BGU Physics Department

Colloquium, Jan 2nd, 2014

Distance to accretion discs; theory versus observation

Jean-Pierre Lasota, Institut d’Astrophysique de Paris & Nicolaus Copernicus Astronomical Center, Warsaw

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 the 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. Since in astronomy one measures fluxes the luminosity of a
radiating body is deduced from its distance. That is why determining distances to celestial objects and systems has been one of the basic
goals of astronomy. In 1999 Hubble Space Telescope 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 (310 million light years) whereas the outburst models require it to be much closer to us, maybe even in our
Galaxy. To conclude I will briefly discuss some psychological, sociological and philosophical aspects of these (apparent) conflicts
between theory and observations.