Appendix A: Sources in the field of NGC 3079
A.1. Correlation of HRI and PSPC detections
Thirty-four sources are detected around NGC 3079 in the PSPC and HRI observations. Table A1 shows their characteristics. If the source has only been detected in one of the detectors, a upper limit at the source position has been determined for the other detector. No upper limit is computed for sources detected in the PSPC that are at a large off-axis angle in the HRI, since we cannot reliably measure the count rates (the effective exposure time could be lowered as a consequence of the wobbling motion that can place the source outside the field of view for part of the observation, the detector sensitivity and the PSF are poorly known). We checked the mean offset of the source positions (listed in col. 3 of Table A1 for those sources detected in both instruments) to verify our attitude solution of Sect. 2.3 for all sources. The mean offsets in the north-south and east-west directions were below the systematical error.
Table A1. X-ray properties of the sources in the NGC 3079 field and proposed identifications
The distribution of the hardness ratios listed in Table 4 is shown in Fig. 1. Comparison with the theoretical values calculated for power law and thermal bremsstrahlung spectra (Fig. 1) show that in most cases the sources have hardness ratios consistent with a moderate amount of absorption, as expected from the line-of-sight Galactic H I column density value.
For several sources the derived fluxes or upper limits were comparable in the two separate observations. For four sources detected in both observations, namely H1/P3, H5/P7, H8/P10, and H9/P12 the flux differences exceed 2. For one additional source detected only in one of the observations, namely P17, the 2 HRI upper limit is lower by more than twice the error on the corresponding detection. Since the expected uncertainty in the count to flux conversion factor is of the order of 15% at most (see Sect. 2), the assumption of a wrong spectral model should not cause these flux differences. However, it should be noted that two of these sources, H1/P3 and H5/P7, are at large off-axis angles, so possible effects due to their positioning in the detector might come into play. We have checked for example the position of source H5 in detector coordinates, to understand whether the wobbling motion could affect the effective exposure of this source, and found that it is in fact possible that the source falls outside of the detector field of view for part of the observation, so the flux quoted should in fact be considered a lower limit. The same consideration does not apply to the other sources and it is therefore likely that these sources have varied between the two observations. As discussed in the next section, four of the variable sources are coincident in position with stellar objects or active nuclei (see Table A1), which as a class are known to be variable sources, and would therefore support the idea that the observed variations are real: H5/P7 is identified with QSO 0957+5543 at a redshift z=2.1, H9/P12 with the well studied QSO 0957+5608AB, a double source originating from a quasar at z=1.4 that is gravitationally lensed by a cluster of galaxies at z=0.4.
A.2. Identification of X-ray sources in the field of NGC 3079
For each source detected we prepared APM finding charts (Irwin et al. 1994), from which we can get a positional accuracy for the optical candidates of better than , optical magnitudes in blue (O) and red (E), and the color index O-E. In the charts sources are classified as stellar, non-stellar, or blend. Optical counterparts were searched for each in a circle of the X-ray position error.
Several positional coincidences were found from these charts. Additional candidates resulted from correlations of our source catalog with the entries in the SIMBAD and NED databases and with radio sources in the field given in Irwin and Seaquist (1991).
To improve on the reliability of the identifications, we have estimated the = + 5.37 (see Maccacaro et al. 1988), that we can use to discriminate between stars, for which is generally -1, and AGN for which this quantity is in the range -1.2 to +1.2 (see also Brinkmann 1992 and Pflüger et al. 1996). X-ray fluxes in the 0.1-2.4 keV band have been determined from the count rates assuming a 5 keV thermal bremsstrahlung spectrum, corrected for Galactic absorption (see Table 2). For the optical magnitude we used the `E' magnitude from the APM catalog.
Proposed identifications for 21 of the ROSAT field sources are summarized in Table A1. Column 6 lists the APM field and classification, and in parenthesis the O luminosity and the color (O-E). The optical identification and the spectral type given for stars are taken from the SIMBAD database. Identifications for extragalactic objects, with their redshift when available, is based on the NED database. The separation to the X-ray position is given in col. 7. and references used are indicated in cols. 8 and 9.
In most cases there is only one APM candidate within the error radius of the X-ray source. All proposed identifications are located within the X-ray position errors. Three sources are identified with known QSOs (H4/P6, H5/P7, H9/P12). Two sources (H2/P5 and P27) are identified with radio sources, for the first of the two there is also a APM candidate. Two sources (H11 and (H19/P22)) are identified with stars cataloged in SIMBAD.
The values and the optical colors of the unidentified objects show values similar to those of the identified QSOs. This suggest that most of them are also AGN/QSOs.
-1 are seen in only three unidentified sources, P1, P24 and H22/P29, for which identifications with stars are suggested. The optical candidates of H3, P25, and P24 show rather red colors. The candidates for H3 and P25 are flagged by APM as non-stellar in the red and stellar in the blue. It is likely that an AGN embedded in a host galaxy is the optical counterpart of these sources.
© European Southern Observatory (ESO) 1998
Online publication: November 9, 1998