Converting PHA2 grating files for XSPEC use.

(last modified 7/5/01)

grating2xspec

The PHA2 file produced by the Chandra grating pipeline is a standard type II spectral file. As such it can be read into XSPEC using a command such as

XSPEC> data myfile_pha2.fits{4}

to read the 4th spectrum from the file.

However, the file also contains background spectra obtained from regions above and below the dispersed source spectrum. These are in the BACKGROUND_UP and BACKGROUND_DOWN columns. These columns cannot be read by XSPEC. A background spectrum for use in XSPEC can be made by extracting these columns and creating a new type II spectral file. The grating2xspec perl script does this (shift-left on the link to download the script).

This script requires HEASOFT to be installed (ie FTOOLS and XSPEC) so runs on all systems on which you can run XSPEC. Help on the script can be obtained by typing...

unix> grating2xspec -h

grating2xspec version 1.00 2001-04-28 kaa

usage: grating2xspec [-h] filename option

This script uses the input pipeline PHA2 spectrum to make another type II spectrum file containing the background. The input file is also copied and the copy modified so that the BACKFILE column points to the correct background spectrum. The option argument is either up, down, or both. If up then BACKGROUND_UP is used, if down then BACKGROUN_DOWN and if both then the _UP and _DOWN spectra are summed and the BACKSCUP and BACKSCDN keywords are summed.

e.g. grating2xspec foo_pha2.fits both

creates a background file foo_bkg2.fits from the sum of the BACKGROUND_UP and BACKGROUND_DOWN columns in foo_pha2.fits. It also creates a copy of the input file as foo_npha2.fits which includes pointers to the background file. To read say the 3rd spectrum into XSPEC simply give the command

XSPEC> data foo_npha2.fits{3}