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