epniose task removes soft X-ray noisy frames from EPIC-pn camera.
The task calculates the number of events per frame between 20 to 30
adu and removes those frames above a certain thershold defined by
noisecut parameter. Once the noisy frames have been removed,
the exposure time is updated accordingly.
To do this filtering, the epnoise
task logic has been divided
in two different steps.
During the first step, epnoise
is run using as input the
output files of epframes
and badpixfind
tasks. Then,
epnoise
identify the noisy frames, creates or updates the
column NEVT_FRM, containing the number of events per frame and CCD
with PHA values below a certain threshold, and write keywords
containing sugestions for subsequent
filtering. Where the keywords are:
Pixels which are affected by bright celestial sources in this energy range are removed through a mask generation. To create this mask for removing bright sources, epnoise calculate the median of the full image and apply a cut using the sigmacut parameter. Then a mask for badpixel is created and added to the previous mask. The savemasks parameter writes to disk the masks of all CCDs.
After this first step of epnoise, the rest of the EPIC-pn processing chain is executed, propagating the new column (NEVT_FRM) and the new keywords (LAMBDA,NORM,NEVT_CUT,NEVT_ALT).
During the second step, epnoise
filter the final event list
using the NEVT_CUT threshold for each CCD. The epnosie
filter the
final event list cretaing the following expression per CCD (NEVT_FRM
= NEVT_CUT). Update the STDGTI extension adding the gaps
corresponding to the frames that have been removed. Then, update the
ONTIME and LIVETIME keywords.