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’