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

6. Summary of results

The spatial analysis on the MECS data indicates that the distribution of the hard emission in the four galaxies observed with BeppoSAX is different in different cases: NGC 1553 is clearly extended, while NGC 3379 is most likely dominated by a nuclear source. A significant contribution from a point source cannot be excluded by the data of NGC 3115, although a more extended component is also observed. The data of NGC 4125 are not conclusive.

For three sources (NGC 1407, NGC 1553 and NGC 4125) the data require a two component spectrum, with a soft and a hard component, although neither can be determined with high accuracy. The soft component is well parameterized by a thin plasma spectrum, with kT keV, and in all three cases its contribution is comparable or higher than that of the hard component in the soft energy band.

At hard energies, the extended nature of the emission in NGC 1553 ensures that a nuclear source, if present, does not contribute significantly to the observed luminosity. Both NGC 3115 and NGC 3379 data instead could be parameterized by a power law spectrum, consistent with a significant contribution from a nuclear source.

An intriguing result is provided by the lack of a very soft component in NGC 3115 and NGC 3379, commonly found in the lowest galaxies (see discussion in Sect. 1). While the poor statistics does not allow us to speculate on whether this is a "significant" result, in the sense of singling out these two galaxies for their lack of a very soft component, the existence of a nuclear component probably further complicates matters. In fact, the soft band ROSAT data of either galaxies do not show a significant contribution from extended components: the galaxies appear rather compact both in the PSPC image of NGC 3115 and in the HRI image of NGC 3379 (cf. Robert & Warwick 2000), suggesting that the nuclear source dominates at all energies. We have estimated the contribution from a soft component allowed by the present data. As discussed before, one component is adequate, and in fact the addition of a second component to either set of data does not give any additional information: the second component either mimics the parameters of the first component or has a zero normalization. To estimate a possible contribution from a soft component, we have therefore fixed the power law parameters at the best fit values and added a raymond with kT = 0.3 keV. We find that in either cases a soft component is allowed, and would have an unabsorbed flux erg cm^-2 s^-1 (NGC 3115 and NGC 3379 respectively). We will discuss this further in Sect. 9.

© European Southern Observatory (ESO) 2000

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