XIS Blank Sky Background Files: Cycle 3 Versions

Note (added on Nov. 27):
The blank sky background files released here are based on the data with a relatively short exposure of 96 ks.    Those who want to simulate spectra for a longer exposure than 100 ks should use the night earth background files and add the CXB and Galactic diffuse background as model componets.

The XIS team has released a set of blank sky background files suitable for the preparation of AO-3 proposals.

The spectral files have been created from the Lockman Hole observations on May 3, 2007. The aim point of the observation was RA = 162.9375, Dec = 57.26667.

The spectra were extracted from a circular region of a radius of 6 mm centered at the XIS/HXD nominal positions. Because the unit of the BACKSCAL keyword changes with the coordinate system, spectra were created for both the DET and SKY coordinates. Two spectra, one for the BI sensor (XIS1) and the other for the sum of FI sensors (XIS0,3), have been extracted for each coordinate system and for each extraction region. Net exposures are 96.1 ks for both of FI and BI sensors.

This release consists of the following 8 files.

  1. XIS nominal position (r=6mm):

  2. HXD nominal position (r=6mm):

These (either the sky or detector coordinate versions) are suitable for simulation purposes. Use with a matching set of RMF and ARF files.

Caution in simulating XIS spectra using the XSPEC "fakeit" command
We would like to draw your attention on the importance of the keyword "BACKSCAL" (background file scaling factor) in both the background fits files and the simulated fits files.    As written in the XSPEC manual, the observed spectrum C(E) is calculated from the data spectrum D(E) and the background spectrum B(E) as follows:
    C(E) = D(E) - (b_D/b_B) B(E),
where b_D is BACKSCAL written in the spectral fits file D(E) and b_B is that written in B(E).  (This is a simplified explanation; see the XSPEC manual for details.)  Therefore, unless correct BACKSCAL values are used, background subtraction could become largely erroneous.   The background files supplied by the XIS team have BACKSCAL values of 0.1873 (detector coordinate) or 0.0832 (sky coordinate).   On the other hand, simulated data files with the "fakeit" command have a BACKSCAL value of 1.0.   Thus, a correct BACKSCAL value, which corresponds to the extraction region you simulate, needs to be set to the data fits file manually.   Note that the BACKSCAL values depend on the coordinate system assumed..   Otherwise, the background subtraction could be wrong by almost an order of magnitude.