EXCALIBUR - ABSOLUTE CALIBRATION OF THE X-RAY-SPECTRUM OF THE CRAB-NEBULA

The next generation of X-ray telescopes (XSPECT/SODART on SPECTRUM-x-GAMMA, AXAF and XMM) will measure the broad band X-ray spectrum of thousands of sources with a photon noise of 1 percent or better. At present, the spectrum of the traditional standard X-ray source, the Crab Nebula, is known only to an accuracy of 10 percent above 2 keV and to about 50 percent below. Therefore, to meet the requirements for the absolute calibration of these X-ray telescopes, a special dedicated calibration mission must be carried out. We suggest an instrument consisting of two pinhole cameras (one with a CCD detector, and the other with an imaging proportional counter with an active area of 20 cm2). We show that this instrument will provide the spectrum of the Crab Nebula with an accuracy better than 2 per cent over the energy range 0.8 - 20 keV, with an energy resolution of 60 eV at 1 keV and 1 keV at 10 keV.