1994A&A...284..386P

The highly inclined spiral galaxy NGC 4258 (M106) has been observed in X-rays with the PSPC of the Roentgen observatory ROSAT. Following X-ray components have been identified in the central 40x40' field: (1) several pointlike X-ray sources, some of which can be identified as blue stellar objects and/or coincide with compact radio sources. Two of them are 18' apart, symmetrically aligned, and equidistant to the nucleus of NGC 4258 with a position angle connecting the tips of the anomalous arms. (2) extended (∼3') X-ray emission from an uncataloged cluster of galaxies with gas at temperature >20 million K. (3) emission from UGC 7356, a dwarf companion of NGC 4258 with a X-ray luminosity of 1.3x10³⁸ erg/s . (4) complex emission from NGC 4258 with total X-ray luminosity of 5.6x10³⁹ erg/s. The X-ray emission from the outer regions of NGC 4258 can be separated into 3 components: -seven pointlike sources in the spiral arms of NGC 4258 with total X-ray luminosity of 6x10³⁸ erg/s. -extended emission from a 3.6 million K gas in the halo of NGC 4258. On the east side of the galaxy the emission is detected in the soft and hard band, on the west side only in the hard band. This can be explained by shadowing of the X-ray emission of the halo gas by cool gas in the disk of NGC 4258, that has been observed in Hi. The luminosity of this emission is 5.1x10³⁹ erg/s. -hard X-ray emission specifically in the northwest side of the galaxy is strongly enhanced where the anomalous spiral arms have been detected in Hα and radio. The soft halo gas emission has been modeled successfully by an isothermal gas sphere with a constant density profile or, even better, with barometrically stacked shells. The diffuse halo emission and the hard X-ray emission connected with the anomalous arms strongly argue for a ballistic interpretation for the origin of these arms. While the diffuse hard X-ray emission in the northwest follows closely the anomalous arm contours and the plateau, X-ray emission corresponding to the anomalous arm in the southwest is more concealed by the halo emission. A search for X-ray emission from SN1980k in NGC 4258 and for residual X-ray emission from the outer disk of NGC 4258 was not successful. The Seyfert 1.9 nucleus of NGC 4258 is not detected as a point source, as it is likely to be heavily absorbed. The X-ray properties of NGC 4258 as a Seyfert 2 galaxy are discussed in the light of the unified AGN scheme.