atomic radiation processes in agn julian krolik johns hopkins university
DESCRIPTION
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 bremsstrahlungTRANSCRIPT
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