Line Transfer and the Bowen Fluorescence Mechanism in
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 H 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 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 H H Line absorption/scattering ( ~ 10 -16 cm 2) Continuum absorption ( ~ 10 -18 cm 2) Electron scattering ( ~ 10 -24 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 100 - 1000). 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 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: Continuum opacity Line optical depth Line profile Intensity Line source function
Theory of Line Transfer ( The source function contains intrinsic as well as scattering terms. destruction probability intrinsic source distribution (e. g. , recombination collisional excitation) ( obtain solution redistribution function the by rewriting transfer equation as a second order differential equation, and discretizing the spatial (optical depth), angle, and frequency coordinates - Feautrier (1964) method
Single-Ion Line Ratios ( H-like oxygen at k. T = 10 e. V (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 reemitted. 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 2 s 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 O VIII, N VII, and some unspecified form of background continuum ( = 10 -5). 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 = 104, most of the O VIII Ly line is destroyed only
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 AZ 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
- Slides: 13