Data analysis and intercalibration

The joint data analysis of WFC and GRBM is possible thanks to the availability of PHA spectra and response matrices of the two experiments in the same formats. For WFC data analysis we used the public response functions and Crab Nebula spectra available at http://www.sdc.asi.it
The WFC instrument consists of two coded aperture cameras, each with a field of view of 40$^\circ \times 40^\circ$ full width at zero response and an angular resolution of 5 arcmin. WFCs have an energy resolution $\approx$ 20% at 6 keV, were operated in normal mode with 31 channels in 1.5-26 keV and 0.5 ms time resolution ([Jager et al. 1997]). The on-axis effective area of WFCs No. 1 and 2 averaged in the 2-26 keV energy band is 118 cm².
The fits to the WFC Crab Nebula on-axis spectra are consistent with the canonical photon index, normalization and column density.

As it has been done for the BeppoSAX NFI ([Cusumano et al. 1998]), an intercalibration between WFC1/WFC2 and the GRBM detectors LS1 and LS3 can be performed by means of simultaneous Crab nebula measurements. We attempted both fits with a single power-law and fits with a broken power-law. The single power-law fits give a photon index of 2.05$\pm$0.08 for WFC1/LS1 and 2.07$\pm$0.1 for WFC2/LS3. If we untie the normalizations of the two data sets we obtain very close values, consistent between 1 sigma. This means that no normalization correction factor has to be introduced when simultaneously fitting data of the two instruments.
As expected, the best fit of the WFC + GRBM Crab Nebula spectrum is a broken power-law. For WFC1/LS1 the break energy is 66$\pm$29 keV, the low energy photon index is 2.06$\pm$0.07 and the high energy photon index is 2.18$\pm$0.6. Similar values are obtained for LS3. These values are fully consistent with those found by other experiments in similar energy bands (see above).

The results on the Crab nebula spectrum (4.12.2 show that the inter-calibration of the two detectors is very good, both for LS1/WFC1 and LS3/WFC2.

  

Figure 4.11: WFC1 + GRBM/LS1 photon Crab spectrum