Type II FITS Spectral Files

A Chandra grating type II spectral file (usually having a name ending in *pha2.fits) contains 12 separate counts spectra corresponding to +/- 1st, 2nd, and 3rd orders for the MEG and HEG. Each of the 12 spectra are associated with vectors BACKGROUD_UP and BACKGROUND_DOWN, corresponding to upper and lower strips for background. Header parameter TG_M indicates the order of the spectrum (+/-1,+/-2,+/-3) and TG_PART indicates the the grating arm (1=HEG, 2=MEG, 3=LEG).

Further details on how to examine type II spectral files with PRISM, ChIPS and SHERPA can be found at this link. In XSPEC one can read in the various spectra by specifying a number between 1 to 12 after the spectral file name. In the sequence 1 to 12, the orders are: [-3,-2,-1,+1,+2,+3,-3,-2,-1,+1,+2,+3], where 1 to 6 refer to the HEG and 7 to 12 refer to the MEG. For example, the command

XSPEC>data 1:1 rootname_pha2.fits{3} 2:2 rootname_pha2.fits{4}

reads in the HEG -1 and +1 orders to data groups 1 and 2 respectively.