Resonance scattering in the X-ray emission line profiles of ζ Pup Maurice Leutenegger With David Cohen, Steve Kahn, Stan Owocki, and Frits Paerels
Dec 25, 2015
Resonance scattering in the X-ray emission line profiles of ζ Pup
Maurice Leutenegger
With David Cohen, Steve Kahn, Stan Owocki, and Frits Paerels
Resonance scattering in the X-ray emission line profiles of ζ Pup
X-ray emission from O star winds X-ray line profiles: summary of theory and
observation Data suggesting resonance scattering Profile formation with resonance scattering Application of RS profile model to data Implications
X-ray emission from O stars
The driving force in radiative lines is unstable
Tenuous streams undergo runaway acceleration before colliding with dense clumps, resulting in reverse shocks
Snapshot from simulation of Feldmeier (1995)
X-ray Doppler profiles(Owocki & Cohen 2001)
Wind modeled as a two-component fluid Cool bulk of wind (absorbs X-rays) Small fraction is heated in shocks (emits X-rays)
X-ray Doppler profiles: emission from thin shells
Example profiles
Parameter dependence
Two parameters influence radial distribution of X-ray emitting plasma: “Turn-on” radius (expected to be ~ 1.5 stellar radii) Filling factor (power law in radius)
Parameter dependence
The cool part of the wind absorbs X-rays as they leave the wind Characteristic continuum optical depth:
Example profiles
Qualitative summary of model profile behavior:
Degree of blueshift measures characteristic continuum optical depth to X-rays
Width measures the onset radius of X-ray emission
Comparison with data
Possible explanations for X-ray profile shapes
(more than one may apply)
Mass loss rates too high – characteristic optical depths really are low
Porosity reduces macroscopic effective optical depth
Resonance scattering causes emission to be intrinsically shifted towards line center
Empirical evidence suggesting resonance scattering?
Empirical evidence suggesting resonance scattering?
Resonance scattering (Ignace et al 2002, Leutenegger et al 2007)
For an optically thick resonance line in a moving stellar atmosphere (Sobolev theory): radial photon escape is due to the radial velocity
gradient (dv/dr) lateral photon escape is due to the spherical
divergence of the wind (v/r) Far out in the wind dv/dr goes to zero, so lateral
escape is favored If the observed X-ray emission comes from far
out in the wind, the profiles are more symmetric
Angular dependence of normalized escape probability (optically thick)
How does resonance scattering affect the model profiles?
Including resonance scattering in N VI leads to much better fit
Including resonance scattering in N VI leads to much better fit
Also improves fit to O VII
Also improves fit to O VII
What about other lines?
We can only infer the importance of RS by comparing two lines from the same ion – one must be a resonance line, and the other must not
But if RS is important in N VI and O VII, it should be important for other strong resonance lines as well!
If resonance scattering is important how do we measure anything?
Problem: profile shape is roughly degenerate for high continuum optical depth with resonance scattering and low continuum optical depth without resonance scattering
Use non-resonance lines Even some resonance lines will not be optically
thick (Make predictions for line optical depth)
Summary
Different profile shapes in resonance and intercombination lines from the same ion can only be explained by resonance scattering
Resonance scattering can explain at least some of the unexpected lack of asymmetry in other profiles
Either porosity or reductions in mass-loss rates (relative to density-squared diagnostics) are still likely to be important in addition to RS
Characteristic optical depth to resonance scattering
Expected values of optical depth