Warning about the GIS background files (December 14, 1994)
This document has become obsolete due to release of the new blank sky event lists. This is kept only for archival purpose.
The ASCA GOF has released GIS background files created by summing several GIS blank sky observations taken during the ASCA performance verification phase. These files are available from the legacy.gsfc.nasa.gov anonymous ftp account in the directory /caldb/data/asca/gis/bcf/bgd.
Recently, Y. Ueda and other GIS team members have pointed out the effects of moderately bright point sources in the blank sky fields. They found point sources in these blank fields are not completely averaged out by the summation of many fields.
The DET XY positions of the three most significant point sources are the following :
We recommend that Guest Observers take extra care when using background data near these positions. In particular, the sources (1) and (2) are NGC6552, which has a very strong iron emission line (Fukazawa et al. 1994, PASJ 46, L141; Reynolds et al. MNRAS, 268, L55). Consequently, background spectra made from these regions show an iron emission line, and this could affect background subtraction.
The ASCA GOF is planning to make new background files by removing apparent point sources. These new files will be accompanied by 'a mask file' which will describe the regions excluded and be used to calculate the correct exposure.
In addition, we have noticed that the background files under /caldb/data/asca/gis/bcf/bgd/94may include data taken in March 1994, when an on-board computer error caused the GIS3 PHA spectra to be quantized to 128 ch (5, 13, 21, ,, 1021ch) rather than the original 1024 channel (0-1023ch).
These GIS3 data should not have been included. However, the effect of the bad data on the actual background subtraction is not very significant because (1) the spectrum is 'smoothed' using random numbers when converted to PI from PHA, and (2) the bad data is just 7 % of the total.
The 1994 March bad data will be excluded when we make the new background files.
Ken Ebisawa and ASCA GOF
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