3.3.7.2 EPIC internal `quiescent' background

This quiescent component is associated with high energy particles interacting with the structure surrounding the detectors and the detectors themselves. The intensity of the quiescent component is monitored regularly for both MOS and PN cameras during CLOSED filter observations. The component shows only small intensity variations in time which are typically observed on long time-scales. The intensity of this component during any given observation is within $\sim$ 10% of the mean. The intensity observed in one MOS camera is usually well correlated with the intensity measured in the other, although some exceptions have been observed. In Figs. 34 and 35 we show the spectrum of the quiescent component for the MOS1 and PN camera, respectively.

**Figure 34:** Background spectrum for the MOS1 camera during an observation with the filter wheel in the closed position. The prominent features around 1.5 and 1.7 keV are respectively Al-K and Si-K fluorescence lines. The rise of the spectrum below 0.5 keV is due to the detector noise.
$\begin{figure}\begin{center} \leavevmode \epsfig{width=0.78\hsize, file=figs/sm4.ps} \end{center} \end{figure}$

**Figure 35:** Background spectrum for the pn camera during an observation with the filter wheel in the CLOSED position (top: single events, bottom: double events) in the energy range 0.2-18 keV. The prominent features around 1.5 keV are Al-K $\alpha$ , at 5.5 keV Cr-K $\alpha$ , at 8 keV Ni-K $\alpha$ , Cu-K $\alpha$ , Zn-K $\alpha$ and at 17.5 keV (only in doubles) Mo-K $\alpha$ fluorescence lines. The rise of the spectrum below 0.3 keV is due to the detector noise. The relative line strengths depend on the (variable) incident particle spectrum.
$\begin{figure}\begin{center} \leavevmode \epsfig{width=0.85\hsize, file=figs/bkgspec_pn.ps} \end{center} \end{figure}$

The spectra are quite flat and present a number of spectral features due to fluorescence from the detectors and the structure surrounding them. Al-K $\alpha$ and Si-K $\alpha$ lines are clearly visible in the MOS spectrum. In case of the PN Al-K $\alpha$ and an intense complex due to Cu-K $\alpha$ , Ni-K $\alpha$ and Z-K $\alpha$ lines around 8 keV is visible. An important point is that the intensity of this complex is not constant over the PN detector. More specifically there exists a central circular region (see Fig. 36) where the complex is virtually absent.

**Figure 36:** Background images for the pn camera with spatially inhomogeneous fluorescent lines: smoothed image in the Ti+V+Cr-K $\alpha$ lines (top left), full resolution image in Copper (7.8 - 8.2 keV) (top right), Nickel (7.3 - 7.6 keV) (bottom left) and Molybdenum (17.1 - 17.7 keV) (bottom right). The absolute normalisation of the images can be inferred from the spectra in Fig. 35. The inhomogeneity is caused by the electronics board mounted below the CCDs; in case of the energy range 4.4 - 5.7 keV probably due to a supporting screw - however at a very low level.
$\begin{figure}\begin{center} \leavevmode \epsfig{width=1.\hsize, file=figs/bkgmap_pn.ps} \end{center} \end{figure}$

Average count rates due to the internal "quiescent" background, for both MOS and PN, in different modes and filters, as well as energy ranges and pattern selection, can be found at:
http://xmm2.esac.esa.int/external/xmm_sw_cal/background/bs_countrate.shtml.
These count rates have been worked out using out-of-field events from Blank Sky event files.

XMM-Newton Users Handbook

3.3.7.2 EPIC internal `quiescent' background