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

Brief Outline  Radiative transfer effects 8 Motivation H Detailed treatment generally ignored in global modeling (e.g., in XSTAR, Cloudy, etc.) H How do they affect the global emergent spectrum?  Theory of resonance line scattering 8 Line production/destruction mechanisms  Line overlap and the Bowen fluorescence mechanism 8 He II / O III in the UV (classical Bowen fluorescence) 8 O VIII / N VII in the X-ray  Simple spectral model

Radiative Transfer Effects  Transfer effects are important when  > 1 8 There are three important “levels” of opacity sources H Line absorption/scattering (  ~ cm 2 ) H Continuum absorption (  ~ cm 2 ) H Electron scattering (  ~ cm 2 )  Most codes assume complete redistribution / escape probability methods for treating resonance line transfer 8 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   ). 8 It is also difficult to apply this method when other opacity sources (e.g. continuum absorption, line overlap) are important as well. 8 In this formalism, a correct treatment of radiative transfer is nearly hopeless when there are abundance and temperature gradients.

Theory of Line Transfer  Has been worked out by various authors 8 Unno (1952, 1955); Hummer (1962); Auer (1967); Weymann & Williams (1969); Ivanov (1970, 1973); Hummer & Kunasz (1980)  Problem 8 Solve for the intensity given by the following transfer equation: Line source function Intensity Continuum opacity Line optical depth Line profile

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 redistribution function intrinsic source distribution (e.g., recombination collisional excitation)

Single-Ion Line Ratios  H-like oxygen at kT = 10 eV (weakly temperature dependent) 8 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.

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

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

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. 8 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. 8 Partial redistribution in a Voigt profile is assumed for all the lines.

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: 8 scatter and eventually escape the medium through the boundaries 8 absorbed by the underlying continuum 8 absorbed by N VII, followed by cascades to the upper levels

Emergent O VIII / N VII Spectrum  A hypothetical medium containing only O VIII, N VII, and some unspecified form of background continuum (  = ). An abundance ratio of O/N = 5 is assumed. 8 At  = 100, the higher-order lines are almost completely suppressed, while the Ly  lines are still unaffected. 8 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. 8 At  = 10 4, most of the O VIII Ly  line is destroyed

A Few Other Important Line Overlap  Fe XVIII - O VIII Ly  8 the Fe XVIII source function dominates over that of O VIII 8 the line separation is quite large; important for large turbulent velocity.  Fe XVII - O VII Ly-n (n > 5) 8 similar to the previous case - the Fe XVII source function dominates. multiple levels of O VII contribute to the total opacity.

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. 8 Incorporate Compton scattering. H Important in very highly ionized medium where the metal abundances are extremely low, i.e., when A Z   b-f ~  T. 8 Comprehensive / global spectral modeling including all important metal transitions. H e.g., Fe XIX - XXIV lines with O VIII continuum ( < 14.2 Å) 8 relativistic effects