XMM Users' Handbook

   
EUROPEAN PHOTON IMAGING CAMERA (EPIC)

Principal Investigator: Dr. M. J. L. Turner, Leicester University

Two of XMM'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 EPIC cameras offer the possibility to perform extremely sensitive imaging observations over a field of view of 30' and the energy range from 0.1 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, albeit with a very low duty cycle).

  

Figure 15: The field of view of the two types of EPIC cameras; EPIC MOS (left) and EPIC 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 focal plane refer to the text.

The detector layout and the baffled X-ray telescope FOV of both types of EPIC cameras are shown in Fig. 15. In both cases the sensitive area of the detector is about 30' across. Note that Fig. 15 is just a rough sketch. It does not show details, such as:

• 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. The offset of the chip array amounts to about 1.5' both parallel and perpendicular to the readout direction. This ensures that $\geq90\%$ of the energy of an on-axis point source are collected on one pn CCD chip.

• Onboard XMM there will be two EPIC MOS cameras, which will be rotated by $90^\circ$ with respect to each other.

• The size of the gaps between the individual chips is exaggerated.

• The dead spaces between the MOS chips are not gaps, but unuseable 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).

All EPIC CCDs operate in photon counting mode, producing event lists.¹ 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, for off-axis angles in excess of 2.5' the RGS spectrum might slip off the detector array (because the RGS FOV is about 5' across in the cross-dispersion direction).