2. Observations and data analysis
NGC 3079 was observed from October 20 to 25, 1992 with the ROSAT HRI and from November 13 to 15, 1991 with the ROSAT PSPC, for a total observing time of 20.7 ks (HRI) and 19.0 ks (PSPC). The observations were split into 11 and 9 observation intervals (OBIs) for HRI and PSPC, respectively. To analyse the data for times of high detector background, we investigated rates of the master veto rate counter for the PSPC and the invalid counts for the HRI integrated over 60 s. The rates reach maxima of 280 cts s-1 (82 cts s-1), but stay below 200 cts s-1 (60 cts s-1) for 98% (99%) of the time for the PSPC (HRI). We therefore decided not to reject events due to times of high background.
2.1. Attitude corrections
To improve on the attitude solution we have adopted two subsequent techniques. For the HRI data we analyzed individual OBIs and aligned the data with the help of 7 bright point sources (namely H1, H2, H8, H9, H15, H20, H23, cp. Sect. 3.1) to the average position. While the offset in the position of these sources was for most OBIs, for OBI 5 the offset was . The photons were corrected for the offsets.
We also looked for possible optical counterparts (see Appendix A.1 and A.2) with the aid of APM finding charts (Irwin et al. 1994), and compared the optical and X-ray positions, to determine a possible systematic error on the absolute attitude solution. We found optical candidates for 9 sources in the HRI and 7 in the PSPC. We determined a systematic shift of 3:004 to the east and 3:009 to the north and an additional counterclockwise rotation of 0:O39 for the HRI observation. The corrected center of the HRI pointing direction is = , (J2000.0). For the PSPC, we determined a shift of 8:008 to the east and 6:005 to the south and an additional counterclockwise rotation of 0:O2. The corrected center of the PSPC pointing is = , (J2000.0). Source lists and images have been corrected for these systematic effects. The remaining position uncertainty is less than for both instruments.
The attitude corrections determined above for HRI and PSPC are in good agreement with boresight parameters determined from a larger sample of observations that now are used for the SASS re-processing (M. Kürster, private communication).
2.2. Iso-intensity contour maps
For the PSPC data, contour plots have been obtained from images that were the result of the superposition of sub-images with bin size in the 8 standard bands (R1 to R8, cf. Snowden et al. 1994), corrected for exposure, vignetting, and dead time and smoothed with a Gaussian filter having a FWHM corresponding to the on-axis point spread function (PSF) of that particular energy band. The FWHM values used range from to . The average background was calculated from a source free region to the north of NGC 3079.
For the HRI, contour plots have been obtained from images with 2:005 bin size which were corrected for dead time and smoothed with a FWHM Gaussian filter. To reduce the background due to UV emission or cosmic rays we used only those events detected in the HRI raw Pulse Height Amplitude channels 2-8.
The resulting images are discussed in Sect. 3.1.
2.3. Source detection
We performed source detection and position determination with the EXSAS local detect, map detect, and maximum likelihood algorithms (Zimmermann et al. 1992). Maximum likelihood values () can be converted into probabilities () through , so that corresponds to a Gaussian significance of about 3.6 and corresponds to a Gaussian significance of about 3.9 (Cruddace et al. 1988; Zimmermann et al. 1994).
Once the count rates of sources are obtained (see below for the details on how they have been obtained in the two instruments), they are converted into fluxes in the ROSAT band (0.1-2.4 keV) assuming a 5 keV thermal bremsstrahlung (cf. Table 2). To estimate possible errors in the X-ray luminosities due to the selection of a wrong model or temperature, conversion factors for a 0.5 keV thermal bremsstrahlung and for a 0.3 keV and a 3 keV thin thermal plasma spectrum have also been calculated. For this range of temperature and models, the resulting values change by 15%.
Table 2. Conversion factors from count rates to fluxes (0.1-2.4 keV) for the ROSAT HRI and PSPC detector (broad band) in units of erg cm-2 cts-1, corrected for Galactic absorption
Different considerations were applied to the HRI and the PSPC data to better suit the properties of these instruments.
© European Southern Observatory (ESO) 1998
Online publication: November 9, 1998