Astron. Astrophys. 364, 53-69 (2000)

4. ASCA data

We have used the "screened" data produced by the REV2 version of the processing. Both sets of focal plane instruments were used (see Table 2). GIS data were obtained in PH mode, and SIS data in BRIGHT mode. Full references to the ASCA instruments and observing modes can be found in the on-line documents at http://legacy.gsfc.nasa.gov/docs/asca . Since calibration is poor for energies below 0.5 keV (SIS) and 0.8 keV (GIS), and above 5 keV for SIS, we have restricted the analysis to energies between 0.5-5 keV and 0.8-10 keV for SIS and GIS respectively.

Each galaxy is discussed briefly below, and the results from the spectral analysis are given in Sect. 5 and tabulated in Table 3.

Table 3. Spectral results.
Note: The models assumed are: raymond (R) with abundances fixed at 100% cosmic values unless otherwise indicated; bremsstrahlung (B); power law (P). The 90% confidence interval for temperature(s) or photon index are for 1 interesting parameter ( they are derived from the +2.7 level) and are calculated only for 1. The N_H is not well defined by these data. We have fixed it at the line-of-sight galactic value.
) The best fit gives abundances of 10% cosmic.
) The best fit gives abundances of 0.1% cosmic.
) We find two minima of almost equal significance, well outside the 68% error contours. We have not derived errors for these parameters.

4.1. NGC 1407

Both Einstein and ROSAT observations of this galaxy show the presence of additional, possibly unrelated, fainter sources around the galaxy, which are also visible in the ASCA images (Fig. 5). Moreover this galaxy is located within a group, so low surface brightness emission from intergalactic gas is expected. The radial photon distribution indicates a slowly decreasing surface brightness emission almost out to the edge of the field (the emission flattens only towards the NE edge off the field, see Fig. 9). We have carefully considered which is the best region to study the emission from this source. In the GIS we have defined a circle of 6´ radius, that corresponds to a flattening of the surface brightness profile before it further decreases, and is also the recommended size for inclusion of all the flux from a strong point source. With this choice, we can retain a high signal-to-noise and obtain the background locally in an adjacent region that should include the contribution from the group, which we can subtract from the galaxy's emission. We have also considered a "field" background by choosing a small region in the NE corner of the detector, where the profile shows that the surface brightness is constant with radius. This background was also used to study the emission from the region outside the galaxy ( the local background considered above).

Fig. 9. Radial distribution of the total emission observed with GIS in the 0.8-10 keV energy range. LEFT: data for NGC 1407. In order to avoid regions outside the field of view, only angles -90o to 180o are averaged together at radii , and angles -30o to 100o outside of r (angles are counterclockwise from N). RIGHT: data for NGC 4125. The data are azimuthally averaged.

For both SIS, we have used a radius of , the maximum that is allowed by the offaxis position of the source in the detector, that avoids including areas outside of the field of view in the source region. As for the GIS, the background is estimated locally around the source, in a not-concentric annulus as large as it was compatible with the detector size.

While it is true that the `unrelated' sources are included in the background regions, their contribution should not be in excess of 5%, so that they will not significantly affect the background characteristics.

4.2. NGC 4125

As shown by the profile in Fig. 9, source counts are visible out to a radius of , then the profile appears to flatten, although a second plateau is visible outside . The source is relatively centered on the GIS field of view, but not in the SIS. We have therefore used a radius circle for the GIS, and chosen the background from a concentric annulus around the source. In the SIS, we have used the maximum radius, of , to avoid regions outside the CCD field of view, and a local background from two regions close to the source.

© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000
helpdesk.link@springer.de