Notes on IPC and PSPC Spectral Fitting

The first step in any spectral analysis is to extract the photon distribution from both a source and a background region. Any size or shape of region can be specified. The photon distribution can be based on either the pulse height analyzer (PHA) channels or the pulse invariant (PI) bins. To convert the PHA distribution into a PI distribution, the instrument gain must be known. For ROSAT PSPC data, this conversion is done during the on-line (SASS) processing. For IPC observations, the instrument gain varies both across the detector and in time, and is applied during off-line processing (PROS) using the BAL histogram.

By default, spectra are extracted in PI bins for PSPC data and in PHA bins for IPC data.

The background is normalized to the source counts using the ratio of the source-to-background areas. The user has the option to adjust the normalization factor. For IPC data, the background should be obtained within the same field if possible, since the software assumes that the on-source BAL histogram applies to the background region as well.

The errors will be calculated directly from the counts, with the error on the background included. The user has the option of including a systematic error, which is added in quadrature; by default, IPC data has a 3% systematic error included.

Special techniques are required when using Einstein IPC data. If the source is a point source, the mirror scattering corrections have been obtained for a 3$^{\prime}$ radius circle (or 22.5 pixel). The corrections will be properly applied only if the source counts are obtained from a 3$^{\prime}$ radius circle. As a consequence of the ongoing inflight calibration effort, there are several versions of the PSPC spectral response matrix available for fitting data in PROS. These are given in Table 7.1:

      Table 7.1:  Available calibration files for the ROSAT PSPC.


     Response matrix   area file   filter area file      Comments
    ___________________________________________________________________
    
                       PSPC-B        PSPC-B   (used since Jan 1991)
                       PSPC-C        PSPC-C   (used prior to Jan 1991)
    ___________________________________________________________________
    ___________________________________________________________________
    
    dtmat.ieee       offar.ieee      filter.ieee      Default in PROS 1.0
                                                      Pre-flight versions.
                                                      non-detector specific
    
    dtmat_5.ieee     offar.ieee      filter.ieee      First post launch matrix
                                                      non-detector specific
    
    dtmat_6.ieee     offar2_5.ieee   filter2_5.ieee   Default in PROS 2.0
                     offar1_5.ieee   filter1_5.ieee   Same as pspcb_mar11 
                                                      and pspcc_mar11 in XSPEC.
    
    dtmat_36.ieee    offar2_6.ieee   filter 2_5.ieee  Default in PROS 2.2 and 2.3
                     offar1_6.ieee   filter 1_5.ieee  same as 93jan12 in XSPEC.
    _____________________________________________________________________

Data taken with the boron filter are automatically fitted using the correct area files. Type help pspc_fitting for more information on these calibration files, and how to specify calibration files other that the default ones.

Note that for spectral analysis of off-axis or extended sources, PROS computes an ``off-axis histogram" of the extracted photons, and the fitted model is appropriately corrected for vignetting.

For PSPC data the background spectrum can be corrected for the differential vignetting of the mirror. The charged particle component can also be removed for PSPC data. (Type help pspc_fitting for more details).

Although the PSPC detectors have 256 PI channels, these are binned into 34 channels for both on-line (SASS) and off-line (PROS) spectral analysis. The correspondence between the 256 channels and the 34 channels is given below.

BINNING SCHEMES
    
    
    34-bin   Energy    256-channels     34-bin    Energy     256-channels
    number  Low - High   included       number   Low - High    included  
    ------ ----- -----  ----------      ------  ----- -----   -----------
      1    0.07 - 0.09     7 - 8          18    0.84 - 0.91    84 - 90   
      2    0.09 - 0.11     9 - 10         19    0.91 - 0.99    91 - 98   
      3    0.11 - 0.14    11 - 13         20    0.99 - 1.07    99 - 106  
      4    0.14 - 0.17    14 - 16         21    1.07 - 1.15   107 - 114  
      5    0.17 - 0.20    17 - 19         22    1.15 - 1.23   115 - 122  
      6    0.20 - 0.24    20 - 23         23    1.23 - 1.32   123 - 131  
      7    0.24 - 0.28    24 - 27         24    1.32 - 1.41   132 - 140  
      8    0.28 - 0.32    28 - 31         25    1.41 - 1.50   141 - 149  
      9    0.32 - 0.37    32 - 36         26    1.50 - 1.60   150 - 159  
     10    0.37 - 0.42    37 - 41         27    1.60 - 1.70   160 - 169  
     11    0.42 - 0.47    42 - 46         28    1.70 - 1.80   170 - 179  
     12    0.47 - 0.52    47 - 51         29    1.80 - 1.91   180 - 190  
     13    0.52 - 0.58    52 - 57         30    1.91 - 2.02   191 - 201  
     14    0.58 - 0.64    58 - 63         31    2.02 - 2.13   202 - 212  
     15    0.64 - 0.70    64 - 69         32    2.13 - 2.24   213 - 223  
     16    0.70 - 0.77    70 - 76         33    2.24 - 2.36   224 - 235  
     17    0.77 - 0.84    77 - 83         34    2.36 - 2.48   236 - 247