1. Atomic Radiation Processes in AGN Julian Krolik Johns Hopkins University

2. Basic Atomic Radiation Processes Collisions between electrons and individual atoms or ions lead to photon creation So luminosity L ε = n e n H j  (T, X; n e,N H )V

3. Elementary Process I: Radiative Coulomb Scattering j fb » j ff µ I Z k T ¶ e I z = k T j ff » Z 2 ® f s ¾ T µ k T m e c 2 ¶ 1 = 2 m e c 3 also known as free-free/free-bound or bremsstrahlung

4. Elementary Process II: Inelastic Scattering + Radiative Relaxation j a » Z 2 ( ² = I Z ) µ k T m e c 2 ¶ 1 = 2 ¾ T ® 2 f s m e c 3 ( n X = n H ) exp ( ¡ ² = k T )

5. Typical Heat Balance in Photoionized Gases H ~ F ion σ ion cn HI = I H n e n p α rec C = n e n H j a ~ tight temperature control, T ~ 1—3 x 10 4 K because  /k ~10 5 K

6. Which Atoms and Ions? Ionization balance: specific conditionsatomic physics “Ionization parameter”

7. Ionization Parameter Also Controls Heavy Element Ionization Balance recombination time ionization parameter Measurements of changes in absorption constrain density, ionization state

8. A Useful Different Form for the Ionization Parameter L e t ¥ ´ L =( 4 ¼r 2 cn k T ) ' p r = p g line emission range

9. Radiative Relaxation Rates If E1 permitted, If E1 forbidden, M1 permitted, If E1, M1 forbidden,

10. Collisions Can Limit Radiation R coll ~ n e πa 0 2 v th,e ~ 10 -8 n e T 4 1/2 s -1 So collision rate faster than radiation rate when Presence or absence of forbidden lines directly bounds the density

11. Relation of Cooling Rates to Abundances L l = n e n x h ¾ ex v i, b u t If this line dominates the cooling, any increase in n X /n H 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.

12. Free-Bound Leads to Recombination Cascade In 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,....

13. Resonance Lines Can Be Very Optically Thick But thermal motions can Doppler shift the photon out of resonance:

14. 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

15. K-shell Photoionization = Soft X-ray Opacity

16. K-shell Photoionization: Fluorescence R a t e ( A uger ) / Z 3, w h i l e R a t e ( ° uorescence ) / Z 6 ; ° uorescencepro b a b i l i t y ' 0 : 35 f or F e, Z = 26 h º > K + X ! X + 1 ¤ + e ¡ ! 8 < : X + 2 + 2 e ¡ A uger X + 1 + e ¡ + h º K ® ° uorescence h º K ® ( F e ) = 6 : 4 k e V

17. K-shell Opacity + Fe Fluorescence + Compton Recoil Make Compton Reflection Amplitude and shape of Compton reflection bump constrain solid angle, ionization state of reflector

18. Our Best Diagnostic of the Innermost Disk: Fe K  Profiles a = M = 0 : 998 j /r ¡ 1 : 5 f orr > r ms