XMM Users' Handbook

   
Points of concern

Some more parameters that might influence certain XMM observations, and should therefore be taken into account, are:

• Seam losses between abutted CCDs

  Although small, there are gaps between the chips of the different X-ray detectors onboard XMM. The two EPIC MOS cameras are mounted orthogonal with respect to each other, so that in final images, after adding up the data from two X-ray telescopes, the gaps should not be visible after correction for exposure. There will only be a reduced total integration time in areas imaged at the location of chip boundaries. The pn camera has a different chip pattern, leading to mimimal losses in other areas of the field of view. It is also offset with respect to the X-ray telescope's optical axis so that the central chip boundary does not coincide with the on-axis position. The inter-CCD gaps of the EPIC MOS chip array are 400 $\mu$m (11'') wide. Those between neighbouring CCDs within one quadrant of the pn chip array are 40 $\mu$m (1.1'') wide, the gaps between quadrants about 150 $\mu$m (4.1'').

  The nine CCDs in each RGS also have gaps of about 0.5 mm in between them. Table 9 lists the energies affected by the gaps in RGS-1. However, the two RGS units will be offset with respect to each other along the dispersion direction to ensure uninterrupted energy coverage over the passband.

  By choosing the instrument that is most important for the planned research as the primary instrument, XMM users can ensure that the programme source is placed properly, away from chip boundaries.

• Charge transfer efficiency (CTE)

  While being transported from their original location to the readout nodes, the charges of CCD pixels are transfered from pixel to pixel up to several hundred times. During each transfer, a small fraction of the charge (which depends mostly on impurities in the semi-conducting layer) can be lost. This effect can be quantified roughly and corrected for in the offline data calibration process.

• ``Out of time'' events

  The XMM X-ray detectors do not have shutters and are therefore at all times exposed to incoming radiation from the sky. Trying to prevent photon pile-up (see above), the CCDs are read out frequently. During readout, photons can still be received. However, they hit pixels while their charges are being transfered to the readout nodes, i.e., when they are not imaging the location on the sky they would normally observe during the exposure. Thus, events hitting them during readout are ``out of time'' (and also ``out of place''). One cannot correct for this effect in individual cases, but only account for it statistically.

  The MOS CCDs have frame store areas, which helps suppress the effect of out-of-time events. The frame shift times of a few ms are much shorter than the maximum frame integration time of 2.8 s. Therefore, the surface brightness background of smeared photons is only a fraction of a percent divided by the ratio of the PSF size to the CCD column height.

  For pn the full window mode has a readout/shift ratio of 1/11, so the surface brightness of the background is 9% divided by the ratio of the size of the PSF to the CCD column height. This is noticeable for any sources brighter than about 10^-13 erg s^-1 cm^-2. The large partial window mode counteracts this by using half the image height as a storage area, but the reduced smear is penalised by a loss in live time. Another strategy is to use a programmable ``wait time'' to extend the full frame imaging, but at the cost of count rate capability before pile-up.

• RGA rib scattering

  Light scattered off the stiffening ribs of the grating plates of the RGAs will produce diffuse ghost images in the EPIC FOV in the $\rm\pm Y$ direction (i.e., the RGS cross-dispersion direction). The intensity of these images is of the order of 10^-4 relative to the intensity of the focused image. For off-axis sources at azimuth angles corresponding to the $\rm\pm Y$ direction, the intensity of the ghost images increases to a few times 10^-4.

  Rib reflection is currently implemented in SciSim (v. 2.0.1), but the reflection coefficient is too high, and the diffuse nature of the scattered light ghosts is not modeled. Simulations with SciSim will therefore overestimate the surface brightnesses of RGA scattered light due to this effect.

Except for event selection, which is performed onboard for EPIC MOS, the above effects are dealt with in the offline data analysis with the SAS.