X-ray Observations of LINER and Starburst Galaxies

Abstract

We present the results of ASCA observations of a heterogenous sample of 15 spiral galaxies. 8 are LINERs or low-luminosity AGN (LLAGN), 5 are starburst galaxies and 2 are normal spiral galaxies. We find that in all cases the ASCA spectra can be described by a canonical model consisting of a power-law with a photon index ~ 1.7-2.0, plus a soft optically thin emission component with kT ~ 0.6-0.8 keV. The implied element abundances are often sub-solar. The soft component is usually extended and the nuclear, point-like emission is sometimes absorbed by column densities in the range log nH ~ 21-23 cm-2. The relative luminosities of the soft and hard components vary from galaxy to galaxy. For the LINERs, the 2-10 keV luminosity of the hard component is typically log L ~ 40-41 ergs/s whereas the 0.5-2.0 keV luminosity of the soft component is typically log L ~ 39-40 ergs/s. For starbursts, the 2-10 keV luminosity of is log L ~ 39-40 ergs/s, somewhat lower than the corresponding luminosity of most of the LINERs in our sample. The hard component is similar to the observed X-ray spectra of quasars and also to the intrinsic X-ray spectra of classical Seyfert galaxies. Most of the galaxies in our sample exhibit no significant (dI/I > 20%) short-term variability (with timescales of a day or less) whereas long-term variability is common. We present a case study of the LINER M81 in detail where there is evidence of large-amplitude (dI/I ~ 70%) variability over several weeks. There is also clear evidence for a broad, complex Fe-K emission line which is compatible with an origin in an accretion disk viewed at ~ 40 deg. These results suggest a strong connection between classical AGN, LINERs, and starburst galaxies.