Restoration of the ASCA source position accuracy

We present a calibration of the absolute pointing accuracy of the Advanced Satellite for Cosmology and Astrophysics (ASCA) which allows us to compensa te for a large error (up to 1') in the derived source coordinates. We param eterize a temperature dependent deviation of the attitude solution which is responsible for this error. By analyzing ASCA coordinates of 100 bright ac tive galactic nuclei, we show that it is possible to reduce the uncertainty in the sky position for any given observation by a factor of 4. The revise d 90% error circle radius is then 12", consistent with preflight specificat ions, effectively restoring the full ASCA pointing accuracy. Herein, we der ive an algorithm which compensates for this attitude error and present an i nternet-based table to be used to correct post facto the coordinate of all ASCA observations. While the above error circle is strictly applicable to d ata taken with the on-board Solid-state Imaging Spectrometers (SISs), simil ar coordinate corrections are derived for data obtained with the Gas Imagin g Spectrometers (GISs), which, however, have additional instrumental uncert ainties. The 90% error circle radius for the central 20' diameter of the GI S is 24". The large reduction in the error circle area for the two instrume nts offers the opportunity to greatly enhance the search for X-ray counterp arts at other wavelengths. This has important implications for current and future ASCA source catalogs and surveys.