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.