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Astron. Astrophys. 338, 8-14 (1998)

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6. X-ray

The cluster ACO 3627 was observed with the ROSAT PSPC for an effective exposure time of 11,257 ksec on September 23 and 24, 1992 and March 13 to 15, 1993. In this observation WKK 6092 is well detected as an X-ray source about 6 arcsec to the north of the optical position. This is well within the typical pointing accuracy of the ROSAT observatory and the Seyfert galaxy can actually be used for the astrometric correction of this ROSAT pointing. The Seyfert galaxy is located at a distance of about 20 arcmin from the X-ray cluster centre (note: not the central radio source). It lies in the outer very low surface brightness region of the cluster (cf., images in Böhringer et al. 1996) and stands out clearly above the cluster background and the general X-ray background. In the PSPC observation the Seyfert is located very close to the support ribs of the PSPC window. With the galaxy approximately in the same direction from the pointing centre (cluster centre) as the position angle of the wobbling of the ROSAT telescope, the X-ray source is, however, not so severely obscured and a good flux measurement is possible.

About [FORMULA] source counts are detected for the Seyfert, yielding a count rate of [FORMULA] s-1, an unabsorbed (0.5 - 2.0 keV) flux of [FORMULA] erg s-1 cm-2 and a luminosity of [FORMULA] erg s-1 in the 0.5 to 2.0 keV band. No significant variation of the count rate could be detected within and between the observing intervals.

The background substracted spectrum of WKK 6092 is shown in Fig. 6. It can be perfectly fitted with an absorbed power law model with best fitting values for the photon power law index of about [FORMULA] and the absorption column density of about [FORMULA] cm-2 yielding a reduced [FORMULA] of 0.8. But the uncertainties are very large with values of the order of 70% ([FORMULA]). The photon index is equal to the canonical value and the absorption column density is close to the value found for the cluster, and independently from 21 cm observations at this position (Kerr et al. 1986, cf., also Sect. 3.2). We can therefore only state that the spectral fits are consistent with the standard expectation for this object. An upper limit (at a 2 [FORMULA] level) for the internal absorption of the X-ray emission from the Seyfert galaxy can be set to about [FORMULA] cm -2.

[FIGURE] Fig. 6. X-ray spectrum WKK 6092.

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© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998