next previous
Up: The Hamburg/RASS


3. Identification criteria

The basic criterion for the classification of optical counterpart candidates is the appearance of the high-resolution density spectra. A small collection of typical spectra are shown in Fig. 2 (click here). In some of the brighter density spectra emission lines (cf. classification CV, QSO in Fig. 2 (click here)) and absorption features (stars of spectral type FG, K and M, some WDs) can be distinguished, and the shape of the continuum can be derived (cf. blue continua of AGN, WDs). Extended direct images help to identify galaxies which often show featureless spectra (cf. galaxy, blue galaxy). At lower densities, near the plate limit, only basic colour information is available and we classify the spectra in "extreme blue and weak tex2html_wrap_inline1107'', "blue and weak tex2html_wrap_inline1109'' and "red and weak tex2html_wrap_inline1111'' spectral categories. We associate the blue spectra with X-ray emitting AGN, while the red and weak spectra are considered as unidentified sources. To identify the most plausible counterpart, we required the candidate to have in addition to an acceptable positional distance to the X-ray source to have a log(tex2html_wrap_inline1113/fB) value consistent with its object class (Stocke et al. 1991).

Figure 2:   Examples of high resolution objective prism spectra for several important object classes. The inset in the upper right of each panel contains the low resolution spectrum. The object classes GALAXY and BLUE GAL are only given if the direct image is extended

For the final evaluation and the determination of the most plausible optical counterpart all obtainable information is put together: sky positions from the direct plates, classifications and optical magnitudes (or upper limits) from the spectral plates, hardness ratios and X-ray to optical flux ratios (log(tex2html_wrap_inline1113/fB)). For white dwarfs (WDs) and some kinds of cataclysmic variables (CVs) information about the softness of the X-ray spectrum (from the hardness ratios) is useful for their classification. Information about the extent of X-ray sources is also helpful for identifying some object classes (e.g. clusters of galaxies). Based on this information and applying various classification criteria (Bade et al. 1992b) a final classification code is determined, and an entry in a "master catalogue of identifications'' is constructed.

The classification is coded by a two or three-digit number. The first digit identifies the class of objects, with "1'' to "3'' identifying extragalactic objects, and "5'' to "7'' stellar objects (Table 1 (click here)). A third digit is present for classes that could be subdivided, e.g. the stars. The second digit describes the reliability of the classification:

"highly probable'', the proposed counterpart fulfills all requirements of its class and no other plausible counterpart is found in the X-ray error circle.
"probable'', the proposed counterpart fulfills the requirements of its class, but there are some limitations. Either the objective prism spectrum is not typical, or small conflicts with the X-ray information (spectral or spatial information, distance to X-ray position) are present or there is another (considerably less) plausible counterpart in the error circle.
"possible'', the proposed counterpart fulfills some requirements of its class, but there are doubts arising from insufficient objective prism data, conflicting X-ray data (e.g. extended emission for an AGN) or another neighbouring plausible counterpart.

In Table 2 (click here) the portion of the reliability classes is tabulated for the relevant object classes.



Objects Number Fraction Comments


AGN/QSO 1574 tex2html_wrap_inline1121
2 Galaxies 138 tex2html_wrap_inline1123 extended optical image, contain some AGN
3 Galaxy clusters 113 tex2html_wrap_inline1125
5 M-dwarfs 155 tex2html_wrap_inline1127
6 White Dwarfs 31 tex2html_wrap_inline1129
7_1 K Stars 136 tex2html_wrap_inline1131 can also contain some stars of early M type
7_2 F or G Stars 4 tex2html_wrap_inline1133 Classification needs further support from high resolution spectra
7_3 CVs 16 tex2html_wrap_inline1135
7_4 Bright Stars 956 tex2html_wrap_inline1137
8 Unidentified 619 tex2html_wrap_inline1139 Majority consists probably of optically weak AGN and clusters
0 Empty 105 tex2html_wrap_inline1141 empty on objective prism and direct HQS IIIa-J plates

Table 1:   Classification codes with contents


Object class

"highly probable'' "probable'' "possible''


tex2html_wrap_inline1143 tex2html_wrap_inline1145 tex2html_wrap_inline1147
Galaxies tex2html_wrap_inline1149 tex2html_wrap_inline1151 tex2html_wrap_inline1153
Galaxy clusters tex2html_wrap_inline1155 tex2html_wrap_inline1157 tex2html_wrap_inline1159
M-dwarfs tex2html_wrap_inline1161 tex2html_wrap_inline1163 tex2html_wrap_inline1165
White Dwarfs tex2html_wrap_inline1167 tex2html_wrap_inline1169 tex2html_wrap_inline1171
K Stars tex2html_wrap_inline1173 tex2html_wrap_inline1175 tex2html_wrap_inline1177
F or G Stars tex2html_wrap_inline1179 tex2html_wrap_inline1181 tex2html_wrap_inline1179
CVs tex2html_wrap_inline1185 tex2html_wrap_inline1187 tex2html_wrap_inline1189
Bright Stars tex2html_wrap_inline1191 tex2html_wrap_inline1193 tex2html_wrap_inline1195

Table 2:   Percentage of the reliability classes in the object classes

3.1. Object classes

The following classes of objects were defined (Table 1 (click here)):

next previous
Up: The Hamburg/RASS

Copyright by the European Southern Observatory (ESO)