Title: Distance to accretion discs; theory versus observation

Speaker: Jean-Pierre Lasota-Hirszowicz (IAP, Paris) 

Time & Place: Thursday, 3:00pm, February 20th, Lecture Hall, 3rd floor

Abstract: Accretion discs around compact objects such as white dwarfs, neutron stars and black holes are ubiquitous and have been intensely studied in last 40 years. Their basic properties are quite well understood. In particular the instability which drives disc outbursts observed in binary systems containing white dwarfs (« dwarf  novae »), neutron stars and black holes (« X-ray transients ») has been clearly identified.This (thermal-viscous) instability is supposed to be present in accretion discs below some critical luminosity and for a long time this had been confirmed by observations:bright disc-containing systems never exhibit outbursts. In 1999 Hubble Space Telescope FGS observations of the famous dwarf nova SS Cygni put this system at a distance at which according to theory it was too bright to have outbursts. I will describe the subsequent « fight » between theory and observations and its (happy for the  model) outcome. Next, I will present the recent case of the outbursting black hole system known as HLX-1 which observations put in a galaxy at 95 megaparsecs whereas the outburst models require it to be much closer to us, maybe even in our Galaxy. I will discuss observations that could solve this distance problem.

Biog: Born in 1942, Jean-Pierre defended his doctoral thesis in 1971 at the Institute for Theoretical Physics, University of Warsaw. Recruited in 1983 by the CNRS, he was director of the Department of Relativistic Astrophysics and Cosmology at the Observatoire de Paris from 1987 to 1999, and since 2003 he is Head of Mission to the INSU. He is currently emeritus research director at IAP of Paris.