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Session 90 - High Energy Phenomena in AGNs.
Oral session, Friday, January 09
The results of ASCA (0.4-10.0 keV) and ROSAT (0.1-2.2 keV) observations of a sample of low-luminosity AGN (M51, NGC 4258, NGC 3147), LINERs (NGC 3998, NGC 4579, NGC 4594, NGC 3079, NGC 3310), and starburst galaxies (M82, NGC 253, NGC 3628) show that although these galaxies have heterogeneous optical classifications, their X-ray spectra can be characterized by a "canonical" model. The X-ray flux above 2.5 keV is often fit well with an absorbed (typically Nh 10^22 cm^-2), featureless continuum that can be modeled by either a power-law with an energy index of 0.8-1.0 or a thermal bremsstrahlung with T \sim 10^8 K. The hard component, with a 2-10 keV luminosity of 10^40-41 ergs s-1, is similar to the hard X-ray spectra observed in "normal" AGN. However, large-amplitude ( \delta I/I > 1) short- term (i.e., time scales of hours) variability, observed in "normal" AGN is not detected in these sources. X-ray variability in AGN tends to increase with decreasing luminosity, so if the hard component is due to a micro- AGN, then either the hard X-ray flux is scattered by pc-scale gas or the AGN is in a different mode of accretion, a scenario suggested by the very low Eddington luminosities of these sources. The emission below 2.5 keV, usually dominated by gas within the disk or nuclear region (i.e., central few kpc) of the galaxy, is fit well with a T \sim10^7 K component. The 10^7 K gas is most likely ISM heated by a starburst or AGN-driven outflow. In cases where the statistics are sufficient for the abundances to be determined, the ratio of alpha-process elemental abundance to Fe abundance is larger than expected from simple models, possibly due to Type-II SN enrichment and/or dust depletion.
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