An improvement of the ${\vec Beppo}$SAX LECS and MECS positioning accuracy

Abstract

We present a study of the source positioning accuracy of the LECS and MECS instruments on-board BeppoSAX. From the analysis of a sample of archival images we find that a systematic error, which depends on the spacecraft roll angle and has an amplitude of17 00 for the LECS and27 00 for the MECS, aects the sky coordinates derived from both instruments. The error is due to a residual misalignment of the two instruments with respect to the spacecraft Z axis arisen from the presence of attitude inaccuracies in the observations used to calibrate the pointing direction of LECS and MECS optical axes. Analytical formulae to correct LECS and MECS sky coordinates are derived. After the coordinate correction the 90% confidence level error radii are 16 00 and 17 00 for LECS and MECS respectively, improving by a factor of2 the source location accuracy of the two instruments. The positioning accuracy improvement presented here can significantly enhance the follow-up studies at other wavelengths of the X-ray sources observed with LECS and MECS instruments.