Generates an OGIP-compliant ([3]) ancillary response file (ARF). The ARF contains a table listing area values (the column SPECRESP) for a number of different energy ranges (the columns ENERG_LO and ENERG_HI). The ARF can be used in conjunction with the response matrix file (RMF, generated by rmfgen) to allow fitting of particular spectral models against the observed spectral data. The actual choice of fitting package is entirely up to the user, the only prerequisite is that it is OGIP-compliant. The most widely used package is expected to be Xspec.
Specifically, arfgen takes calibration information provided by the CAL, performs the necessary corrections for instrumental factors depending on the user's requirements, and the state of spectral data, and formats the output such that it is compliant with [3]. It is this operation that is described here; details of individual CCF constituents and how the CAL processes such data are beyond the scope of this document, and the reader should refer to the Calibration Access and Data Handbook for such information ([1]).
The following table summarises the factors used to generate the ARF, and the status of the corresponding code needed to model that factor in arfgen:
|
Item |
Description | Status |
| 1 | Telescope Effective Area | Implemented |
| 2 | Filter Transmission | Implemented |
| 3 | Quantum Efficiency | Implemented |
| 4 | Complex region selection | Implemented |
| 5 | Spatial variation of source | Implemented |
| 6 | Source position specification | Implemented |
| 7 | Encircled energy correction (point source) | Implemented |
| 8 | RMF dataset i/p optional | Implemented |
| 9 | Pattern support | Implemented |
| 10 | Flux loss due to pile-up | Not implemented |
| 11 | Bad pixel correction | Implemented |
| 12 | Bad column correction | Implemented |
| 13 | CCD gaps | Implemented |
| 14 | Out of FOV/observing window | Implemented |
| 15 | Out-of-time event smearing (PN) | Implemented |
Item 10 is actually implemented within rmfgen for EPIC-pn spectra.
Whether a factor is taken into account by arfgen in the generation of the ARF can be controlled by individual parameters with the prefix model. For example, to correct for filter transmission, the parameter modelfiltertrans should be set to true.
