Scale_factor = mean_bkg_rate_obs * exposure_time_obs
mean_bkg_rate_fwc * exposure_time_fwc
where mean_bkg_rate_obs is the mean background rate in the observation
which can be calculated from the discarded line rate by:
mean_bkg_rate_obs = (c0 + c1 * DLMEAN_obs)
where c0 and c1 are coefficients that have been calculated for each observing mode and are stored in the EPN_FWC_xxxx.CCF file. The mean discarded line rate for the observation, DLMEAN_obs, is calculated when the input product is produced by evselect and stored in a header keyword, DLMEAN.
When the QPB product is created by qpbselect it consists of all the selected FWC counts with the output product exposure time adjusted for the scaling factor such that the final count rate e.g. as seen in a spectral fitting program is correctly scaled for the background conditions of the input product. The applied scaling factor, is stored in the EWEIGHT column of the event file and is the same for each event.
The task runs evqpb which takes as input a science EPIC event list. By default it obtains attitude information, needed to convert events from detector to sky coordinates, from header keywords in the input event file. Alternatively, time-dependent attitude information can be taken from a supplied attitude file if the parameter useodfatt is set to true (note that the SAS_ODF environment variable needs to be set to point to the ODF data in this case). evselect is then run, using the event file produced by evqpb, with the same selection expression as was used to produce the original science product, e.g. subsetting on PI channel, pattern and FLAG.
The input product must contain the DLMEAN keyword, which will be automatically written if the product has been created with evselect version 3.69 or later.
