Case Study 2 : ACIS-BI Observation of a Warm Absorber

We investigated whether an appropriate selection of grades and split threshold could lead to improvements in resolving absorption features in quasar spectra. We considered an ionizing incident quasar continuum transmitted through a warm absorber. The warm absorber model used in this study consists of a simple power law spectrum incident on the absorber with a photon index  = 1.2. We chose an absorber with an ionization parameter of 0.3, a column density of 5.$\times$10²² cm^-2and a total hydrogen density of 8.$\times$10³ cm^-3. We convolved the simulated absorbed quasar spectrum with two different spectral resolution functions. The first resolution function was created using the grade selection g0 and a split threshold of 6 ADU. The second spectral resolution function was created using the grade selection g02346 and a split threshold of 13 ADU. A slight improvement in resolving the absorption features is achieved when selecting only single events and low split thresholds, with the loss however of considerable effective area. Figure 6.11 shows the modeled spectrum of an absorbed quasar convolved with the two different spectral resolution functions mentioned above.

Figure 6.11:  By selecting only single events and low split thresholds a
slight improvement is achieved in resolving the absorption features
in an absorbed quasar spectrum.