Case Study 1 : ACIS/HEG Observation of Fe-K$_{\alpha }$ line profile in the Seyfert Galaxy NGC 4151

The improved energy resolution of ACIS/HEG at 6.4 keV (compared to previously attainable ASCA SIS resolution of about 180 eV) is needed to identify the presence of gravitational and Doppler effects and verify whether or not multiple reprocessors contribute to the broadening of the Fe-K$_{\alpha }$ line. For grating observations the intrinsic energy resolution of a CCD is mostly used to separate spectral orders. For this case study we have sacrificed intrinsic spectral resolution for increased effective area by excluding only ACIS grade 255 events resulting in an effective area at 6.4kev of about 30 cm². We considered a model for the incident spectrum of NGC 4151 in the vicinity of the Fe line based on ASCA observations, (see Table 6.17) and convolved it with the expected grating energy resolution of about 25 eV at 6.4 keV and added Poisson noise. The expected dependence of the effective area of the ACIS/HEG combination between 5 and 7 keV with grade is shown in figure 6.9. In figure 6.10a we show a simulation of the incident spectrum of NGC 4151 in the vicinity of the Fe-K$\alpha$ line. By excluding only grade 255 events in the analysis of the ACIS/HEG observation of NGC4151 we maximize the effective area without compromizing the spectral resolution. Figure 6.10b shows a simulation of an ACIS/HEG observation of NGC 4151 analyzed with different grade selection schemes.
 

Figure 6.9:  Expected dependence of the effective area
of the ACIS/HEG combination between 5 and 7 keV with grade selection.

 

Table 6.17:  Spectral Parameters of NGC4151

 
 

Figure 6.10:  (a) Simulation of the incident spectrum of NGC 4151
in the vicinity of the Fe-K alpha line.

(b) Simulation of an ACIS/HEG observation of NGC 4151
analyzed with different grade selection schemes.