HETGS Scattering Analysis using ACIS-2C0


Input was level 1 events in 5×5 PHA format
Used IDL predominantly, custom driver with somewhat generic utilities
Parameter setting routines depend on phase, detector, grating, etc.:
Reduction procedure
  1. Set event PH to sum of 25 pixel PHs
    "ASCA" grading criterion computed from 3×3 but not used
    Many cases of 2nd event in 5×5 window frame
    This figure shows the pulse height distribution for the entire 5 image data set. The input is monochromatic at 1.775 keV and background is negligible. Thus, the pileup peaks are apparent and there is a tail of incomplete charge collection events below 1 keV. The ACIS-2C0 gain parameters vary significantly over the entire array, so peaks are broad.
  2. Randomized events by ± 0.5 pixel before rotations applied
  3. Offset by FOA loc, then rotated 0.6° from Chip XY to XRCF YZ
  4. Offset by FAM shift; coordinates now in pixels from 0th order
  5. Rotated by grating dispersion angle (HEG: -5.2°, MEG: 4.7°)
    New coordinates are dispersion and cross-dispersion distances
    NOTE: Rotation about (virtual) 0th order location, not chip FOA,
    so the rotations are distinguishable (as with HSI)
  6. Coordinates are scaled to physical units (mm)
  7. "Unwrapped" 2C to center dispersion line
    Depend only on cross-dispersion (xd) distance
    Result is modulo W (= 114*0.024 mm) so that -W/2 < xd < W/2


    Grayscale image showing all the data combined after unwrapping. The dispersed orders show up as bright vertical lines because the horizontal scale is compressed. Holes in the middle of the images result from event selection to avoid pileup. Click on the image for more complete annotation and a larger version which is stretched to show scattering better.

  8. Set wavelength based on grating equation assuming sin phi = tan phi
  9. Converted xd values to angles (in arcsec) using mirror focal length
  10. Determined exposure for each wavelength bin
    Set up dummy events at corners
    Sent events through same transformations as photon events
    Added chip exposure time to allowed wavelength bins
  11. Accumulated exposures, events into encompassing arrays
    Exposure function gives the resultant exposure time as a function of wavelength, adding all 5 chips.

Analysis of events binned into an array with 0.05 Å by 0.5 arcsec pixels
Interpretation of scattering profile results