1999A&A...352...64V

Spatial, spectral, and timing analysis of deep ROSAT HRI and PSPC observations of the active spiral galaxy NGC 4258 are reported. The diffuse emission of NGC 4258 has a 0.1-2.4keV luminosity of L_x∼2x10⁴⁰erg/s (corrected for Galactic foreground absorption) and fills ∼40% of the D₂₅ diameter. One half of the HRI count rate of this region is caused by a ridge of high surface brightness emission following the anomalous spiral arms of the galaxy. PSPC spectra of the diffuse emission suggest the superposition of two thermal emission components. A 0.2keV component can be attributed to gas in the outer disk and eastern halo hemisphere, a second component of higher absorption and temperature (0.5keV) most probably traces hot gas along the anomalous spiral arms. The point source catalogue of NGC 4258 contains 15 source candidates within the optical extent of the galaxy. At the position of the nucleus, the HRI results allow for a point-like source on top of diffuse emission, and, if indeed present, the luminosity of such a source is estimated to (9±4)x10³⁷erg/s. Excluding the nuclear region and two sources which were flagged as extended, the sources have luminosities between 7x10³⁷erg/s and 9x10³⁸erg/s, accounting for an integrated L_x of 2x10³⁹erg/s. A deficiency of the 0.1-0.4keV band emission in the outer disk of NGC 4258 is detected and can be explained by a shadowing of the diffuse soft band background, which originates from sources behind the disk of NGC 4258 and is absorbed by cold gas in the outer disk. A lower limit of 4.4x10^–4cts/s/arcmin² for the diffuse 0.1-0.4keV background is estimated. The new results are compared to previous ROSAT and ASCA observations of NGC 4258 as well as to results from other wavelengths and to other spiral galaxies.