XMM-Newton Users Handbook The RGS Background

The RGS instrumental background has different components, each with its own characteristics and time dependency. The main components are listed below. The background characterisation has been performed with in-flight data by analysing clear sky images (where there is no source emission) and fitting a model.

The average background spectrum has been analysed for quiet-time periods. This has been performed using clear sky images (or “blank” images) and selecting low background periods (after inspection of the light curves). The average first and second order RGS1 and RGS2 background spectra are shown in Fig. 87. The total exposure time is about 170 ks. The spectra have been extracted in a standard way for the energy, and with the full field-of-view in the cross-dispersion direction. The background count rate in the quiet period corresponds to 1-5x10$^{-5}$ counts s$^{-1}$ per RGS resolution element for a point source.

The fraction of the total exposure time which corresponds to quiet background periods is unpredictable, as it is basically linked to the solar activity itself. During the first years of XMM-Newton operations there have been both “active” and “quiet” background epochs, during which about 50% and nearly 100% (respectively) of most exposures where taken with quiet background. A significant fraction of the data collected so far has much higher background than the one shown in Fig. 87. This should be taken into account for exposure time estimates, specially if detection of weak features is the main objective of the observations.

Figure 87: The average quiet background spectra from first (top) and second (bottom) order. RGS1 is shown in black and RGS2 in red. An enhancement of the count rate below 7 Å in each RGS is due to a change in the width of the pulse height filter at that wavelength. There is a bump around 32 Å in the RGS1 spectrum. The origin has not been fully understood but the most likely explanation is a somewhat higher dark current for CCD2 in this RGS. The lower background in the first order spectrum of RGS2 in this range seems to be related to the use of “single node readout” mode in this RGS.

Each of the background components described above has a very different impact on the RGS sensitivity. The first four are either rejected on board or have low intensity. The major constituents are therefore a tail on the CCD response due to readout noise, and the soft proton radiation.

European Space Agency - XMM-Newton Science Operations Centre