# GTI extension¶

The GTI extension is a binary FITS table that contains the Good Time Intervals (‘GTIs’) for the event list. A general description of GTIs can be found in the OGIP GTI standard.

This HDU contains two mandatory columns named START and STOP. At least one row is containing the start and end time of the observation must be present. The values are in units of seconds with respect to the reference time defined in the header (keywords MJDREFI and MJDREFF). This extension allows for a detailed handling of good time intervals (i.e. excluding periods with cloud cover or lightning during one observation).

High-level Science tools could modify the GTIs according to user parameter. See e.g. gtmktime for an application example from the Fermi Science Tools.

## Mandatory columns¶

• START tform: 1D, unit: s
• Start time of good time interval (given in instrument specific time reference, see below)
• STOP tform: 1D, unit: s
• End time of good time interval (given in instrument specific time reference, see below)

• MJDREFI type: int, unit: days
• MJDREFF type: float, unit: days
• TIMEUNIT type: string
• TIMESYS type: string
• TIMEREF type: string