Weight = mean_bkg_rate_obs * exposure_time_obs mean_bkg_rate_fwc * exposure_time_fwcwhere 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.
The weight, which is the same for each event, is stored in the EWEIGHT column of the event file. When the QPB product is created by qpbselect events are scaled by the weight. As this leads to non-integer values of counts, output spectra and images have units of counts per second, after dividing by the exposure time.
The task runs evqpb which takes as input a science EPIC event list and an attitude file from the ODF (note that the SAS_ODF environment variable needs to be set to point to the ODF data). evselect is them 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.
XMM-Newton SOC -- 2021-01-26