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New ASCA GIS Background files in the OGIP CALDB

          New ASCA Calibration Files in the CALDB

A new set of ASCA GIS background file have recently been installed in
the OGIP caldb at NASA/GSFC:

The datasets are a superposition of 15 blank sky fields observed during 
the PV phase. There is a README file in the above directory (also appended 
below) which gives further details on the individual files & their use.

These files have been given a "good" quality flag by the ASCA GOF, and
therefore supersede earlier versions in the directory:

Ian M George

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README FOR GIS BACKGROUND FILES (version 2) 05/18/94 written by K.E.

The files in this directory, listed below, contain GIS background. Each 
file is a superposition of 15 blank sky fields observed during the 
ASCA PV phase (May 93 - Mar 94).  The files having *all* in their name
(each for the two detectors) includes all the events, regardless of the
magnetic cut-off rigidity (COR) values.  Since the GIS internal 
background increases as COR decreases, other files are sorted by COR 
values (in units of GeV/c). *v2* stands for version 2 of the BGD files.

blanksky_g2_10cor12_v2.evt      blanksky_g3_10cor12_v2.evt
blanksky_g2_12cor14_v2.evt      blanksky_g3_12cor14_v2.evt
blanksky_g2_14cor_v2.evt        blanksky_g3_14cor_v2.evt
blanksky_g2_4cor6_v2.evt        blanksky_g3_4cor6_v2.evt
blanksky_g2_6cor8_v2.evt        blanksky_g3_6cor8_v2.evt
blanksky_g2_8cor10_v2.evt       blanksky_g3_8cor10_v2.evt
blanksky_g2_all_v2.evt          blanksky_g3_all_v2.evt
blanksky_g2_cor4_v2.evt         blanksky_g3_cor4_v2.evt

Like  the AO data, these background data were taken after the Spread 
Discriminator was turned on (May 27, 93).

The difference from old blank sky files, which are under the directory
../94apr,  is the following;

(1) Addition of the two more blank sky fields, then total exposure
    time increased from 300 ksec to 350 ksec.
(2) RTI (Rise Time Invariance) column is correctly filled, thus
    particle background is efficiently removed with the 'gisclean'
    command in XSELECT.

Excluding the calibration source and background events very close to the
detector edge, a total of about 72000  events/sensor have been collected. 
The distribution of exposure with respect to cut-off rigidity is as 

14 < COR	22 ks		6 %
12 < COR < 14   126 ks		36 %
10 < COR < 12 	123 ks		35 %
8 < COR < 10 	37 ks		11 %
6 < COR < 8 	17 ks		5 %
4 < COR < 6 	17 ks		5 %
COR < 4		4 ks		1 %

In order to prevent the extraction of astrophysical information, all 
the X & Y values (sky pixel) are set to 1, and the TIME values are 
made constant. Furthermore, events are sorted in ascending order of 

To use these files in XSELECT (for energy spectral analysis), 
Guest Observers should choose the file 
which best matches the COR range in their source events file and 
read it in using the "read events" command.  If COR selection is not done
in the data selection, which is appropriate for most moderately bright
sources (see the NOTE below), users should use blanksky_g2/3_all_v2.evt.  
Next, extract a background spectrum. The best (and easiest!) way to 
do this is to use the same region filter file as was used for the 
source spectrum, making sure that the spatial binning and coordinate 
system (DETX, DETY) are the same.  If you did 'gisclean' for your 
data, you should do gisclean for the BGD data too.
In XSPEC, read in the source spectrum with the 
"data" command, and the background spectrum with the "backgrnd" 

The ASCA GOF is planning to supply event lists of night Earth data. 
Such data may be used for the study of X-ray background.

If you have questions, please write to Ken Ebisawa at: 

ebisawa@bluejay.gsfc.nasa.gov (English)
ebisawa@astro.isas.ac.jp	(Nihongo)

1) 'gisclean' is effective to remove particle background near the 
   edge of the detector.  To see the effect of the gisclean,
   you may compare images created with and without gisclean.
   You will notice the 'bgd ring' will become less conspicuous
   after gisclean.

2) Here are count rates (including cosmic X-ray BGD and particle BGD)
   of BGD files in this directry (after gisclean). If the source count 
   rates are much larger than the BGD count rates, BGD subtraciton will 
   not very important. If the difference of the counts in different 
   COR sorted files is much smaller than the source count rate, 
   COR selection will not be important. 

       	     whole area Center  Outer
All COR         1.61	1.65	1.57
     COR < 4	1.94	1.91	1.98
4 <= COR < 6	1.78	1.70	1.86
6 <= COR < 8	1.71	1.72	1.70
8 <= COR <10	1.71	1.74	1.67
10<= COR <12	1.60	1.65	1.54
12<= COR <14	1.58	1.63	1.52
14<= COR 	1.46	1.53	1.37

Unit = 10^-2 cts/sec/cm2
Center = r < 15 arcmin
Outer  = 15 < r < 20 (excluding the calibration source)
GIS 1 pixel = 0.25 mm = 0.24558 arcmin

Because of the vignetting of the telescope, X-ray BGD is higher in the
central area.  On the other hand, particle BGD is larger close to the 
edge, in the outer area.  For high COR, the particle BGD is relatively
small and the total BGD is larger in the center.  For low COR, particle
BGD is stronger, then the total BGD counts are larger in the outer area.
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