SAS watchout

Caution when analysing EPIC spectra extracted from annular source regions

Recent analysis of EPIC point-source spectra extracted from annular regions show discrepancies in spectral shape. This issue is likely due to inaccuracies in the currently implemented encircled energy correction.

The discrepancies become significant for annular regions with inner radii above ~ 15", and increase with larger inner radii.

The spectral changes are seen above ~ 4 keV and are instrument dependent. They are in general characterised by a spectral softening for EPIC-pn, and a hardening for the EPIC-MOS instruments. In terms of flux, the resulting discrepancies may be of the order of 15% in the 5-10 keV band.

It is emphasized that this issue affects point-source analysis only, i.e. where the arfgen task is run with the detmaptype parameter set to psf. Moreover, annular extraction regions for point source analysis are in general only required for bright objects; they allow to exclude data from a potentially piled-up PSF core.

This issue is currently under active investigation.