mkgisbgd -- Create exposure corrected blank sky spectrum and image from point-source removed blank sky event files


        mkgisbgd filelist specfile gtifile output


mkgisbgd creates 'pure' blank sky background spectrum and image without point sources.

mkgisbgd requires blank sky event files in which point sources are removed, and the removed detector area is indicated in the 'mask' image which should be in the primary of the event file. These files may be taken from the ASCA CALDB, at User may create such source removed event files by themselves for mkgisbgd.

In this background dataset, GIS high latitude observations taken between 1993 June and 1995 December are used. Total exposure time is approximately 2.1 Msec. The 'mask files' are supplied by Yoshitaka Ishisaki (Tokyo Metropolitan University, Sources brighter than ~1e-13 erg/s/cm2 in 2-10 keV are removed. Find Y. Ishisaki's doctoral thesis entitled "Spectra and Large-scale Isotropy of the Cosmic X-ray Background from ASCA Observations" (available at for more details.

The blank sky event lists are 'screened' with the standard criterion used in the ASCA REV2 pipe-line processing. Namely, these event files (having the 'evt' suffix) are taken from the 'screened' directory in the ASCA archives, and subsequently point sources are removed using the 'fmaskfilt' ftool with the mask images. The REV2 stander screening criterion is the same as the 'ascascreen' standard criterion. Therefore, most users who adopt the standard screening criterion will not need further screening of the event files.

We also have made good time interval files corresponding to the ascascreen 'strict' criterion, which are in the same CALDB ftp area (gis_strict.gti). Users who use the 'strict' criterion for their sources should apply this GTI file when running mkgisbgd.

The exposure time of the blank sky observation is calculated taking account of the effect of masking the point sources. The algorithm of exposure time calculation is found in the article "Method of GIS Background Subtraction", Ikebe et al., ASCA Newsletter number 3 (available at

The detector region to use is taken from the WMAP of the input spectral file. The BACKSCAL keyword of the input spectral file is copied to the output spectral file to make sure the correct background normalization. The output background file is ready to be used in XSPEC for the background of the input spectral file.


filelist [file name]
ASCII file for the input blank sky event file (one file for each line). The template is found at\ g2filelist and g3filelist. Note that in these file-lists the ftp address of the blank sky files are shown. mkgisbgd, powered by the new cfitsio feature, can read remote compressed files in the anonymous ftp area. A test run of mkgisbgd at ISAS, Japan, using these remote files at GSFC in Maryland finished within a reasonable time. Users may either access the remote compressed files at GSFC or copy all the blank files to their own sites. Note that compression ratio is so high that the compressed files are one quarter to even one-tenth of the original files in size.

specfile [file name]
Input GIS spectral file. The WMAP, BACKSCAL, DATCHANS of the input file are used by mkgisbgd. The output file has the same BACKSCAL value and the same number of the spectral channels as indicated by DATCHANS. Input WMAP specifies the detector region to use, within which the spectrum and image are accumulated.

gtifile [file name]
Optional GTI file name. If 'NONE' or 'none' all the input events are used. Users may specify the 'gis_strict.gti' found in the anonymous ftp area for the strict data selection criterion.

output [file name]
Output file name. The output file has a primary and three extensions. The primary is the exposure corrected detector image, which may be used for image analysis. The first extension is the spectral file to be used in xspec as the background for the input spectral file. The second and third extensions are non-exposure-corrected detector image and the detector exposure map respectively.

Lower boundary of the PI range (inclusive). The option to use only a narrow energy band may be useful to create band limited background images.

Upper boundary of the PI range (inclusive).

(clobber [yes or no])
Clobber the output file if exists or not.


         mkgisbgd g2file_list 50msec.spec NONE 50msec.bgd clobber=yes

Using the point-source-removed blank sky event files listed in 'g2file_list', make a background file '50msec.bgd' corresponding to the spectral file '50msec.spec'. No GTI file is specified ('NONE'). If the output file '50msec.bgd' exists, that is overwritten. Content of the g2file_list may look like, bfn1_g2_no_source.evt bfn2_g2_no_source.evt bfn3_g2_no_source.evt bfn4_g2_no_source.evt draco1_g2_no_source.evt draco2_g2_no_source.evt draco3_g2_no_source.evt lss101_g2_no_source.evt lss102_g2_no_source.evt lss103_g2_no_source.evt .....


Not reported.



         Ken Ebisawa ( at ASCA GOF.
         Questions and comments to


Sep1999 ftools.asca