The XIS team has released an updated set of files containing the night earth data.
A gzipped tar file is available for download, containing 12 files.
For each unit of XIS, there are three files.
Intervals when the telescopes were pointed at the night earth (elevation less than minus 5 degrees, and pointed at night side rather than day) were collected from observations between 2005 Sep 1 and 2006 May 30. The event files have been carefully screened for telemetry saturation and other artefacts. Resultant exposure times are roughly 800 ksec each.
For convenience, the XIS team has prepared spectral files extracted for 7' radius circular region at the center of each XIS. The difference between the two versions is in the BACKSCAL keyword - xspec will correctly scale these files for different extraction regions, provided that the same coordinate systems were used in extracting the source and the background spectra.
For the FI sensors (XIS0, XIS2, and XIS3), the count rates below 0.4 keV in these files are not reliable.
For simulation purposes, we recommend the use of the spectral files that match the response files. However, for certain observations, a more detailed examination of the non X-ray background may be useful, for which these night Earth data can be used. Please note that the night earth spectra do not use the same extraction region as the ARF files released for simulation purposes.