XMM-Newton Users Handbook

3.3.7 EPIC background

The EPIC background can be divided into two parts: a cosmic X-ray background (CXB), and an instrumental background. The latter component may be further divided into a detector noise component, which becomes important at low energies (i.e. below 300 eV) and a second component which is due to the interaction of particles with the structure surrounding the detectors and the detectors themselves. This component is characterised by a flat spectrum and is particularly important at high energies (i.e. above a few keV).

The particle induced background can be divided into 2 components: an external `flaring' component, characterised by strong and rapid variability, which is often totally absent and a second more stable internal component. The flaring component is currently attributed to soft protons ($E_p$ smaller than a few 100 keV), which are presumably funnelled towards the detectors by the X-ray mirrors. The stable component is due to the interaction of high-energy particles ($E$ larger than some 100 MeV) with the structure surrounding the detectors and possibly the detectors themselves.

A table summarising the temporal, spectral and spatial properties of the EPIC background components mentioned above is available at http://www.cosmos.esa.int/web/xmm-newton/background.

The SOC has performed an analysis of the seasonal and long-term behaviour of the XMM-Newton background from the beginning of the mission (available as XMM-SOC-GEN-TN-0014). Main conclusions are:

• The data clearly show that the distance to the Earth is the dominant parameter (as expected, the background is lower close to apogee). There are quite often differences of about one order of magnitude or even more, between the background at the end of the science window of the revolution and at apogee.
• In addition to this:
  • There is a marked asymmetry in the behaviour of the background when moving away from perigee and when approaching to it (the ascending and descending parts of the orbit).
  • The background reaches higher values at the end of the revolution than at the beginning. The high background time does not extend usually beyond orbital phase $\sim$0.2 (i.e. 9-10 hours after perigee passage) ² . There are however epochs of the year in which the background starts to rise a few hours after apogee.
  • The behaviour of the background presents a pronounced seasonal effect, but it has long-term changes from year to year, due to the XMM-Newton orbit evolution and/or the solar cycle.
  • Around solar maximum, the probability of solar flaring activity increases, whereas the level of quiescent background decreases.