Ancilliary Response Files (ARFs) and Response Matrix Files (RMFs) are available for download from the HEASARC Calibration Database (caldb) for all three instruments (Table 1.1). These are XSPEC compatible files which can be used for data simulation. In some cases ARFs and RMFs are supplemented, or in the cases of BAT and UVOT, replaced by RSP files. These are response files that have been normalized to their appropriate effective areas. As such RSP files should be used in XSPEC without ARFs.
Table 1.1: ARF, RMF and RSP calibration file downloads for the BAT, XRT and UVOT.
|XRT||Windowed Timing grade 0-2||swxwt0to2s6_20010101v012.arf, swxwt0to2s6_20070901v012.rmf|
|Photon counting grade 0-12||swxpc0to12s0_20010101v011.arf, swxpc0to12s6_20070901v011.rmf|
The BAT response matrix was generated from flight data for the on-axis response with all 32768 detectors enabled. In reality, gamma-ray bursts will be detected off-axis, and with fewer detectors, so the "typical" matrix represents the ideal case. The BAT effective area curve is displayed in Figure 2.1.
BAT spectra should be simulated as described in the fhelp file for batphasimerr. Batphasimerr is distributed with the Swift software.
Table 2.1 describes the FWHM spectral resolution of the BAT. This quantity was derived by fitting a Gaussian to model emission lines of negligible intrinsic width. The shape of the cutoff at energies greater than 100 keV prevent a meaningful measure of the FWHM above 80 keV using a Guassian model.
Table 2.1: Comparison of the FWHM spectral resolution of the BAT as a function of +energy.
|E = 20 keV||E = 50 keV||E = 80 keV|
The XRT effective area is determined by three main components: the mirror area, the filter transmission and the CCD quantum efficiency (QE). The ancilliary response (ARF) files contain the mirror area (which was initially created by a ray-tracing code) and the filter transmission (which is identical to the XMM-MOS medium filter), as well as any corrections for vignetting and point spread function losses. The CCD QE is accounted for by the Redistribution Matrix Files (RMFs) and depends on the operating mode and choice of grades selected.
The RMFs were created by a Monte-Carlo simulation code which models the physical interaction of an X-ray in the CCD and the appropriate readout mode (Osborne et al. 2005, SPIE, 5898, 340; Godet et al. 2007, SPIE, submitted). The standard grades which maximize the QE are grade 0-2 for WT mode and grade 0-12 for PC mode (see Burrows et al., 2005, Space Science Review, 120, 165 for a description of the grade definitions). Using in-orbit calibration observations, corrections were made to the ARF files to remove residuals around the gold edges (2.2-3.0 keV), near the oxygen edge (0.54 keV) and the silicon edge (1.84 keV). (see Campana et al.).
The Swift XRT RMFs and ARFs provided here for simulation purposes are appropriate for WT mode grade 0-2 and PC mode grade 0-12. The figure below compares their effective areas.
Effective areas of the filters (fig. 4.1) have been generated by scaling the manufacturer's specifications to the observed fluxes measured on orbit (see Poole et al. 2007, astro-ph 0708.2259). The grism effective areas (fig 4.2) are based on observed calibration spectra.