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Astron. Astrophys. 361, 491-499 (2000)

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3. The catalog of X-ray sources

Three detection procedures (local, map, and maximum likelihood) were applied to the merged pointings using EXSAS commands (Zimmermann et al. 1994). In the local and map detection a square shaped detection cell is slid over the image. Source counts are determined within the cell, background counts either from an area surrounding the cell (local background) or from a (smoothed) background image within the same cell. In the maximum likelihood detection the distribution of the detected photons above the background is compared in a maximum likelihood ratio test with the analytical point-spread function.

The analysis was performed in the five energy channel ranges Soft = (channel 11-41, 0.1-0.4 keV), Hard = (channel 52-201, 0.5-2.1 keV), Hard1 = (channel 52-90, 0.5-0.9 keV) and Hard2 = (channel 91-201, 0.9-2.0 keV) and broad (0.1-2.4 keV). The five source lists were merged to one final source list taking detections at off-axis angles [FORMULA]50´ into account. The maximum likelihood algorithm was used to determine the final source position, the counts in five energy bands and the source extent. A one-dimensional energy and position dependent Gaussian distribution was applied in order to obtain the source extent. The source extent (Ext) is given as the Gaussian [FORMULA]


Hardness ratios [FORMULA] and [FORMULA] were calculated from the counts in the bands as [FORMULA] and [FORMULA]. The existence likelihood ratio and the extent likelihood ratio was calculated according to Cash (1979) and Cruddace et al. (1988). We selected for our final source catalog only detections with an existence likelihood ratio [FORMULA], which is equal to a probability of existence [FORMULA]. We give the value for the extent only in case the extent likelihood ratio is [FORMULA]. A [FORMULA] source error radius was calculated, adding quadratically a systematic error of 5" (cf. Kürster 1993).


The positional error derived for large off-axis angles [FORMULA] may be somewhat underestimated due to the asymmetry of the point-spread-function. But the positional error should not be larger than [FORMULA]1´.

We finally screened the catalog of ROSAT sources by displaying the positions of these sources on the hard, soft, and broad band ROSAT PSPC image. We could confirm 91 of the detected sources. This screened catalog of pointlike and moderately extended sources is given in Table 6. We give in the first column of the catalog for each confirmed source the sequence number and in the second column the source number (from the catalog of unscreened detections). We always refer to the source number in the text.

In Fig. 1 we show the ROSAT PSPC image of the NGC 3109 field in the hard band (0.5-2.0 keV). We note that the dwarf spheroidal galaxy Antila which is [FORMULA]1.o2 to the south of NGC 3109 is outside of the observed field of view. In Fig. 2 we mark on the ROSAT PSPC image of the central 20´ of NGC 3109 the sources for with accurate hardness ratios [FORMULA]0.2 have been determined.

[FIGURE] Fig. 1. Exposure corrected ROSAT PSPC image of the NGC 3109 field in the hard band (0.5-2.0 keV). North is up and east is to the left.

[FIGURE] Fig. 2. ROSAT PSPC image (0.5-2.0 keV) of NGC 3109. The 22 sources with accurate hardness ratios [FORMULA] (and in addition the AGN candidate 62) are marked with circles and the source number is given. Also shown is the ellipse of the stellar surface density profile of this galaxy (Demers et al. 1985).

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© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000