WIDE FIELD CAMERA CRAB SPECTRAL CALIBRATION

Date of latest modification: Februari 16, 1998

Disclaimer: this is not a definitive statement, but just a progress report on the current situation.

Fitting Crab spectra obtained from correlated images for both wide field camera 1 and 2 (w1 and w2) gives the following results. Only observations with Crab on axis were considered, which were deconvolved with the appropriate response matrices (wfc1_iros_dec97.rmf and wfc2_iros_dec97.rmf (available at the ftp site), respectively).

Fits were made in two ways, i.e.,
1. using nearly all channels, and
2. using only energy channels 6-8, 10-23 (3.0 - 12.5 keV for wfc1, 3.3 - 11.3 keV for wfc2) to avoid bands for which the response matrices are not yet reliable (i.e., the lowest and highest energies and the channels near the Xe L edge(channels 9 and 10) ).

Furthermore, the WFC team of SRON, Utrecht, The Netherlands advises to add a systematic error of 2.5% in quadrature . This of course reduces the chi square considerably, as shown in fits of the second type.

A simple powerlaw model with Wisconsin absorption was applied. Fits made with the XSPEC package are shown with the value for the galactic absorption NH fixed to 0.33 x 10^22 cm, as was also done for the Narrow Field Instruments.

For each fit the flux between 2.0 and 10 keV is given; this value should be 2.08 x 10^-8 erg/cm^2/s (assuming a column density nh = 0.33 x 10^22, photon index 2.1 and normalisation 9.7). From the values given below, we find that at present the fluxes are underestimated by a factor of say 0.8. This means that you should multiply the flux found with the WFC software by a factor of about 1.2.