What is the confidence of the absolute timing of BAT data, and how is the absolute timing on Swift verified?
For an introduction to the time systems and keywords used in Swift FITS files, see our Timing Guide. In order to achieve an accurate absolute time for Swift data, you will first want to apply the UT Correction Factor (UTCF). The value of UTCF at the start time of the observations appears in the FITS files in the keyword UTCFINIT and is accurate to +/- few tenths ms. You can get a more accurate correction than that by using the tool “swifttime” which is part of the Swift software package, or using the Web tool X-time which now also converts Swift times between several time systems. There is a description of the testing of the XRT absolute timing accuracy in SPIE. This of course relies on the absolute accuracy of the spacecraft clock. The requirement was that the clock (Mission Elapsed Time plus UTCF) be kept within 200 microseconds of UTC, and it has been comfortably doing so throughout the mission, sometimes much better. The MOC measures this clock offset routinely, and this data is st
