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 0.2 to 12 keV, with moderate spectral ( $E/\Delta E \sim 20-50$ ) and angular resolution ( $\sim 6''$ FWHM; $\sim 15''$ 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 that correlates the on-board time to the universal time. The accuracy of the absolute and relative timing reconstruction in the EPIC-pn camera is regularly monitored through observations of the Crab Pulsar. Regularly updated results of this monitoring are published in the EPIC Calibration Status Document (XMM-SOC-CAL-TN-0018).

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 micrometeorite 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. 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://www.cosmos.esa.int/web/xmm-newton/mos1-ccd6.
At about 06:51 hrs.UT on 11 December 2012, during XMM-Newton revolution 2382, an event was registered in the focal plane of the EPIC MOS1 instrument. The characteristics of the event were reminiscent of the aforementioned March 2005 event, and therefore tentatively attributed to micro-meteoroid impacts scattering debris into the focal plane. After this event, MOS1-CCD3 has been significantly damaged, and declared not usable for science. Scientific observations are continuing normally with MOS1, but now without data from CCD3 as well. The event has also increased the noise level in CCD4.
For a complete assessment of the scientific impact of this event, readers are referred to the following web page:
http://www.cosmos.esa.int/web/xmm-newton/mos1-ccd3.

**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 diameter area. For the alignment of the different cameras with respect to each other in the XMM-Newton focal plane refer to the text.
$\begin{figure}\begin{center} \leavevmode \epsfig{width=1.0\hsize, file=figs/epic_FOV.ps} \end{center} \end{figure}$

**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 coordinates with North to the top and East to the left. In each case the camera detector coordinate frames are noted.
$\begin{figure}\begin{center} \leavevmode \epsfig{width=1.0\hsize, file=figs/MOS1_UHB_15.ps} \end{center} \end{figure}$

**Figure 18:** *The field of view of the EPIC MOS cameras (cntd. from previous figure): MOS2.*
$\begin{figure}\begin{center} \leavevmode \epsfig{width=1.0\hsize, file=figs/MOS2_UHB_15.ps} \end{center} \end{figure}$

**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 coordinates with North to the top and East to the left. Again the camera detector coordinate frame is noted. The nominal boresight is marked with a small box. Note that the approximate distance between the boresight and the nearby chip gaps is $\sim$ 40 arcsec, which can be of importance when observing extended sources. 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.
$\begin{figure}\begin{center} \leavevmode \epsfig{width=1.0\hsize, file=figs/PN_UHB_15.ps} \end{center} \end{figure}$

All EPIC CCDs operate in photon counting mode with a fixed, mode dependent frame read-out frequency, 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, 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.

Users are encouraged to be familiar with the EPIC Calibration Status Document, XMM-SOC-CAL-TN-0018, that is regularly updated to reflect any changes in the instrument calibrations. Additionally, specific documents deal with the calibration status of fast modes in EPIC-MOS (http://xmmweb.esac.esa.int/docs/documents/CAL-TN-0082.pdf) and EPIC-pn (http://xmmweb.esac.esa.int/docs/documents/CAL-TN-0083.pdf). They are also updated whenever new calibration results are available.

Subsections

European Space Agency - XMM-Newton Science Operations Centre