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Problem on GTI of the burst option

(PDF version)
Dec 10, 2008, the XIS team

The burst option introduces a dead time in each exposure. However, XIS software does not take into account the dead time. Consequently, counting rates are underestimated. e.g. In the case of a 2s burst option (with the full window option), the counting rate is under estimated by a factor of 4.

Solution

The XIS team will prepare a tool to generate a GTI file for the burst option soon. Until then, if an observation is conducted with a full-window burst option, one can try the tentative solution in the following page. The tentative solution cannot be applied directly to a burst observation with a window option.

Tentative solution for the full-window burst observation by using ftools

Take a 2s burst option as an example.
Extract frame information from the FRAME extension of the uf event files.
fextract "(uf event file name)[FRAMES][col FRAME_END_AETIME]" tmp1.fis
For this observation, each exposure began at "FRAME_END_AETIME-2s" and ended at "FRAM_END_AETIME". Make an GTI file for this observation from tmp1.fits.
fcopy "tmp1.fits[FRAMES][col START=FRAME_END_AETIME-2; STOP=FRAME_END_AETIME]" tmp2.fits
Change the extension name of tmp2.fits from "FRAMES" to "GTI".
fparkey GTI "tmp2.fits[FRAMES]" EXTNAME
Extract a light curve with XSELECT by using tmp2.fits as a GTI file.
Use the following command in XSELECT before extracting a light curve:
XSEL> filter time file tmp2.fits
Extract a light curve with a command 
XSEL> bin curve exposure=0.0


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