3 The ROSAT All-Sky Survey data

The starting point of the sample construction is the RASS standard analysis source list. This source list was constructed during the second RASS processing (RASS II) by Voges et al. (1996, 1999) using subsequently the LDETECT, MDETECT, and Maximum-likelihood algorithms (referred to as Standard Analysis Software System). While only highly significant sources (maximum likelihood parameter $L \ge 15$) with count rates above 0.05 ctss^-1 and with interactively confirmed existence entered into the published RASS bright source catalogue (Voges et al. 1999), the primary, ROSAT Team internal source catalogue down to a source likelihood of L = 7 is used here. At this low likelihood the significance of some of the sources is below $3\sigma$ and not all of the sources are expected to be real. However, this low threshold guarantees that no sources are missed in the final sample, after the new flux cut is introduced. In total 54076 sources were found by the standard analysis in RASS II down to a likelihood of 7 in the study area of REFLEX. One complication in using RASS data is the non-homogeneous sky coverage. Since the sky was scanned in great circles perpendicular to the ecliptic, the shortest exposures are near the ecliptic equator, while this piles up at the ecliptic poles. In addition the satellite had to be switched-off frequently during the passages through the radiation belts. This affects in particular the southern sky data, since due to the South Atlantic Anomaly of the Earth's magnetic field the radiation belts are more prominent in the southern sky at the flight altitude of ROSAT. Some minor regions are underexposed because the data have been rejected for reasons of bad quality of the attitude solution. This leaves low exposure areas in the primary data. The resultant exposure distribution is shown in Fig. 1. The mean exposure is 335 s and the median is 323 s (compared to NORAS with mean and median exposures of 397 and 402 s, respectively). Table 2 gives the fractions of the sky area which are underexposed. Only a few percent of the sky area are strongly underexposed and only about 12% has less than half the median exposure. Such structure imposed by the exposure drop-outs is therefore not so dramatic, but has to be taken into account for any statistical analysis of the cluster population. Its actual effects depend on the X-ray flux limit of the sample.

Figure 1: Exposure time distribution of the ROSAT All-Sky Survey as analyzed in RASS II in the area of the REFLEX survey

Figure 2: Exposure distribution in the area of the REFLEX survey. Four grey levels have been used for the coding of the exposure times, with increasing intensity for $t_{\rm exp} < 100$ s, $100 < t_{\rm exp} < 200$ s , $200 < t_{\rm exp} < 300$ s, $t_{\rm exp} \ge 300$, respectively. The coordinate system is equatorial for the epoch J2000

   

Table 2: Fractions of the REFLEX survey area with low exposure

In particular for the clustering analysis the distribution of underexposed areas has to be known, so that it can be taken into account. This distribution is shown in Fig. 2. The underexposed area is not contiguous, but it is more or less confined to four strips in the southern sky. These strips reflect the shut-off times of the ROSAT-detector during the passage of the radiation belts in the South Atlantic Anomaly.