ASCA Observation of NGC 1808

The Japanese X-ray satellite ASCA observed NGC 1808, which is known to exhibit both Seyfert and starburst activities. In the soft X-ray band, ASCA detected emission lines from highly ionized Mg and Si, which confirmed the presence of a thin thermal plasma, due possibly to starburst activity. The thin thermal plasma consists of two temperature components of kT ~ 0.78 keV and kT ~ 0.35 keV. We found that the intrinsic absorption toward the 0.78 keV plasma is N_H ~ 7times 10(21) cm(-2) , which is greater than the galactic foreground value of 2.8times 10(20) cm^ {-2} (Dickey, Lockman 1990, AAA 53.155.163). This result implies that most of the X-ray emission from this plasma comes from the central region. The absorption toward the 0.35 keV plasma, on the other hand, shows a low value compared to the galactic absorption. This indicates that the plasma is more extended like an X-ray halo. The metal abundance of about 1 solar value found in the thin thermal plasma is larger than that of the archetypical starburst galaxy M82. In addition to the thin hot plasma, ASCA detected a hard X-ray component from NGC 1808, with a luminosity of 2times 10(40) erg s(-1) (0.5--10 keV band), obscured by an absorbing column of 1.0times 10(22) cm(-2) . The hard X-ray flux and spectrum may also be attributable to starburst activity. However, the hard X-rays exhibited a long-term variability from the Ginga and ASCA observations, suggesting the presence of a low-luminosity AGN in NGC 1808.