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XMM­Newton CCF Release Note
XMM­CCF­REL­270
EPIC MOS Fixed O#set Tables
M.J.S. Smith
September 11, 2010
1 CCF Components
Name of CCF VALDATE EVALDATE Blocks CAL XSCS
Changed Version Flag
EMOS1 DARKFRAME 2010­09­02T18:20:17 OFFSET CCD2 NO
0033.CCF OFFSET CCD4
EMOS2 DARKFRAME 2010­09­02T18:20:17 OFFSET CCD4 NO
0031.CCF OFFSET CCD6
OFFSET CCD7
2 Changes
The OFFSET blocks in the DARKFRAME CCFs reflect the uploaded MOS fixed o#set tables.
Analysis of MOS background maps shows a continued decrease in background for several MOS1
and MOS2 CCDs by # 1 ADU per year. Periodically the uploaded MOS o#set tables are revised
so as to bring the o#set in line with the background measured in the background maps. In this
instance, the uploaded FF­mode fixed column o#sets for MOS1 CCDs 2 and 4, and MOS2 CCDs 4,
6 and 7 have been lowered by 1 ADU.
3 Scientific Impact and Estimated Quality
Lowering of the overall o#set level for the CCDs mentioned above will restore the fixed o#set to the
level of the actual CCD o#set. Too high o#sets would result in the loss of events (or partial event
charge) below threshold, whereas too low o#sets would increase noise above threshold, potentially
creating false events or adding charge to real events.
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Note that the values in the OFFSET extensions of the DARKFRAME CCFs are not used to
determine the E1 event energies; this is already done on board in the EDU. The main reason
for reflecting the on board o#set values in the DARKFRAME CCFs is correctly to determine the
reconstructed event energies, for which knowledge of the contemporary on board o#sets is required.
Reconstructed event energies of 4095 ADU (so­called truncated events) can be useful in e.g. flare
screening.
4 Expected Updates
The background of all CCDs changes in time and will need to be compensated through changes of
the fixed o#sets. These will have to be reflected in the OFFSET extensions of the DARKFRAME
CCFs.
5 Test Procedures and Results
Correct functionality tested with cifbuild and emproc (SAS version 10.0.0). Reducing data with
mismatched uploaded fixed o#sets and DARKFRAME CCF issue may result in SAS warnings
** emevents: (spGatti11), reconstructed energy larger than 4095.
As expected, use of the CCFs in this release result in correct reconstructed energies and no such
warnings are issued.