NGC-1808 - X-RAY-EMISSION FROM THE NUCLEAR STARBURST

We present results from ROSAT PSPC observations of the nuclear starbur st galaxy NGC 1808. The bulk of the X-ray emission comes from the (unr esolved) nucleus of NGC 1808. The X-ray spectrum of this galaxy shows almost complete absorption below 0.5 keV, indicating a high intrinsic hydrogen column density towards that source (8 x 10(21) cm(-2) in comp arison to 3 x 10(20) cm(-2) for the Galactic foreground). This is cons istent with results from high-resolution HI observations of NGC 1808, and also from extinction studies comparing hydrogen recombination line s. Model fits for the X-ray spectrum of NGC 1808 indicate the dominanc e of emission of hot thermal gas with a plasma temperature of approxim ate to 0.5 keV or 6 x 10(6) K, tracing the existence of supernova remn ants (SNRs) or hot bubbles in the central starburst of NGC 1808. The t otal X-ray luminosity of 1.4 x 10(41) erg s(-1) can be explained by a supernova rate of approximate to 0.09 yr(-1) and a star-formation rate (SFR) of approximate to; 10 M.yr(-1) in the nuclear starburst, in agr eement with results from optical and FIR observations. The PSPC data a lso show X-ray emission from the disk of NGC 1808, comprising approxim ate to 7% of the total emission, which is probably associated with HII regions reaching out to 3 kpc from the nucleus in the disk of NGC 180 8. Despite the indication of outflow activity in NGC 1808 (optical dus t lanes), no extended X-ray halo is detected in the ROSAT PSPC data. T he comparison of ROSAT soft X-ray observations with data from the Ging a satellite at higher energies indicates the existence of a compact nu clear source (Seyfert core) besides the nuclear starburst from their d ifferent spectral characteristics. Altogether 19 X-ray sources are fou nd in the 2 degrees PSPC field, including NGC 1808 and NGC 1792, a gal axy possibly interacting with NGC 1808. In addition, we identify anoth er strong X-ray source in the field with an optically faint object whi ch is likely to be a Seyfert galaxy on the basis of its spectrum. In c ase of NGC 1792, the major contribution to the X-ray spectrum is expla ined by a population of massive X-ray binaries.