XIS Non X-ray background Files: Cycle 5 Versions
The XIS team has released a set of non X-ray background (NXB) files suitable for the preparation of AO-5 proposals.
The NXB 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, the spectra were created for both the DET and SKY coordinates.
The XIS team also prepared the NXB spectra for the entire filed of view. In this case, users should note that regions irradiated by the onboard calibration sources were excluded.
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. Note that the spectra for the FI sensor is summed, not averaged.
The exposure times are ~350ks. Proposers may think the exposure is not enough for simluations of long expososure observations. However, in most cases, users usually make use of xisnxbgen in XIS spectral analyses. This NXB generator extracts events from the NXB database of ~300 days long by default, which results in NXB spectra with an exposure of ~350ks. Thus the XIS team concluded that proposers should use the provided NXB spectra, if they will analyze their data in the ordinary way.
A gzipped tar file is available for download, containing the following 12 files.
- XIS nominal position (r=6mm)
- Sky coordinate
- nte_xisBI_xispos_6mm_sky.pi (XIS1)
- nte_xisFI_xispos_6mm_sky.pi (sum of XIS0 and XIS3)
- Detector coordinate
- nte_xisBI_xispos_6mm_det.pi (XIS1)
- nte_xisFI_xispos_6mm_det.pi (sum of XIS0 and XIS3)
- HXD nominal position (r=6mm)
- Sky coordinate
- nte_xisBI_hxdpos_6mm_sky.pi (XIS1)
- nte_xisFI_hxdpos_6mm_sky.pi (sum of XIS0 and XIS3)
- Detector coordinate
- nte_xisBI_hxdpos_6mm_det.pi (XIS1)
- nte_xisFI_hxdpos_6mm_det.pi (sum of XIS0 and XIS3)
- Entire XIS FOV
- Sky coordinate
- nte_xisBI_entireFOV_sky.pi (XIS1)
- nte_xisFI_entireFOV_sky.pi (sum of XIS0 and XIS3)
- Detector coordinate
- nte_xisBI_entireFOV_det.pi (XIS1)
- nte_xisFI_entireFOV_det.pi (sum of XIS0 and XIS3)
These files (either the sky or detector coordinate versions) files are suitable for simulation purpose only. Please use with a matching set of RMF and ARF files.
A second gzipped tar file contains a set of 12 files that have been reduced in size using a variable rebinning scheme. These should be used with the corresponding RSP files.
Important note on 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 be
significantly incorrect. The NXB
files for the 6mm region have BACKSCAL values of 0.187 (detector
coordinate) or 0.0833 (sky coordinate). The NXB files for the entire
field of view have BACKSCAL of 0.901 (detector coordinate) or
0.419 (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.
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