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

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7. Candidate AGN

In Sect. 5 we have used the hardness ratios [FORMULA] and [FORMULA] to classify 3 ROSAT sources (with numbers 36, 41 and 63) and projected onto the HI extent of NGC 3109 as candidate AGN.

We have searched in the optical B-band COSMOS finding charts of these sources for optical counterparts. In addition we have generated deep I-band finding charts for source 63 and other ROSAT sources and we have looked for faint unresolved and extended objects within the positional error circles of these ROSAT sources (cf. Fig. 6). We have found several optical candidates in the positional error circle of each ROSAT source. Therefore optical spectroscopy of all candidates is required to firmly identify the optical counterpart of these ROSAT sources.

If the background nature of candidate AGN "close to" NGC 3109 is established then these AGN can be used to probe the gas column density of NGC 3109 in the direction of these AGN (taking the additional galactic gas column into account). For two sufficiently X-ray bright candidate AGN RX J1003.2-2607 (source 63) and RX J1002.5-2602 (source 36) we perform X-ray spectral fitting with the EXSAS spectral analysis task (Zimmermann et al. 1994). We assume a galactic foreground column density of [FORMULA] and determine the additional hydrogen column density (due to NGC 3109) assuming reduced metallicities ([FORMULA]0.2 solar). For both sources we find that the hydrogen column deduced from the X-ray spectral fit is consistent with the hydrogen column deduced from the 21-cm survey of HSM80 and JC90 (cf. Table 5 and Fig. 8).

[FIGURE] Fig. 8. ROSAT PSPC X-ray spectra and best-fit spectral models (assuming galactic and NGC 3109 intrinsic absorption) of the AGN RX J1003.2-2607 (upper panel) and RX J1002.5-2602 (lower panel).


Table 5. NGC 3109 intrinsic hydrogen column densities in the direction of the candidate AGN RX J1003.2-2607 and RX J1002.5-2602 as deduced from X-ray spectral fitting. Also given is the hydrogen column density derived from the 21-cm data of HSM80 and CJ90. 68% confidence errors are given. A galactic hydrogen column density of [FORMULA] and a metallicity [FORMULA]0.2 solar is assumed in the X-ray spectral fit. Also given is the spectral photon index [FORMULA] and the unabsorbed X-ray flux (0.1-2.0 keV).
(a) No constraint on [FORMULA]; (b) constraint on powerlaw photon index: [FORMULA]; (c) HI model of HSM80; (d) HI model of JC90.

This result is also consistent with the location of these two sources in the [FORMULA] - [FORMULA] plane (cf. Fig. 5). We do not include in the spectral analysis source 53 which has been classified as a candidate X-ray binary in Sect. 5.

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© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000