SAS watchout page

The emission line energies in my MOS spectrum seem a bit too low. Any calibration issues?

If your MOS exposure was taken in Timing Mode, this is possible. An error has recently been found in the MOS timing response as generated by SASv6.0. It associates incorrect energies with line features below 1 keV by shifting them to lower energies and applying too much redistribution. Fig.1 compares the SASv6.0 result (red curve) with the SASv5.4 result (black curve) for a 0.8 keV line feature.

Fig.1: Comparison of the SASv6.1 (green curve) with the SASv6.0 (red curve), and SASv5.4 (black curve) results for a 0.8 keV emission line

New canned response matrices (available from the EPIC calibration web pages) correct this error and also anticipate the improved response for MOS which will be incorporated into SAS6.1. This is demonstrated in the green curve in Fig.1, in which the low-energy shoulder has been reduced, while the flat shelf increases (roughly from 0.1 to 1%).

Currently SAS 6.0 leads to obviously very bad fits at low energies, while the MOS-timing canned-response produces fits in excellent agreement with MOS small-window mode down to 100 eV (as shown in Fig.2).

Fig.2: Results of a fit on the radio-loud AGN PKS2155-204 with MOS data in Small Window (red curve) and Timing (black curve) mode