Data analysis

A systematic comparison of LECS and MECS X-ray source positions with those collected from various optical catalogs is presented.

We have independently analyzed sky images of the four BeppoSAX X-ray telescopes (LECS, MECS1, MECS2 and MECS3) using event files processed at the ASI Science Data Center (ASDC).

We have restricted our analysis to observations performed after May 6th 1998, since before this date various calibration problems have affected the absolute spacecraft attitude reconstruction (see the Appendix for a detailed discussion on this topic).

This time selection has not been possible for MECS1, since this unit failed on May 6th 1997. We have therefore considered MECS1 observations from the beginning of the mission (May 1996) until the failure of the unit. Due to the worse spacecraft attitude reconstruction during this time interval, the MECS1 data are affected by larger systematic errors on the absolute sky position reconstruction.
 
 

LECS and MECS fields have been selected according to the following criteria:

• observation later than May 6th 1998 (with the exception of MECS1)
• exclusion of extended or confused target sources
• the target of the observation is sufficiently bright (count rate $> 10^{-2} {\mathrm ~cts~s}^{-1}$ for the LECS and the MECS units)
• the position of the optical counterpart of the target is known to within 1 arsec
• the whole range of spacecraft roll angle values (from $-90^\circ$ to $+270^\circ$) is covered

 

This selection results in a sample of 77 MECS1 images and 72 LECS, MECS2 and MECS3 images.

Source detection of the targets has been performed using the DETECT routine of the XIMAGE package (Version 2.66a).

Images were accumulated in the 2-10 keV energy band (channels 44-220) for the MECS and in the 0.1-10 keV (channels 10-950) for the LECS.

Special attention has been devoted to the quality of the detections. For each source different sizes of the slide-cell of the DETECT routine have been attempted and all detections have been checked interactively.