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.