RGS sensitivity limits

As a measure of the RGS sensitivity the detection threshold for a few prominent lines in the 0.35-2.5 keV range has been modeled using SciSim. As examples, the following lines were chosen: the O VII line complex (at 0.57 keV), the Ne X line (at 1.022 keV) and the Si XIII line (at 1.86 keV). The RGS line sensitivity plots displayed in Figs. 55-57 provide an estimate of the exposure time necessary to detect a line above a certain underlying continuum at a significance level of 5- $\sigma$ . The calculations were performed for one RGS. It is in all cases assumed that the line emission emanates from a point source.

**Figure 55:** *The RGS sensitivity limits of one RGS for a 5-* $\sigma$ *detection on the O VII emission line complex at 0.57 keV of a point source.*
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**Figure 56:** *The RGS sensitivity limits of one RGS for a 5-* $\sigma$ *detection on the Ne X emission line at 1.022 keV of a point source.*
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**Figure 57:** *The RGS sensitivity limits of one RGS for a 5-* $\sigma$ *detection on the Si XIII 1.86 keV emission line of a point source.*
$\begin{figure} \begin{center} \leavevmode \epsfig{width=0.6\hsize, angle=270, file=figs/rgs_limits_1860.eps} \end{center} \end{figure}$

The calculations are based on the line flux integrated over the HEW of the line profile, assuming Gaussian error propagation. For such an estimate, four contributing components must be considered:

The X-ray background varies over the sky. In order to provide a graphical representation of RGS's sensitivity which is independent of the source coordinates on the sky, the X-ray background was not considered directly. However, at the discussed energies the extragalactic X-ray background can well be reproduced by a power-law spectrum. This allows us to consider the X-ray background simply by adding an additional component to the continuum flux of the source. In general, the ``continuum flux'' in the plots can therefore be assumed to contain in practice a contribution from both the source continuum and the X-ray background at the position of the source.

The particle-induced background, which depends on the design of the instruments, is considered in the calculation of the curves. We assume a particle-induced background of $3.0\times10^{-9}$ counts mm^-2 s^-1 keV^-1. This is a mean number, which accounts for the normal fluctuation in solar activity. For the given lines we assumed a geometrical area for particle hits of 7.2, 10.8 and 12.5 mm².

The plots allow a first estimate of the exposure time in the sense that unreasonable sources can be rejected during a pre-selection of possible targets. They give a basic idea of whether or not it is possible to detect lines. They are not intended to (and cannot) substitute detailed simulations with realistic spectra with SciSim for sources where a detection and/or separation of one or more emission lines is the main scientific goal.

XMM Users' Handbook

RGS sensitivity limits