2000A&A...360...24P

Spatial and spectral analysis of deep ROSAT HRI and PSPC observations of the near edge-on starburst galaxy NGC 253 reveal diffuse soft X-ray emission, which contributes 80% to its total X-ray luminosity (L_X=5x10³⁹erg/s, corrected for foreground absorption). The nuclear area, disk, and halo contribution to the luminosity is about equal. The starburst nucleus itself is highly absorbed and not visible in the ROSAT band. The emission from the nuclear area stems from a heavily absorbed source with an extent of 250pc (FWHM) about 100pc above the nucleus along the SE minor axis (``nuclear source", X34), and the ``X-ray plume". The nuclear source is best described as having a thermal bremsstrahlung spectrum with a temperature of T=1.2keV (N_H=3x10²¹cm^–2) and L_X^exgal=3x10³⁸erg/s (corrected for Galactic foreground absorption). The spectrum of the hollow-cone shaped plume (opening angle of 32° and extent of ∼700pc along the SE minor axis) is best modeled by a composite of a thermal bremsstrahlung (N_H=3x10²⁰cm^–2, T=1.2keV, L_X^exgal=4.6x10³⁸erg/s) and a thin thermal plasma (Galactic foreground absorption, T=0.33keV, L_X^exgal=4x10³⁸erg/s). The diffuse nuclear emission components trace interactions between the galactic super-wind emitted by the starburst nucleus, and the dense interstellar medium of the disk. Diffuse emission from the disk is heavily absorbed and follows the spiral structure. It can be described by a thin thermal plasma spectrum (T = 0.7keV, intrinsic luminosity L_X^intr = 1.2x10³⁹ erg.s^–1), and most likely reflects a mixture of sources (X-ray binaries, supernova remnants, and emission from HII regions) and the hot interstellar medium. The surface brightness profile reveals a bright inner and a fainter outer component along the major axis with extents of ±3.4kpc and ±7.5kpc. We analysed the total halo emission separated into two geometrical areas; the ``corona" (scale height ∼1kpc) and the ``outer halo". The coronal emission (T=0.2keV, L_X^intr=7.8x10³⁸erg/s) is only detected from the near side of the disk (in the SE), emission from the back (in the NW) is shadowed by the intervening interstellar medium unambiguously determining the orientation of NGC 253 in space. In the NW we see the near edge of the disk is seen, but the far component of the halo, and vice versa in the SE. The emission in the outer halo can be traced to projected distances from the disk of 9 kpc, and shows a horn-like structure. Luminosities are higher (10 and 5x10³⁸erg/s, respectively) and spectra harder in the NW halo than in the SE. The emission in the corona and outer halo is most likely caused by a strong galactic wind emanating from the starburst nucleus. As an additional contribution to the coronal emission floating on the disk like a spectacle-glass, we propose hot gas fueled from galactic fountains originating within the boiling star-forming disk. A two temperature thermal plasma model with temperatures of 0.13 and 0.62keV or a thin thermal plasma model with temperature of 0.15keV and Gaussian components above ∼0.7keV and Galactic foreground absorption are needed to arrive at acceptable fits for the NW halo. This may be explained by starburst-driven super-winds or by effects of a non-equilibrium cooling function in a plasma expanding in fountains or winds. We compare our results to observations at other wavelengths and from other galaxies.