Dead Time Correction

Dead time is negligible for the MECS in its whole dinamic range (up to a few Crab).

Dead time is instead not negligible in the LECS for sources brighter than a few tenth of Crab (F>10-9 erg cm² s¹). For example, the needed deadtime correction for Crab data is ~ 7.5 %.

The lecspipe calculates automatically the deadtime correction, and writes it as the value of the spectral file header keyword DEADC. The spectrum extracted through the lecspipe or the scs has therefore the correct exposure time keywords.
Unfortunately, XSELECT does not update the EXPOSURE keyword to take the dead time effect into account, and therefore LECS spectra extracted with XSELECT outside the lecspipe have a wrong exposure time, if they are directly read in XSPEC.

For sources fainter than a few tents of crab this is not really a problem. For brighter sources the observer should update the EXPOSURE keyword using the fparkey FTOOLS, like in the following example:

fparkey value=truetime fitsfile=lecs.pha keyword=EXPOSURE
where truetime=exposure*deadc

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