PhotoionizationTim
Kallm
an NA
SA/G
SFC
•W
hat is photoionization?
•Rem
oval of a bound electron by a photon•
Loosely refers to any situation where external photons are the
dominant source of ionization (and heating)
•O
utline:
•Review
of coronal plasma
•Effect of photoionization
•Background, definitions
•Exam
ples
Coronal ionization
•A
ssume all processes are in a steady state, so that for each ion
species the rate of creation= rate of destruction
•A
lso assume that electron velocity distribution is M
axwellian,
kTion ~kT
electron , so that electron collisions are much m
orefrequent than ion collisions.
•Ion destruction is due to electron im
pact ionization by thermal
electrons
•Ion creation is due to recom
bination (radiative and dielectronic)
•The fraction of an ion peaks w
hen the electron temperature is
kTe~(0.7)I
Photoionization•
What happens w
hen an external photon source illuminates the
gas?•
The photons ionize the atoms in the gas.
•The photoelectrons created in this w
ay collide with am
bientelectrons (m
ostly) and heat the gas•
The gas cools by radiation•
The gas temperature adjusts so that the heating and cooling
balance
In a photoionized gas the temperature is not a free param
eterand
The ionization balance is determ
ined by the shape and strengthof the radiation field
Processes
•Photoionization (+heating)
•Recom
bination (+cooling)
•D
ielectronic recombination (+cooling)
•Collisional ionization (cooling)
•Collisional excitation(cooling)
•Com
pton scattering
•O
thers (more later)
Ionization and Thermal Balance
For each ion:
Ionization = recombination
~photon flux ~electron densityFor the gas as a w
hole
Heating = cooling
~photon flux ~electron density=> A
ll results depend on the ratio photon flux/gas density or"ionization param
eter"
Consequences of Photoionization
•Tem
perature lower for sam
e ionization than coronal, T~0.1 Eth /k
•Tem
perature is not a free parameter
•Tem
perature depends on global shape of spectrum
•A
t high ionization parameter, the gas is fully ionized, and the
temperature is determ
ined by Compton scattering and inverse
T=<E>/4k•
Ionization balance is more 'dem
ocratic'
•M
icrophysical processes, such as dielectronic recombination,
differ
•O
bserved spectrum differs
Observed Spectrum
: Emission
•In coronal gas, need kTe~DE to collisionally excite lines.
•In a photoionized gas there are few
er lines which satisfy this
condition.•
Excitation is often by recombination cascade
•A
lso get recombination continua (RRCs) due to recom
bination bycold electrons directly to the ground state. The w
idth of thesefeatures is directly proportional to tem
perature•
Due to the dem
ocratic ionization balance, it is more likely that
diverse ions such as N V
II, O V
III, Si XIV
can coexist and emit
efficiently than it would be in a coronal gas
•Inner shell ionization and fluorescence is also im
portant in gasesw
here the ionization state is low enough to allow
ions with filled
shells to exist.
Helium
-like ion level diagram
Density dependence of H
e-like lines
Coronal photoionized
(Porquet and Dubau 1998)
Chandra H
ET
GS spectrum
of Vela X
-1(Shulz et al. 2002)
Chandra H
ET
GS spectrum
of Capella
(Canizares et al. 2000)
Emission spectrum
of NG
C 1068
•N
GC 1068 is the prototype of Seyfert 2 galaxies, i.e. A
GN
inw
hich our direct line of sight to the nucleus is blocked by a thickring of cold m
aterial.
•If so, w
e should see emission from
photoionized material in the
'hole' of the doughnut, even though we don't see the nucleus
directly
•Chandra H
ETGS X
-ray spectra appear to confirm this prediction
Iron K Lines
•W
idely Emitted by all stages of iron, due to the efficiency of the K
shell fluorescence process, and expected to be bright due to therelative abundance of iron.observed from
all classes of(photoionized) X
-ray sources.•
Are likely to probe the hottest and m
ost highly ionized regions ofphotoionized gases, due to the high atom
ic number of iron.
•A
s discovered by ASCA
, this line shows evidence for relativistic
broadening in Seyfert galaxies and some black hole candidates.
•The com
bined effects of special and general relativity broaden andredden the line profile, and the shape depends on the inclination ofthe accretion disk and on the range of radii w
here emission occurs.
Line B
roadening by Black H
ole Disk E
mission
Fabian et al. 2000
Iron K L
ine from Seyfert G
alaxy MC
G-6-30-15
Tanaka et al., 1995
Absorption
•A
bsorption by interstellar material is in every spectrum
, butabsorption is uniquely associated w
ith photoionized sources.
•A
crude approximation for the photoabsorption cross section of a
hydrogenic ion is that the cross section is ~Z-2 at the threshold
energy, and that the threshold energy scales ~Z2.
•In addition, the cosm
ic abundances of the elements decrease
approximately ~Z
-4 above carbon
•So the net cross section scales as E
-3, and large jumps in
absorption are not expected at the thresholds.
•D
etection of such edges are indicative of abundance anomalies or
partial ionization of the gas
Cross section for photoionization for abundant
elements vs. w
avelength (Zom
beck)
Interstellar absorption (Morrison and M
cCam
mon; Z
ombeck)
Example 4: Photoabsorption spectrum
of theSeyfert 1 galaxy M
CG-6-30-15
•M
CG-6-30-15 is a relatively bright Seyfert 1 galaxy
•A
SCA discovered the first relativistically broadened iron K
linesfrom
this source, and it remains one of the m
ost extreme exam
plesof this phenom
enon.•
ASCA
also discovered features which w
ere interpreted asabsorption by O
VII and O
VIII photoionization 'edges', i.e.
Photons absorbed in photoionizing these ions from the ground
state.•
The first Chandra spectra failed to find the same features.
Chandra HETG
Spectrum of M
CG-6-30-15
Relativistically broadened O V
II Emission
Simple A
bsorption (O V
II, O V
III)l
Including O V
II 1s-np absorption
Iron n=2-3 UTA
s (Fe II-V)
Fe I L shell photoionization
Best fit: OV
II + Fe UTA
Summ
ary: Absorption spectra of M
CG-6-30-15
•The spectrum
in the 15-20 A (0.6-0.8 keV
) band observed with
Chandra and XM
M gratings contains com
plex features which do
not fit with sim
ple photoelectric absorption
•The O
VII absorption edge is not at the energy expected, 16.8
A (739 eV
)•
One possible explanation is relativistically broadened em
ission inthe O
VIII Lalpha line (and N
VII).
•Com
bined effects of the 1s-np absorption + n=2-3 transitions ofiron + n=2 photoabsorption appear to provide a good fit w
ithoutrequiring an exotic explanation.
Summ
ary
•'Photoionization' is likely im
portant in a wide range of
astrophysical situations, including AG
N, galactic binaries (BH
T,X
RB, CV), and the physics of photoabsorption is in every
spectrum.
•Photoionization equilibrium
differs from coronal equilibrium
insignificant w
ays, i.e. Lower tem
perature, more dem
ocratic iondistribution.
•The spectra em
itted by photoionized plasmas contain characteristic
features which have use as diagnostics.
•A
bsorption spectroscopy is (essentially) unique to photoionizedsources, and is m
ore important than w
as thought 5 yrs ago
What’s M
issing
•Tim
e dependence:•
time average spectrum
may not be the sam
e as the response to the time
average ionizing spectrum
•G
as we see m
ay be transiently ionized due to, eg., gas flow
•Radiative transfer
•N
on-thermal gases
•M
ulti-component
Where to go from
here
•Tools
• Cloudy
•X
star
•Photoion
•A
PEC
•Books
•O
sterbrock ‘Astrophysics of G
aseous Nebulae’ (Ferland)
•M
ihalas ‘Stellar Atm
ospheres’