sssbck -- builds the EINSTEIN SSS background PHA files for a given source obsn
sssbck phafile (jcgparm) linfexp varfexp bckfile corfile (chatter) (mode)
This task takes a user-defined PHA file from the Solid State Spectrometer (SSS) onboard the Einstein Observatory, and generates the appropriate two background PHA files. Each of the o/p files is a scaled version of an input calibration file, available from anonymous ftp to the HEASARC Calibration database (ftp://heasarc.gsfc.nasa.gov/caldb/data/einstein/sss/bcf/). The scaling involves : multiplying the background count rate in each channel by 2 variables (a dimensionless parameter related to JCG, and a ratio of the source and background exposures); dividing this result by the source exposure.
This task therefore takes three files as i/p, all of which must be in an OGIP-approved FITS file format (i.e. PHAVERSN = 1992a) :
(i) an EINSTEIN SSS PHA file for a given source observation.
(ii) an EINSTEIN SSS Basic Calibration File (type=bcf). This is a .PHA file containing a component of the SSS background spectrum which is related to cosmic ray and particle events. This component is independent of energy and is fairly stable with time (otherwise referred to as the LINEAR component).
(iii) an EINSTEIN SSS Basic Calibration File (type=bcf). This is a .COR file containing a component of the SSS background spectrum which is of unknown origin, but at energies >3 keV is variable with time (otherwise referred to as the VARIABLE component).
(both background calibration data sets were modelled from 10,000 secs of blank sky observations).
Finally, the source PHA file is updated by writing the names of the two output background files to the keywords BACKFILE and CORRFILE. Note that BACKFILE corresponds to the scaled LINEAR component, whilst CORRFILE corresponds to the scaled VARIABLE component.
Historically, the 'JCG-parameter' has been used in the analysis of Einstein SSS to normalise the background calibration data to that expected in the source observation. This parameter is related to the number of 'bad' counts (total counts - good counts) in the source PHA file. The details of exactly now the JCG-parameter should be calculated are currently unavailable. However, in most cases a value of JCG = 70 appears appropriate. SSSBCK contains a hidden parameter jcgparm, with a default value of 70, in users users wish to change this value.
Lorraine Breedon HEASARC NASA/GFSC http://heasarc.gsfc.nasa.gov/cgi-bin/ftoolshelp