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 ? gt 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 source function
Intensity
Line profile
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 (n7) 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
A Few Other Important Line Overlap

• 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 gt 5)
• similar to the previous case - the Fe XVII source
  function dominates. multiple levels of O VII
  contribute to the total opacity.

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
  (? lt 14.2 Å)
• relativistic effects