Line Transfer and the Bowen Fluorescence Mechanism in Highly Ionized Optically Thick Media

1. Line Transfer and the Bowen Fluorescence Mechanism in Highly Ionized Optically Thick Media Masao Sako (Caltech) Chandra Fellow Symposium 2002

2. Brief Outline • Radiative transfer effects • Motivation • Detailed treatment generally ignored in global modeling (e.g., in XSTAR, Cloudy, etc.) • How do they affect the global emergent spectrum? • Theory of resonance line scattering • Line production/destruction mechanisms • Line overlap and the Bowen fluorescence mechanism • He II / O III in the UV (classical Bowen fluorescence) • O VIII / N VII in the X-ray • Simple spectral model

3. Radiative Transfer Effects • Transfer effects are important when  > 1 • There are three important “levels” of opacity sources • Line absorption/scattering ( ~ 10-16 cm2) • Continuum absorption ( ~ 10-18 cm2) • Electron scattering ( ~ 10-24 cm2) • Most codes assume complete redistribution / escape probability methods for treating resonance line transfer • Although this approximation is appropriate for isolated lines with moderate optical depths ( ≤ 10), it does not adequately describe line transfer when absorption and scattering in the damping wings become non-negligible (i.e., when   100 - 1000). • It is also difficult to apply this method when other opacity sources (e.g. continuum absorption, line overlap) are important as well. • In this formalism, a correct treatment of radiative transfer is nearly hopeless when there are abundance and temperature gradients.

4. Theory of Line Transfer • Has been worked out by various authors • Unno (1952, 1955); Hummer (1962); Auer (1967); Weymann & Williams (1969); Ivanov (1970, 1973); Hummer & Kunasz (1980) • Problem • Solve for the intensity given by the following transfer equation: Continuum opacity Line profile Intensity Line source function Line optical depth

5. Theory of Line Transfer • The source function contains intrinsic as well as scattering terms. • obtain solution by rewriting the transfer equation as a second order differential equation, and discretizing the spatial (optical depth), angle, and frequency coordinates - Feautrier (1964) method destruction probability intrinsic source distribution (e.g., recombination collisional excitation) redistribution function

6. Single-Ion Line Ratios • H-like oxygen at kT = 10 eV (weakly temperature dependent) • When higher order Lyman lines are absorbed, there is a ~80% chance (depending on the principal quantum number) for the line to be re-emitted. The other ~20% of the time, the line is radiated in the Balmer, Paschen, etc. lines, and eventually as either a lower-order Lyman line or 2-photon emission from the 2s level.

7. Bowen Fluorescence Mechanism • Classic He II / O III Bowen fluorescence (Bowen 1934,1935; Weymann & Williams 1969)

8. O VIII / N VII Transfer • O VIII Ly-alpha & N VII Ly-zeta (n=7) wavelength overlap

9. O VIII / N VII Transfer • Line photons scatter around in space and frequency. Every once in a while, an O VIII line photon scatters with N VII. When this happens, the line is lost ~20% of the time. • The N VII line intrinsic source function is negligible compared to that of the O VIII lines. Makes very little difference to the final results. • Partial redistribution in a Voigt profile is assumed for all the lines.

10. Conversion Efficiencies • From the solution to the transfer equation, one can calculate the efficiencies for the various processes. In the previous case, the lines either: • scatter and eventually escape the medium through the boundaries • absorbed by the underlying continuum • absorbed by N VII, followed by cascades to the upper levels

11. Emergent O VIII / N VII Spectrum • A hypothetical medium containing only O VIII, N VII, and some unspecified form of background continuum ( = 10-5). An abundance ratio of O/N = 5 is assumed. • At  = 100, the higher-order lines are almost completely suppressed, while the Ly lines are still unaffected. • At  = 1000, fluorescence scattering is important, and some of the O VIII Ly lines are converted to the N VII Lyman, Balmer, etc. lines. ~33% of this radiation escape as Ly photons. • At  = 104, most of the O VIII Ly line is destroyed

12. Fe XVIII - O VIII Ly the Fe XVIII source function dominates over that of O VIII the line separation is quite large; important for large turbulent velocity. Fe XVII - O VII Ly-n (n > 5) similar to the previous case - the Fe XVII source function dominates. multiple levels of O VII contribute to the total opacity. A Few Other Important Line Overlap

13. Summary, Conclusions, Future Work • Line transfer effects can alter not only line ratios within a given ion, but also across different elements. • Important for deriving CNO abundances from optically thick sources (e.g., in accretion disks). • Work in progress. • Incorporate Compton scattering. • Important in very highly ionized medium where the metal abundances are extremely low, i.e., when AZ b-f ~ T. • Comprehensive / global spectral modeling including all important metal transitions. • e.g., Fe XIX - XXIV lines with O VIII continuum ( < 14.2 Å) • relativistic effects