Astron. Astrophys. 352, 64-84 (1999)

2. Observations and data analysis

2.1. Observations

In total, NGC 4258 was observed with the ROSAT HRI and PSPC for 54.8 ks and 32.9 ks, respectively, and this publication presents the analysis of all ROSAT data available for NGC 4258 with the exception of an observation pointed 7´ east of the center of the galaxy, for which the attitude solution failed. A description of the satellite and the detectors onboard can be found in Trümper (1983) and Pfeffermann et al. (1987).

The ROSAT observations were carried out in 9 observations blocks each subdivided into several observation intervals (OBIs). The 3 PSPC observation blocks and the 6 HRI observations blocks (see Table 1), were spread over years.

Data were retrieved from the ROSAT data archive at the Max-Planck-Institut für Extraterrestrische Physik, the data reduction was performed with the ESO-MIDAS/EXSAS (ESO-MIDAS 1997, Zimmermann et al. 1997) software package.

2.2. Attitude corrections

Attitude solutions of ROSAT pointings used by the Standard Analysis Software System (SASS, Voges et al. 1992) to calculate event files are known to produce residual errors of the order of 6" (boresight error) for the OBIs, and - due to short term fluctuations - systematically broaden the PSF. To improve the solution we adopted two subsequent techniques.

For the PSPC observations, pointed towards the nucleus of NGC 4258, the positions of point sources in the PSPC images taken at different epochs (cf. Table 1) were calculated, and the observations were aligned. For the HRI, to improve the intrinsically narrower PSF compared to the PSPC, all OBIs of block 2 and 3 (both blocks pointed towards the center of NGC 4258) were aligned with respect to the first OBI of block 2, all OBIs of block 6-9 (pointing direction , , approximately 7´ west of the center of NGC 4258) with respect to the first OBI of block 6. For this purpose, the centroid of 7 bright point sources visible in each OBI was traced for blocks 2 and 3, the centroid of 5 bright sources for blocks 6-9. After the individual pointings, blocks 2+3 and 6-9, were corrected, the positions of bright point sources visible in both pointings were used to calculate a transformation between the observations, and the pointings were merged.

In a second step, the HRI and PSPC positions of point sources (X2, X3, X4 and X24; cf. Table 2) were compared to the positions of optical counterparts retrieved from the APM finding charts (Irwin et al. 1994), and the proposed ROSAT attitude solutions were transformed into our improved sky coordinate system. For the HRI, a translation of 3:002 and 0:003 to the east and south, respectively, and a clockwise rotation of 0.6^o were applied. For the PSPC only a translation (9:003 to the west and 29:005 to the south) was necessary, which, due to a wrong identification of a guidestar pattern, clearly exceeds the expectations for typical ROSAT boresight errors, and this problem already was reported for the first observation block of the PSPC (Paper I). The residual errors of the transformations between the X-ray positions and sky positions for both detectors are , taken into account as additional systematical error of the positions in the source catalogues.

Table 2. X-ray properties of detected point sources

2.3. Image generation

2.3.1. Images of the HRI observations

To reduce the background due to UV emission and cosmic rays, HRI images were integrated using raw channels 2-8. We did not apply an additional screening of times with high cosmic background radiation, as this did not change the diffuse emission structures, while the signal-to-noise-ratio dropped. A image (0.1-2.4 keV band) was constructed with a bin size of 2:005 and smoothed with a Gaussian filter of 12" FWHM (Fig. 1). The optical extent of the galaxy is indicated by an ellipse of as suggested by deep optical plates kindly provided by H. Arp (1994). To show the detailed structure of the central region, an additional image of this region was calculated with a bin size of 1" and smoothed with a Gaussian filter of FWHM (Fig. 2).

Fig. 1. ROSAT HRI image of the NGC 4258 pointing for the inner field of view. The image was formed with a bin size of 2:005 and smoothed with a Gaussian filter of FWHM. The grey-scale representation runs several times from bright to dark to make the full dynamical range of the data visible. The X-ray sources (cf. Table 2) which were detected with the HRI and PSPC are marked with squares, those only detected with the PSPC with diamonds (no sources were detected with the HRI only). The optical extent of NGC 4258 ( ellipse) is outlined, the center of the galaxy coincides with the central source. The central source (X18) and a source south of X18 (X14) are not enumerated. The right ascension and declination are given for J2000

Fig. 2. ROSAT HRI image of the inner arcmin region of NGC 4258. The image was formed with a bin size of 1" and smoothed with a Gaussian filter of FWHM. The grey-scale representation runs several times from bright to dark to make the full dynamical range of the data visible. Sources are marked according to Fig. 1 and Table 2, the central source X18 is not enumerated. The position of the supernova 1981 K north of nucleus is marked with a cross

2.3.2. PSPC broad band image

A 0.1-2.4 keV (channels 11-235) PSPC image (Fig. 3) for the same field of view as Fig. 1 was constructed by the superposition of sub-images in 8 standard bands (R1 to R8, cf. Snowden et al. 1994). Each sub-image was corrected for exposure, vignetting and deadtime and smoothed with a Gaussian filter corresponding to the on-axis PSF of the energy band (FWHM ranging from 52" to 24" for the lowest to the highest energy band). Similar to the analysis of the HRI images, the influence of an additional screening of times with high cosmic background contamination was considered unnecessary as such a screening had negligible influences on the images, the count rates calculated from the images and the spectra taken for different test regions.

Fig. 3. ROSAT PSPC broad band contours of the NGC 4258 pointing for the same field as presented in Fig. 1. The contours were calculated from an image of bin size of and smoothed as described in Sect. 2.3.2, the given levels are 2, 4, 8, 16, 32, 64 and  cts s arcmin-2 above the background ( cts s arcmin-2). The optical extent of NGC 4258 ( ellipse) is outlined, sources are marked according to Fig. 1 and Table 2

2.4. Constructing a combined HRI/PSPC point source catalogue

2.4.1. HRI point source detection

Point sources were searched for with the EXSAS local detect, map detect and maximum likelihood algorithms (Zimmermann et al. 1997), using an image of pixel size 5" (raw channels 2-8, cf. Sect. 2.3.1). Sources with a detection likelihood were accepted. Maximum likelihood values (L) can be converted into probabilities (P) through , thus corresponds to 3.6 Gaussian sigma significance (cf., e.g., Cruddace et al. 1988).

2.4.2. PSPC point source detection

Source positions and count rates were calculated in the five standard energy bands: `broad' (0.11-2.40 keV), `soft' (0.11-0.41 keV), `hard' (0.52-2.01 keV), `hard1' (0.52-0.90 keV), and `hard2' (0.91-2.01 keV). All images used for the source detection had a bin size of 5". As with the HRI, the EXSAS local detect, map detect, and maximum likelihood algorithms were applied to each energy band. Sources with a likelihood 9 () were accepted, and the source lists within each of the different energy bands merged, assuming that detected source positions were identical if their separation was less than 3 times the statistical position error. The final source position was taken from the energy band in which the source was found with the highest likelihood.

2.4.3. Combined HRI/PSPC point source catalogue

From the HRI and PSPC catalogues, a combined point source catalogue (Table 2) was constructed. Column 1 gives the source numbers, that are used to identify sources in the figures. The R XJ name (following the naming convention for ROSAT sources according to Zimmermann et al. 1997) is contained in Column 2. Columns 3 and 4 give the source positions (right ascension and declination, equinox 2000). Sources with spatial separations between the HRI and PSPC positions smaller than the sum of the 90% error radii were taken as identical (the `detector flag' in Column 5 has the entry `B' for both). Sources have the entry confused (`C') in Column 6 if the PSPC hard band image shows extended emission surrounding the source, or the HRI image resolves two point sources separated by a distance smaller than the PSPC hard band PSF. If a source existed only in the PSPC source list, an entry `P' (PSPC) is given in Column 6, and there was no HRI suggested source which was not visible in the PSPC image. For `B' sources located outside the central 6´ diameter region, the position information with the smaller statistical position error was selected. Inside the central 6´ diameter region, which is filled with diffuse X-ray emission, the HRI suggested source positions were used. The position errors, including a 3" systematical error, are given in Column 7. The likelihood for the source detections (Columns 8, 9), net counts (Columns 10, 11) and count rates (Columns 12, 13, corrected for exposure, deadtime and vignetting) are given for the HRI and PSPC, respectively. PSPC counts for non-confused sources were determined using the source detection results. For confused PSPC sources with nearby point sources (separation below the FWHM of the hard band PSF), a `multi source fit' technique (cf. Zimmermann et al. 1997) was used with source positions fixed according to the HRI detections. In the case of HRI non-detections at PSPC source positions, HRI counts were calculated at the position suggested by the PSPC. To do so, HRI counts were extracted with a cut radius of 1.5 times the local FWHM of the HRI PSF at the PSPC source position, and background counts subtracted determined at that position in the EXSAS background maps. For sources with detection likelihoods , upper limits, at a 2 confidence level, are given.

The energy resolution of the PSPC detector allows the calculation of `X-ray colors' for the X-ray sources, the hardness ratios HR1 and HR2 (Columns 14, 15). The ratios are defined on the basis of the net counts in the soft, hard, hard1, and hard2 bands. By definition, HR1 = (hard-soft)/(hard+soft), and HR2 = (hard2-hard1)/(hard2+hard1), soft here means the counts in the soft band etc. A brief discussion of the physical implications of the hardness ratios is given, e.g., in Pietsch et al. (1998). In cases, where the error exceeds the counts in one sub-band, only an upper (lower) limit of the hardness ratio is calculated. For that, the counts of the non-detected band are chosen to be equal to the upper limit (). Where the errors exceed the counts in both sub-bands, no hardness ratios are calculated. For confused sources, HR1 is not calculated, since problems might arise due to the large extraction radius needed for the PSPC soft band.

2.5. Time variability investigations

Time variability investigations were performed calculating the counts and count rates of the individual sources (with fixed positions according to Table 2) for the individual observation blocks (cf. Table 1). For the PSPC count rates of confused sources, the same technique was used as mentioned in Sect. 2.4.3. The `likelihood ratio test' (cf., e.g., Cash 1979, Hogg & Tanis 1983) was used to test for variability. Since the comparison of HRI and PSPC count rates depends on the assumed spectral model, the HRI and PSPC observations were analyzed separately.

© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999
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