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2. Identification technique

2.1. The digitized database of Schmidt plates

The HQS provides objective prism plates for 567 fields of the northern high galactic latitude sky with tex2html_wrap_inline969 and direct plates for most of them. In many cases two prism plates per field are available. The field size of the plates is tex2html_wrap_inline1029 and the spectra have a non-linear dispersion with 1390 Å/mm at Htex2html_wrap_inline1039. The Kodak IIIa-J emulsion is used, giving a wavelength coverage of tex2html_wrap_inline1041 between the atmospheric UV-limit at tex2html_wrap_inline1043 and the emulsion sensitivity cut-off at 5400 Å. The limiting magnitude for the spectral plates is tex2html_wrap_inline1045 but can differ between the worst and the best plates up to 1 magnitude. The upper brightness limit due to saturation is reached between 12 < B < 14 for the objective prism plates. The direct plates have a limit of tex2html_wrap_inline1049.

Figure 1:   Digitized objective prism spectrum of 1RXS J095703.2+563138. The left image shows a part of the digitized objective prism plate (low resolution scan) around the X-ray position. The right image displays the spectrum from the high-resolution and low-resolution (inset) scan

The plates are digitized with a PDS microdensitometer. To fasten the digitization process the prism plates are scanned with low resolution, providing density spectra consisting typically of 15-20 pixels. Interesting spectra can be rescanned individually with a tenfold higher resolution. The left part of Fig. 1 (click here) shows a cut of the digitized spectral plate around the position of an arbitrary RASS source, and the right part presents the digitized spectrum in high and low resolution.

The digitized spectral plates are calibrated in the Johnson B-band with photometric sequences from the Guide Star Photometric Catalogue (Lasker et al. 1988) and faint sequences from various sources. Down to tex2html_wrap_inline1055 the photometric accuracy varies in the range 0.2-0.5 mag depending on the sequences available. Below tex2html_wrap_inline1055 the quality of the calibration degrades gradually. The calibration of extended optical sources (galaxies) yields brightnesses systematically too faint often more than 1mag. The HRC marks such brightnesses with an "e'' for extended after the magnitude. The digitized direct plates provide coordinates with an accuracy of tex2html_wrap_inline1061 and images with moderate to good resolution (2'' - 4'').

2.2. The identification process

The identification process is performed in several steps. Originally the positions of X-ray sources were provided by the first processing of the survey data (RASSI, Voges et al. 1992a). This system collected all RASS sources in great circle strips tex2html_wrap_inline1069 wide passing through the ecliptic poles. Data from neighbouring strips were combined to create a RASS X-ray source file for each HQS field. Correlations with stellar positions from the SIMBAD catalogue revealed that the RASSI positions have a 1tex2html_wrap_inline1071-error of tex2html_wrap_inline1073 (Voges 1992). For the search of counterparts we adopted an error circle with radius tex2html_wrap_inline1075 or tex2html_wrap_inline1077, whichever is larger and where tex2html_wrap_inline1071 is the standard deviation of the X-ray position as given by the first RASS processing. In order to account for the larger positional error of the spectra in dispersion direction the error circle was enlarged by tex2html_wrap_inline1081 (2.3'') in this direction (forming an ellipse).

The selection of optical counterparts is made field by field. The first step is the registration of all optical sources inside the X-ray error circle. The method of registration was changed in 1995 to extract the full information from the spectral plates down to their limit. Prior to 1995 all spectra inside the error circle were automatically selected from the digitized low-resolution spectral database and rescanned with higher resolution. Then, objects from the direct plates, too faint to be visible on the spectral plates, were added. Since 1995 the automatic selection was made on the direct plates and scans with high resolution were made at the transformed positions. This procedure has the advantage to provide additional density spectra with low signal to noise for objects which failed to enter the digitized low resolution spectral database.

The classification of the optical sources is made basically with the high resolution objective prism spectra. These spectra are displayed on a video screen and are classified interactively. Only galaxies and galaxy clusters are mainly recognized due to their appearance on the direct plates. The classification criteria are described in Sect. 3 (click here) and a detailed description of the catalogue is given in Sect. 4 (click here).

2.3. Adjustment to the second RASS processing

The RASS-BSC is based on a second processing of the RASS data, yielding tex2html_wrap_inline1093 80000 sources (RASS-II). The main differences in the second data processing as compared to the first are as follows. The photons were not collected in strips but rather merged in 1378 sky-fields of size tex2html_wrap_inline1095, taking full advantage of the increasing exposure towards the ecliptic poles. Neighbouring fields overlap by at least tex2html_wrap_inline1097, in order to ensure detection of sources at the field boundaries, which was a problem in the first processing. The candidate list for the maximum-likelihood analysis was enlarged by lowering the threshold values for the two preceding source detection algorithms, and the acceptance criteria were changed to allow very soft and very hard sources to be included. These might have been rejected previously if the likelihood in the broad energy band was too low. The calculation of the spline-fitted background map was improved, resulting in more accurate count rates. Finally a new aspect solution was incorporated; this reduced drastically the number of sources with erroneous positions and morphology.

In order to ensure the quality of the source detection process a visual inspection of all RASS-BSC sources was performed (Voges et al. 1997a). While the total number of sources increased from 50000 (RASS-I) to 80000 (RASS-II) there are only a few sources in the RASS-BSC, which were not already detected as RASS-I sources. Typical error radii for RASS-II sources are tex2html_wrap_inline1099. Due to the relatively large error circles (tex2html_wrap_inline1101) adopted for the identification of RASS-I sources, the error circles of the corresponding RASS-II sources usually fall within the RASS-I error circles. We then only updated our catalogue with the new X-ray positions. Checks were made, if the criteria leading to a particular classification were altered by the new X-ray data. Finally, a few sources were discarded from the final catalogue because the error circles of the two RASS processings do not overlap.

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