Effective Area and Energy Resolution vs. Grade Selection

The HRMA/ACIS effective area and energy resolution were calculated for the ASCA grade selections g0, g01, g012, g0123, g01234, g012345, g02346. We also considered the special case of including all grades except ACIS grade 255. ACIS 255 (which corresponds to all eight neighbors of the central pixel being above the Split Event Threshold) is considered to contain mostly events that result from interactions of cosmic ray particles with ACIS (and not X-ray photons).

  

Figure 6.1: Effective Area vs. Energy for ACIS CCD I1
calculated for grade selections of
g0, g01, g012, g0123, g01234, g012345, g02346, g01234567 - ACIS255  

  

Figure 6.2: Effective Area vs. Energy for ACIS CCD S3 calculated for grade selections of g0, g01, g012, g0123, g01234, g012345, g02346, g01234567 - ACIS255  

Figure  6.1 and figure 6.2 show the dependence of effective area with grade selection for ACIS CCD's I1 and S3. Tables 6.8 and 6.9 provide numerical forms of the results.

The first significant result is that the HRMA/ACIS effective area varies considerably with grade selection for both front illuminated (FI) and back illuminated (BI) CCD's. A possible use of this effect is in the analysis of grating observations of faint objects where CCD energy resolution only influences the separation of overlapping spectral orders. The exclusion of only ACIS grade 255 will result in a relatively high effective area and a higher signal to noise spectrum of the faint object.

  

Figure 6.3: Energy Resolution (FWHM) vs. Energy for ACIS CCD I1 calculated for grade selections of g0, g01, g012, g0123, g01234, g012345, g02346, g01234567 - ACIS255  

  

Figure 6.4: Energy Resolution (FWHM) vs. Energy for ACIS CCD S3 calculated for grade selections of g0, g01, g012, g0123, g01234, g012345, g02346, g01234567 - ACIS255  

Figure 6.3 and figure 6.4 show the dependence of energy resolution with grade selection for ACIS CCD's I1 and S3. (Tables 6.10 and  6.11 give numeric values of this data.) The energy resolution for FI chips is fairly insensitive to grade selection in the 3-6 keV range. For energies above 6 keV the exclusion of grades begins to improve energy resolution for FI chips. In the energy range of 3-9 keV investigated so far, the energy resolution for back sided CCD's improves significantly with the exclusion of grades.

A possible astrophysical observation in which this effect may be exploited is ACIS imaging of a bright source with the BI chip. For bright sources imaged on a BI chip it may be desirable to sacrifice source counts for spectral resolution by selecting only single events.

Footnotes

...g02346.

Event grades are a characteristic of each event recognized by the ACIS software. They are critically important for separating X-ray produced events from non-X-ray events. The full ACIS grade information is reported as an eight-bit integer (0-255), but typically users analyze data by combining the grades into larger categories. Commonly used are the grade categories defined by the ASCA satellite processing.

Fuller descriptions of grades can be found in an appendix to this document, a document on ACIS Grades by Kenny Glotfelty of the ASC, and the ACIS Operations Manual, SOP-01.