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.
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.