ASCA Observation of NGC 1808

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Abstract
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 NH ∼ 7 × 1021 cm−2, which is greater than the galactic foreground value of 2.8 × 1020 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 2×1040 erg s−1 (0.5–10 keV band), obscured by an absorbing column of 1.0×1022 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.

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