Grating data analysis

The following procedure is provided by V. Gupta:

1. Derive a pha2.fits file and the arf files using CIAO Science Threads.
   

The level=2 file provided by CXC (or the default recipe provided by CXC to create a level=2 file from level=1) keeps the pixel randomization ON. Pixel randomization is done while creating the level=1.5 file from the level=1.0 using tg_resolve_events.
In order to create the level=1.5 file with pixel randomization off, one can use:
pset tg_resolve_events rand_pix_size = 0.0
For details, go to the CXC page here.


Use
ahelp tg_resolve_events
or
ahelp tgextract
to look for the detailed descriptions of the parameters. Following recipe assumes that the gratings spectrum has been extracted with the default values.
Note 1: The pha2 file for the HETG grating consists of information about spectrum of different orders for HEG and MEG. Each row corresponds to photons of different orders. The background counts correspond to the rectangular area above(background_up) and below (background_down) the dispersed spectrum.
Note 2: In case your file is processed with an ASCDSVER lower than R4CU5UPD13, you would require to update the header keywords: BACKSCAL, BACKSCUP, BACKSCDN.
BACKSCUP and BACKSCDN indicate the ratio of the area of the background and the source extraction regions and should always be numbers >1. (Info from CXC available here!)



2. Create an ASCII file from pha2.fits with four columns: wavelength, source_counts, background_counts, effective area.

This can be done with an IDL script.
Note 1: In the pha2.fits file:
• Wavelengths in columns "bin_lo" and "bin_hi" are in angstrom units. The wavelength for each bin can be chosen as lamda_mid=0.5*(bin_lo+bin_hi).
• Source counts given in column "counts".
• Background counts given under the columns "background_up" and "background_down". The background counts for each bin is calculated as: 0.5*(BACKSCAL/BACKSCUP)*(background_up+background_down). (since Background_up and background_down have the same width)
  
• Effective area from the correponding arf file given under column "specresp"
Note 2. Read the exposure time from the header of the pha2 file.


3. Rebin the data and create an ASCII table with three columns: wavelength, background subtracted flux, and errors.
• Rebin the wavelength bins such that each bin has at least 20 counts per bin.
• The effective area in the new bin can be chosen to be the effective area averaged over the bins that are being added to create a new bin.
• Divide counts in new bin by the average effective area, exposure time and binwidth of new bin to obtain the flux in units counts/cm^2/s/A, and calculate the statistical errors for each bin. Difference flux units could be chosen.
• The background flux can be calculated in the similar manner. Subtract the background flux from the source flux and take into account the background errors also.

K. Lewis provides a conveniently reformatted Web-based listing of X-ray line identifications based on data provided by the HEASARC at GSFC (note: Opens in new window).