Thursday, 5 March, 1998 at 1pm (Note new time!!!!)

Liz Puchnarewicz - Mullard Space Science Laboratory

"Investigating the hottest, strongest accretion discs in Seyferts and quasars"

ABSTRACT:"About half of active galactic nuclei show a soft X-ray `excess' above the hard X-ray power-law. One in ten have extreme `ultrasoft components', ie. a very steep and soft component which is largely confined to energies below ~0.5keV. This component is believed to be emitted from the inner edge of an accretion disc and may be the high energy tail of the optical UV `big blue bump'. Until recently, it was found that all ultrasoft AGN, irrespective of source luminosity, black hole mass or accretion rate, had relatively low velocity broad line regions. Whether this is due to some third parameter such as orientation or black hole mass, or whether there is a direct causal effect, remains unclear, but recent X-ray data on three AGN with ultrasoft components and relatively broad lines provide important new clues. We have also extended our investigations to high redshifts, searching the ROSAT archive for the hottest and most luminous ultrasoft excesses identified to date. The results from this search are not easily predicted by any of the most popular models but do place strong constraints on the basic physics of accretion in AGN at the high-temperature extreme."