XMM-Newton Users' Handbook

3.3 EUROPEAN PHOTON IMAGING CAMERA (EPIC)

Two of XMM-Newton's X-ray telescopes are equipped with EPIC MOS (Metal Oxide Semi-conductor) CCD arrays, the third carries a different CCD camera called EPIC pn. In a nutshell, the XMM-Newton EPIC cameras offer the possibility to perform extremely sensitive imaging observations over a field of view of $30'$ and the energy range from 0.15 to 15 keV, with moderate spectral ( $E/\Delta E \sim 20-50$) and angular resolution ($6''$ FWHM; $15''$ HEW). The pn type camera can be operated with very high time resolution down to 0.03 ms in the timing mode and 0.007 ms (but with a very low duty cycle of 3%) in the burst mode. Note however that the absolute timing accuracy is determined by the process which correlates the on-board time to the universal time.

The detector layout and the baffled X-ray telescope FOV of both types of EPIC camera are shown in Figs. 16 (which is just a rough sketch), 17, 18 and 19. For all cameras the sensitive area of the detector is about $30'$ across. The following details should be noted:

• The pn chip array is slightly offset with respect to the optical axis of its X-ray telescope so that the nominal, on-axis observing position does not fall on the central chip boundary (see approximate boresight position for pn marked in Fig. 19). This ensures that $\geq90\%$ of the energy of an on-axis point source are collected on one pn CCD chip.

• The two EPIC MOS cameras are rotated by $90^\circ$ with respect to each other.

• The dead spaces between the MOS chips are not gaps, but unusable areas due to detector edges (the MOS chips physically overlap each other, the central one being located slightly behind the ones in the outer ring).

• At about 01:30 hrs. UT on 09 March, 2005, during XMM-Newton revolution 961, an event was registered in the focal plane of the EPIC MOS1 instrument. The characteristics of the event might be attributed to a micrometeoroid impact scattering debris into the focal plane. In the period immediately following a light flash it became apparent that MOS1 CCD6 was no longer recording events, and that all CCD6 pixels were, in effect, returning signal at the saturation level raising the possibility that CCD6 had sustained significant damage. At the time of writing, scientific observations are continuing normally with XMM-Newton, including MOS1, but with the peripheral CCD6 (see Fig. 20) switched off.

  Observers are invited to read a dedicated web page, available at
  http://xmm.esac.esa.int/external/xmm_news/items/MOS1-CCD6/.

Figure 16: A rough sketch of the field of view of the two types of EPIC camera; MOS (left) and pn (right). The shaded circle depicts a $30'$ diameter area. For the alignment of the different cameras with respect to each other in the XMM-Newton focal plane refer to the text.

Figure 17: The field of view of the EPIC MOS cameras for an observation with a position angle of $\sim$ 80$^{\circ }$: MOS1 (here) and MOS2 (next figure). The two MOS cameras view the same field as displayed in sky co-ordinates with North to the top and East to the left. In each case the camera detector co-ordinate frames are noted.

Figure 18: The field of view of the EPIC MOS cameras (cntd. from previous figure): MOS2.

Figure 19: The field of view of the EPIC pn camera for an observation with a position angle of $\sim$ 80$^{\circ }$. The pn camera views the same field as displayed in Figs. 17 and 18 in sky co-ordinates with North to the top and East to the left. Again the camera detector co-ordinate frame is noted. The nominal boresight is marked with a small box. Position 'X' shows the preferred location to centre on an object in the pn small window mode, however, the user is advised that this requires a knowledge of the position angle of the observation, and will also place the target outside the EPIC MOS small window.

All EPIC CCDs operate in photon counting mode with a fixed, mode dependent frame read-out frequency, producing event lists.² This allows for simultaneous imaging and non-dispersive spectroscopy due to the intrinsic energy resolution of the pixels.

Note: If for any reason a user should decide to observe a target with EPIC not on-axis, but instead off-axis, then for off-axis angles in excess of $2.5'$ the grating spectrum might slip off the RGS detector array (because the RGS FOV is about $5'$ across in the cross-dispersion direction). Observing off-axis can be complicated if the observer wants to avoid all the MOS and pn inter-chip gaps - very careful position angle and off axis angle compromises should be made.