backgrnd: change the background file for a given spectrum

Modify one or more of the files used in background subtraction.

Syntax:           backgrnd [ <filespec>...]

            backgrnd <spectrum number> none

where <filespec> =:: [ <spectrum num>]  <filename>... and where  <filename> is the name of the PHA file to be used for background subtraction. The numbering scheme is as described for the data command, except that the <spectrum num> must have previously been loaded.

An error message is printed if  <spectrum num> is greater than the current number of spectra (as determined from the last use of the data command. backgrnd <n> none indicates that no background subtraction is to be performed for that spectrum. If a file is not found or cannot be opened for input, then the user is prompted for a replacement background file(an  <EOF> at this point is equivalent to backgrnd <spectrum number> none). The current ignore status for channels is not affected by the bkgrnd command. (See the ignore and notice commands). Finally, any grouping specification will be overridden by the grouping in the source spectral file so that the source and background are binned in the same way.

The format of the background file must match that of the spectrum file: for this purpose OGIP Type I and II are considered to be the same format.

For details of how to remove spectra see the data command documentation.

Examples:

Suppose there are currently three spectra. Then

XSPEC12> backgrnd a,b,c

// New files for background subtraction are given for all

// three spectra.

XSPEC12> backgrnd 2 none

// No background subtraction will be done for the second spectrum.

XSPEC12> backgrnd ,d

// d.pha becomes the background for the second spectrum.

XSPEC12> backgrnd 2 e{4-5}

// Rows 4 and 5 of Type II file e.pha become the background for

// the second and third spectrum respectively.