XMM-Newton Science Analysis System: User Guide

5.10 Treatment of extended RGS sources

Figure 41: The bright emission lines of the SNR N132D, which is about 2 arcmin in diameter, fit comfortably within the RGS aperture but overflow the default selection regions defined for point sources. By extending the cross-dispersion selection region to 98%, in this case, most of the source flux is recovered.

Even though the methods currently used by rgsproc were designed for point sources, the 5 arcmin cross-dispersion width of the RGS apertures also makes the instrument suitable for observing extended sources such as supernova remnants or clusters of galaxies. Obviously, such objects can fill part or all of the aperture and invalidate the usual means of differentiating source and background selection regions in terms of the parameters xpsfincl & xpsfexcl that are defined in terms or point-source fractions. For moderately extended sources which do not fill the aperture, such as the LMC SNR N132D shown in Fig. 41, it is possible to adjust the equivalent point-source fractions by a process of trial and error to nominate suitable selection regions. For more extensive sources, when this is not possible, there are RGS background template files and techniques available through the XMM web pages that allow independent estimates of background spectra. Source extension in the dispersion direction causes confusion with the wavelength scale. Since version v11.2, Xspec [26] contains a new rgsxsrc model which specifically deals with spectra of extended sources.