XMM-Newton Science Analysis System


epnoise (epnoise-0.9) [xmmsas_20230412_1735-21.0.0]

General

The task epnoise removes soft X-ray noisy frames from EPIC-pn event lists.

The task calculates the number of events per frame between 20 to 30 adu and removes those frames above a certain threshold defined by the noisecut parameter. Once the noisy frames have been removed, the exposure time is updated accordingly.

To perform this filtering, the epnoise task logic has been divided into two different steps.

During the first step, epnoise is run using as input the output files of epframes and badpixfind tasks. Then, epnoise identifies 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 writes keywords containing suggestions for subsequent filtering. These keywords are:

• LAMBDA: lambda of Poissonian fit
• NORM: normalization of Poissonian fit
• NEVT_CUT: suggested cut value (“10%”)
• NEVT_ALT: alternative cut value (1.0)

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 calculates the median of the full image and applies a cut using the sigmacut parameter. Then a mask for the bad pixels is created and added to the previous mask. The savemasks parameter writes the masks of all CCDs to disk.

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 ({knLAMBDA, NORM, NEVT_CUT, NEVT_ALT).

During the second step, epnoise filters the final (merged) event list using the NEVT_CUT threshold for each CCD, with the following expression per CCD: NEVT_FRM $\geq$ NEVT_CUT. It then updates the STDGTI extension adding the gaps corresponding to the frames that have been removed. Finally, it updates the ONTIME and LIVETIME keywords.