1996A&A...305...74V

The edge-on spiral galaxies NGC 4565, NGC 4656, and NGC 5907 have been observed with the PSPC onboard ROSAT. The targets were selected for low galactic foreground absorption to enable the detection of "soft" X-ray emission, which is a fingerprint for the emission of a million degree gaseous component. For NGC 4565 we find an X-ray luminosity of 1.1x10⁴⁰erg/s. Point sources contribute 6.0x10³⁹erg/s in the bulge and 0.7x10³⁹erg/s in the disk. 1.2x10³⁹erg/s may be due to diffuse X-ray emission in the bulge or disk. A nuclear source (L_x=2.7x10³⁹erg/s) is visible. In the halo of NGC 4565 diffuse emission with a luminosity of 2.6x10³⁹erg/s was detected. The X-ray data indicate low absorption and soft spectra. Attributing the entire diffuse flux to emission of a hot thermal plasma, its density, mass, and cooling time are n=0.6x10^–3cm^–3/sqrt(η), m=2.2x10⁷M_☉xsqrt(η), and τ=4.2x10⁸yxsqrt(η), respectively (with the filling factor η). The disturbed galaxy NGC 4656 shows a luminosity of 1.5x10³⁹erg/s, composed of a weak nuclear point source (L_x=1.3x10³⁸erg/s), five further point sources in the disk (L_x=1.0x10³⁹erg/s), and remaining diffuse emission components (L_x=0.3x10³⁹erg/s). No diffuse emission could be established in the halo of NGC 4656. NGC 5907 has an overall X-ray luminosity of 6.5x10³⁹erg/s. The only point source in the bulge coincides with the nucleus (L_x=2.8x10³⁹erg/s), three point sources in the disk show an integral luminosity of 3.2x10³⁹erg/s. Neither X-ray emission from SN 1940a in NGC 5907 nor diffuse halo emission could be found. The different X-ray components for the three galaxies are discussed and compared to results from other wavelength regimes.