Spectra of many Sources

A common requirement in ROSAT analysis is to improve the signal to noise ratio by combining spectra from many similar sources in the same field, e.g. AGN in a deep exposure field. Strictly speaking this is incorrect because a separate detector matrix is required for each source, depending on the off-axis angle and the source extraction radius. However, if this point is born in mind and the analysis is restricted to sources within the central ring it is possible to use the technique.

Each source required should be extracted using a circular box with a fixed radius. If all the sources are faint then xrtmspec may be used to extract each spectrum. This procedure uses an extraction radius of 0.01 degrees which is too small for bright sources (>100 counts). If one or more bright sources are present, then xrtmspec should be modified to use a larger radius or each source should be extracted separately with xrtsort. Each source must be extracted with the same radius.

The spectra may be added together using:

        > meandat average=no

This program adds the counts from each input spectrum. Subsequent programs will use the off-axis angle from the first file used in meandat to calculate the energy dependent corrections for the combined spectrum, so it is important that the first datafile is at an average off-axis angle for all the sources. The resultant datafile may be subtracted and corrected as described earlier.