Particle Physics Division

Accretion column onto highly magnetic white dwarfs

The Orion experiment [2] was set up to replicate the processes taking place in stellar binary systems containing a highly magnetized white dwarf that captures matter from a small mass secondary star. These systems called magnetic Cataclysmic Variables (mCVs) provide a unique insight to study accretion processes in extreme astrophysical regimes. Because of the very high (B > 10MG) magnetic field, matter captures by the white dwarf is channelled in a very narrow region above the white dwarf magnetic pole, called an accretion column with dimensions typically from 100 to 1000 kilometres.  Radiation collected from these objects mainly comes from this unresolved area near the white dwarf surface and is produced below an accretion shock. Understanding the physics of this radiation hydrodynamic region is primordial since it is the key to infer of the white dwarf properties and the initial conditions of supernovae explosion.

A tiny target in an extremely short time

Using specific scaling laws coupling radiation and hydrodynamic, the scientists were able to determine that to achieve an equivalent in the lab of an accretion column, all parameters have only to be shifted by the right amount in the space-time domain. What happens in a 1000 km accretion column in about one second would be equivalent to what takes place at millimetre size during a few nanoseconds, providing that enough energy is injected by a laser.

In the actual experiment, a 2 kiloJoule laser shot of 1 nanosecond duration was focused onto a 600-micrometre diameter target used to drive a hot plasma at a 200 km/s velocity. Instead of a magnetic field, the plasma was collimated by a plastic tube 3.mm long, ended by a steel obstacle. The laser-produced plasma, tube and obstacle formed the laboratory analogues of the incoming flow, magnetic field collimation and white dwarf surface, respectively, in the astrophysical case.
 
The main difficulty the scientists had to face was actually to see and record what happens in such a short time. To do this, they had to use another very short laser pulse (500 picoseconds) able to generate an X-ray source that probe the plasma in the tube. By measuring the varying transmission by the plasma, the density can then be reconstructed by this ultrafast X-ray radiography. With this complex experiment set-up, a time-sequenced image was produced showing clearly the building of the shock, analogous to the structure predicted by astrophysical models and numerical simulations.

It is the very first time that an accretion shock is directly observed with X- ray radiography in such regimes. The physical properties of the radiative post-shocked region were probed while this structure cannot be observed at the astrophysical scale with telescopes available today. Though strong limitations still exist; the Orion experiment confirms that these complex radiation hydrodynamic flows, which could not be studied in laboratory a few years ago, can now be produced commonly in laboratory. These experiments were first initiated at the LULI200 facility (Laboratoire pour l’Utilisation des Lasers Intenses). The next step is to adapt the target design to the more powerful Megajoule lasers (NIF and LMJ) [3] which provides the laser energy needed to produce a radiative regime similar to the accretion columns of the magnetic cataclysmic variables.

Contacts : J.M. Bonnet-Bidaud, 

Publications :

Publication
“Laboratory analogue of a supersonic accretion column in a binary star system”
J. E. Cross, G. Gregori, J. M. Foster,    P. Graham, J. -M. Bonnet-Bidaud, C. Busschaert,    N. Charpentier, C. N. Danson, H. W. Doyle,    R. P. Drake, J. Fyrth, E. T. Gumbrell,    M. Koenig, C. Krauland, C. C. Kuranz, B. Loupias, C. Michaut, M. Mouchet, S. Patankar, J. Skidmore, C. Spindloe, E. R. Tubman, N. Woolsey, R. Yurchak & É. Falize.,
Nature Communications, 13 June 2016, doi:10.1038/ncomms11899, for an electronic version : Cross_2016_Nature_Communications.pdf

see          - the CEA-CNRS Press Release (13 juin 2016)
               - the CNRS Press release (13 juin 2016)
               -the AWE (Atomic Weapons Establishment) press release) (13 juin 2016)

See also         : Une étoile en laboratoire - Le Monde.fr (12 sept 2016 - in French)

See also         : Instabilities at the surface of magnetic white dwarfs (22 June 2015)

see the numerical simulation of the accretion column (video)

Notes :

[1] 1 nanosecond = 1 billionth of a second. There are as many nanoseconds in one second as they are seconds in about 30 years.

[2] The Orion world-leading laser facility provides a platform for AWE’s plasma physics experiments. Orion delivers a unique combination of 10 long-pulse (nanosecond) and two short-pulse (subpicosecond) beamlines. The short- pulse petawatt beamlines are two of the highest power beams of their type in the world. 15% of Orion’s system time is made available for cutting- edge collaborative academic research, enabling this to be brought to bear on current and future AWE programmes. This is being facilitated working with the Central Laser Facility at STFC Rutherford Appleton Laboratory.

[3] LMJ (Laser Mégajoule) laser installation of 1,4 Megajoules being deployed at Barp (Gironde, France) - NIF (National Ignition Facility), laser installation laser of megajoule type at Lawrence Livermore National Laboratory (California, USA).

Redaction : J.M. Bonnet-Bidaud, E. Falize