Correcting contaminated SUD data

Occasionally, bright X-ray sources contribute a significant number of counts above 24 keV, so leading to an artificially high SUD rate (which is crucial to the determination of the background at the time of observation). The SUD rate minus the contribution from the source can be estimated from off-source data taken before and after the source observation (or at the same phase in the 37-day orbital period of the Ginga satellite). This is possible because the satellite position in orbit, and thus the intrinsic background, depends on two parameters (excluding the phase of the 37-day cycle): the angle from ascending node and the longitude of the ascending node (Hayashida 1996). The program bgdssud maps the SUD rate, from the off-source data, against both parameters (dividing the data into 20° bins) and, using the satellite position during the source observation, the program bgdisud calculates the expected SUD rate and inserts this into the source observation data cube. Background subtraction can be performed as normal. However, the following test is necessary, as this method of correcting SUD rates is somewhat incestuous: the background data should be subtracted using both the original and modified SUD rates, and the r.m.s. deviations should be compared using the program bgdchk (see section 3.1). Examples of bgdssud and bgdisud are given below:

qcl>bgdssud

** BGDSSUD Version 1.2 Tue Aug 19 18:29:57 1997 **
 
BGD_NFILE Number of input data files = 0 =5
BGD_DATIN001 Input file names =bgd_89b_01.cube_cl
BGD_DATIN002 Input file names =bgd_89b_02.cube_cl
BGD_DATIN003 Input file names =bgd_89b_03.cube_cl
BGD_DATIN004 Input file names =bgd_89b_04.cube_cl
BGD_DATIN005 Input file names =bgd_89b_05.cube_cl
Warning: object IMID already exists
Warning object IMID is being overwritten
Warning: object ICOR already exists
Warning object ICOR is being overwritten
Warning: object IPERI already exists
Warning object IPERI is being overwritten
Warning: object ISUD already exists
Warning object ISUD is being overwritten
Warning: object ILONG already exists
Warning object ILONG is being overwritten
Warning: object ILAT already exists
Warning object ILAT is being overwritten
Warning: object IALONG already exists
Warning object IALONG is being overwritten
Warning: object IANGLE already exists
Warning object IANGLE is being overwritten
qcl>

The output file, produced by bgdssud, has the same root name as the first input file (i.e. bgd_89b_01 in this example), but takes the file extension ".dat".

qcl>bgdisud

** BGDISUD Version 1.0 Tue Aug 19 18:34:05 1997 **
 
BGD_DATIN Input file name =gs2023_89b.cube_cl
BGD_DATAF File containing psuedo HK parameters =bgd_89b_01.dat
BGD_DATOUT Output file name = gs2023_89b.cube_cl_ssud =
Warning: object ICOR already exists
Warning object ICOR is being overwritten
Warning: object IPERI already exists
Warning object IPERI is being overwritten
Warning: object ISUD already exists
Warning object ISUD is being overwritten
qcl>

Warning messages are issued by both programs because they overwrite existing arrays. To save an exisiting array, use the qcl COPY command:

qcl>copy
HDF_OLD object to be copied =sud
HDF_NEW new object =sud_new
    Object sud_new created
qcl>

The HDS file will contain an array SUD_NEW which is a copy of the original array, SUD. Arrays can be deleted using the qcl DEL command:

qcl>del sud