1995A&A...294....8J

We present results from ROSAT PSPC observations of the nuclear starburst galaxy NGC 1808. The bulk of the X-ray emission comes from the (unresolved) nucleus of NGC 1808. The X-ray spectrum of this galaxy shows almost complete absorption below 0.5keV, indicating a high intrinsic hydrogen column density towards that source (8x10²¹cm^–2 in comparison to 3x10²⁰cm^–2 for the Galactic foreground). This is consistent with results from high-resolution HI observations of NGC 1808, and also from extinction studies comparing hydrogen recombination lines. Model fits for the X-ray spectrum of NGC 1808 indicate the dominance of emission of hot thermal gas with a plasma temperature of ≃0.5keV or 6x10⁶K, tracing the existence of supernova remnants (SNRs) or hot bubbles in the central starburst of NGC 1808. The total X-ray luminosity of 1.4x10⁴¹erg/s can be explained by a supernova rate of ≃0.09yr^–1 and a star-formation rate (SFR) of ≃10M_☉/yr in the nuclear starburst, in agreement with results from optical and FIR observations. The PSPC data also show X-ray emission from the disk of NGC 1808, comprising ≃7% of the total emission, which is probably associated with HII regions reaching out to 3kpc from the nucleus in the disk of NGC 1808. Despite the indication of outflow activity in NGC 1808 (optical dust lanes), no extended X-ray halo is detected in the ROSAT PSPC data. The comparison of ROSAT soft X-ray observations with data from the Ginga satellite at higher energies indicates the existence of a compact nuclear source (Seyfert core) besides the nuclear starburst from their different spectral characteristics. Altogether 19 X-ray sources are found in the 2deg PSPC field, including NGC 1808 and NGC 1792, a galaxy possibly interacting with NGC 1808. In addition, we identify another strong X-ray source in the field with an optically faint object which is likely to be a Seyfert galaxy on the basis of its spectrum. In case of NGC 1792, the major contribution to the X-ray spectrum is explained by a population of massive X-ray binaries.