XMM-Newton Users Handbook

4.3.2 Other considerations

Some more parameters that might influence certain XMM-Newton 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 on board XMM-Newton. 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 minimal 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 the RGSs. The two RGS units have an offset with respect to each other along the dispersion direction to ensure uninterrupted energy coverage over the passband. Due to operational problems with two CCDs there are two additional gaps, one on each RGS unit. The wavelength ranges affected are from 10.6 to 13.8 Å and from 20.0 to 24.1 Å in RGS-1 and RGS-2, respectively.

• ``Out of time'' events

  The XMM-Newton X-ray detectors do not have shutters and are therefore exposed to incoming radiation from the sky at all times . In order to prevent photon pile-up (see § 3.3.9), the CCDs are read out frequently. During readout, photons can still be received. However, they hit pixels while their charges are being transferred 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''; see § 3.3.10). One cannot correct for this effect in individual cases, but only account for it statistically.

  The MOS CCDs have frame store areas, which help 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 percentage of ``out of time'' events is 6.3% for the full frame mode, 2.3% for the extended full frame mode, 0.16% for the large window mode and 1.1% for the small window mode. The large window mode has a smaller fraction of ``out of time'' events because half the image height is used as a storage area, but the reduced smear is penalised by a loss in live time.

• EPIC pn caveat for bright low energy sources

  The lower event threshold of the EPIC pn camera constrains its spectral capabilities for sources which show a significant flux below the threshold. These are mostly nearby white dwarfs which have absorption column densities below $10^{20}$ cm$^{-2}$ and temperatures below $\sim 40$ eV.

  Observations of the white dwarf GD 153 ($kT=25$ eV) with various EPIC pn read-out modes and filters yield large inconsistencies in the spectra below 0.5 keV. The spectrum of GD153 has its maximum at 75 eV, which is about 115 eV , i.e. the bulk of the photons do not directly produce events above the threshold. There is a strong correlation between count rates and read-out mode and filters. Slower read-out results in higher countrate and harder spectrum. The medium filter reduces the count rate more than expected from the thin/medium ratio.

  The most likely explanation for this effect is pile-up. Three kinds of pile-up at low energies are possible: two source X-ray photons, a source photon with electronic noise and a source X-ray photon with optical photons from the source. Pile-up can bring the energy of sub-threshold events above threshold. Source photons with energies below the threshold (which would nominally not be detected) have a high probability to "gain energy" by fortuitously adding to noise. For weak sources (and/or fast readout) this is most likely the dominant pile-up effect. No calibration is available to correct for this soft pile-up effect.

• EPIC pn timing mode feature

  The EPIC pn camera operated in its timing mode shows at RAWY=19 a bright line in RAWX direction. The feature only shows up for bright point sources. Its origin is related to an on-board clock sequence feature whose effect was only noticed after launch.

  There is no effect on the scientific quality of the data as long as the integration time for spectra and light curves is longer than 5.9ms. Care should be taken for pulse phase spectroscopy with bin sizes below 5.9ms, but only if the pulse period itself is a multiple of the frame time (5.9ms).

• RGA rib scattering

  Light scattered off the stiffening ribs of the grating plates of the RGAs produces diffuse ghost images in the EPIC MOS FOV in the $\pm$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 $\pm$Y direction, the intensity of the ghost images increases to a few times 10$^{-4}$.

  Rib reflection is implemented in SciSim, but currently (v.4.0.4) 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.

• OM exposures

  When using FAST mode the target coordinates must be accurate at better than about 2 arcseconds to fit and track the target in the approximately 10''$\times$10'' wide windows.

  FAST mode OM exposures require a successful Field Acquisition (FAQ). FAQ could fail in crowded fields, where many extended objects are present. This limits the possibility of performing - e.g. - high resolution OM timing of active galactic nuclei in clusters.

  Ring-like loops due to scattering of out-of-field bright stars (see Fig. 93) can heavily affect the detection of faint or extended sources at the boresight. This effect is mainly due to bright stars that happen to fall in a narrow annulus 12'.1 to 13' off-axis.

  OM Grisms produce spectra of all objects in the field of view, therefore the spectrum of the target of interest can be contaminated by the zero and first orders from other objects in the field. This problem can usually be avoided by selecting an adequate position angle for the observation.

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