (long frame time) or S = secondary exposure (short frame time).
ACIS data taken in standard TIMED mode has a CYCLE value of "P", since there is only a single frame time. There is no valid CYCLE value for continuous-clocking mode or for HRC data, so the keyword is omitted from those event files.
The CIAO response tools will issue a warning if the CYCLE keyword is missing from the file header:
*** Warning: CYCLE not found or invalid in /tmp/imap_3.fits --- assuming 'P'
Note that this is a warning not an error. As long as you are not working with interleaved data, you can ignore the warning and continue with your analysis.
If you are working with interleaved mode, you may need to edit the header of the input file to have the correct keyword value for the "secondary" case; contact the CXC Helpdesk for details.