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

5. Summary

Deep ROSAT HRI and PSPC observations of the Seyfert II galaxy NGC 4258 are reported. The deep observations are used for a spectral analysis of different X-ray emission regions, the construction of a detailed point source catalogue and timing analysis of the point sources.

NGC 4258 is detected with a total X-ray luminosity of  erg s^-1 in the 0.1-2.4 keV ROSAT band. The emission components are made up of point-like sources in the bulge and disk of the galaxy ( erg s^-1), a `ridge' of high surface brightness, diffuse X-ray emission following the anomalous spiral arms of the galaxies, further diffuse X-ray emission from the bulge and inner disk of the galaxy, and an outer disk/halo component of the X-ray emission.

The diffuse emission of NGC 4258 can be decomposed into two thermal components, the superposition of an outer disk/halo component and a component attributed to the anomalous spiral arms. For the outer disk/halo component Galactic foreground absorption was assumed, and the spectral models then suggest a temperature of 0.2 keV and a luminosity of  erg s^-1. For the spiral arm component the derived is  cm^-2 and  keV, yielding a luminosity of  erg s^-1 (corrected for Galactic foreground only) or  erg s^-1 (corrected for total fitted absorption). A combination of the spectral capabilities of the PSPC and the spatial resolution of the HRI suggests a spectral hardening of the X-ray emission along the anomalous spiral arms. The findings can partly be explained by the existence of the halo component. However, the results also could support the ballistic picture of the anomalous arms.

The point sources have individual luminosities from  erg s^-1 to  erg s^-1, and for three sources time variability can be established. The light curves of the time variable sources and their maximum luminosities between  erg s^-1 and  erg s^-1 suggest the detection of X-ray binaries radiating at or above the Eddington limit of an accreting neutron star. The brightest source is probably correlated to a giant HII complex in the outer spiral arms of the galaxy, and with a maximum luminosity of  erg s^-1, this source is suggested to be a black hole binary with an accreting object in excess of .

No X-ray emission could be detected from SN 1981 K, which is located in a region of enhanced diffuse X-ray emission. From the HRI observations we derive a upper limit of  erg s^-1.

The point source population of NGC 4258 is compared to ROSAT results from 11 other nearby spiral galaxies; there is no evidence for a different source population in the case of the Seyfert galaxy NGC 4258.

A shadowing of the 0.1-0.4 keV X-ray background by the outer disk of NGC 4258 is detected. With the help of the ROSAT HRI measurements we estimate a lower limit of  cts s^-1 arcmin^{- 2} for the diffuse 0.1-0.4 keV background.

© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999
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