Comparison with optical positions

We have compared the BeppoSAX X-ray detections with the accurate positions obtained from various optical catalogs.

The results are shown in Figure 1 where the deviations in RA and Dec between the LECS and MECS X-ray sky positions and the optical positions are plotted. Note that differences have been computed subtracting the X-ray value from the optical one and that RA differences are corrected by the factor cos(Dec) and represent the true separation in the sky:
 

$~~~~~~~~~~~~~~~~~~~~~~~~ \Delta {\mathrm RA = (RA_{opt} - RA_{sax})~ cos(Dec)}$
$~~~~~~~~~~~~~~~~~~~~~~~~ \Delta {\mathrm Dec = Dec_{opt} - Dec_{sax}}$
 

We have then computed the radial distance between the X-ray and optical position and the ``radius'' within which 68% and 90% of the objects are included. The results are summarized in Table 1.
 
 

Table 1: LECS and MECS 68% and 90% error radius values

Instrument	68% error radius	90% error radius
LECS	23''	29''
MECS1	38''	53''
MECS2	30''	42''
MECS3	32''	43''

From Figure 1 and Table 1 some issues can be pointed out.
 

First, MECS1 X-ray positions are significantly affected by larger errors than MECS2 and MECS3, as a result of the worse absolute attitude reconstruction of the spacecraft during the first two years of the mission. An analysis of MECS2 and MECS3 images from May 1996 to May 1997 confirms this interpretation: MECS2 and MECS3 error radius values during this period are nearly identical to those of MECS1.
 

Second, RA and Dec deviations for MECS2 and MECS3, and marginally for MECS1, are not uniformly distributed around the zero values, suggesting the presence of some problems in the telescopes misalignment calibrations.
 
 

Figure 1: The deviation in RA and Dec between X-ray positions (LECS, MECS1, MECS2, MECS3) and optical positions. The short-dashed circle has a radius of 30 arcsec.