Atomic Radiation Processes in AGN
Julian KrolikJohns Hopkins University
Basic Atomic Radiation Processes
Collisions between electrons and individual atoms or ions lead to photon creation
So luminosity Lε = nenH j (T, X; ne,NH)V
Elementary Process I:Radiative Coulomb Scattering
j f b » j f fµ IZkT
¶eI z=kT
j f f » Z2®fs¾Tµ kTmec2
¶1=2mec3
also known as free-free/free-bound or bremsstrahlung
Elementary Process II:Inelastic Scattering + Radiative Relaxation
j a » Z2(²=I Z )µ kTmec2
¶1=2 ¾T®2fsmec3(nX =nH ) exp(¡ ²=kT)
Typical Heat Balance in Photoionized Gases
H ~ FionσioncnHI = IHnenpαrec
C = nenHja ~
tight temperature control, T ~ 1—3 x 104 K because /k ~105 K
Which Atoms and Ions?
Ionization balance:
specific conditions atomic physics
“Ionization parameter”
Ionization Parameter Also Controls Heavy Element Ionization Balance
recombination time ionization parameter
Measurements of changes in absorption constrain density, ionization state
A Useful Different Form for the Ionization Parameter
Let ¥ ´ L=(4¼r2cnkT) ' pr=pg
line emission range
Radiative Relaxation Rates
If E1 permitted,
If E1 forbidden, M1 permitted,
If E1, M1 forbidden,
Collisions Can Limit Radiation
Rcoll ~ neπa02vth,e ~ 10-8neT4
1/2 s-1
So collision rate faster than radiation rate when
Presence or absence of forbidden lines directly bounds the density
Relation of Cooling Rates to Abundances
L l =nenxh¾exvi, butIf this line dominates the cooling, any increase in nX/nH simply permits the same heating to be balanced at a lower temperature.So only weak lines are sensitive to abundance---but it’s difficult to measure them well. And ionization corrections can be very model-dependent.
Free-Bound Leads to Recombination CascadeIn H atoms or H-like ions,
So most recombinations at high l
E1 demands Δl = ±1, so most Δn = 1
But ion collisions can drive (n,l) to (n,l’)
Predictable ratios of Hα/Hβ, etc.; departures signal other effects, e.g., extinction, optical depth in the lines,....
Resonance Lines Can Be Very Optically Thick
But thermal motions can Doppler shift the photon out of resonance:
At each scatter, the photon energy can shift roughly one thermal width.
The probability that in any single scatter, the photon leaves with such a large frequency offset that its optical depth is < 1 is then
Photon trapping can make collisional deexcitation easier
Large optical depth leads to saturation at the thermal intensity
K-shell Photoionization = Soft X-ray Opacity
In weakly-ionized, Solarabundancegas, · (²) / ²¡ 3
But as » increases,opacity disappears atlow energy ¯rst
! warmabsorbers
K-shell Photoionization: Fluorescence
Rate(Auger) / Z3, whileRate(°uorescence) / Z6;°uorescenceprobability ' 0:35 for Fe, Z = 26
hº>K +X ! X +1¤+e¡ !
8<:X +2+2e¡ AugerX +1+e¡ +hºK®°uorescence
hºK ®(F e) = 6:4 keV
K-shell Opacity + Fe Fluorescence + Compton Recoil Make Compton Reflection
Amplitude and shape of Compton reflection bump constrain solid angle, ionization state of reflector
Our Best Diagnostic of the Innermost Disk:Fe K Profiles
a=M = 0:998j / r¡ 1:5 for r > rms
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