Suppression of X-rays from radiative shocks by their thin-shell instability
Abstract
We examine X-rays from radiatively cooled shocks, focusing on how their thin-shell instability reduces X-ray emission. For 2D simulations of collision between equal expanding winds, we carry out a parameter study of such instability as a function of the ratio of radiative versus adiabatic-expansion cooling lengths. In the adiabatic regime, the extended cooling layer suppresses instability, leading to planar shock compression with X-ray luminosity that follows closely the expected (L_X ∼ dot{M}^2) quadratic scaling with mass-loss rate dot{M}. In the strongly radiative limit, the X-ray emission now follows an expected linear scaling with mass-loss (L_X ∼ dot{M}), but the instability deforms the shock compression into extended shear layers with oblique shocks along fingers of cooled, dense material. The spatial dispersion of shock thermalization limits strong X-ray emission to the tips and troughs of the fingers, and so reduces the X-ray emission (here by about a factor 1/50) below what is expected from analytic radiative-shock models without unstable structure. Between these two limits, X-ray emission can switch between a high-state associated with extended shock compression, and a low-state characterized by extensive shear. Further study is needed to clarify the origin of this `shear mixing reduction factor' in X-ray emission, and its dependence on parameters like the shock Mach number.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- March 2014
- DOI:
- 10.1093/mnras/stt2475
- arXiv:
- arXiv:1401.2063
- Bibcode:
- 2014MNRAS.438.3557K
- Keywords:
-
- shock waves;
- hydrodynamics;
- instabilities;
- stars: mass-loss;
- X-rays: binaries;
- X-rays: stars;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- doi:10.1093/mnras/stt2475