• • Outline: What is photoionization • • • • • • Examples Background, definitions Effect of Review of coronal plasma dominant source of (and heating) Loosely refers to any situation where external are the Removal of a bound electron by photoionization P Tim Kallman NASA/GSFC hotoionization ? • • • • • kTe~(0.7)I The fraction of an ion peaks when the electron temperature is Ion creation is due to recombination (radiative and dielectronic) electrons Ion destruction is due to electron impact ionization by thermal frequent than ion collisions. kT Also assume that electron velocity distribution is species the rate of creation= destruction Assume all processes are in a steady state, so that for each ion ion

~kT electron , so that electron collisions are m C oronal ionization uch m M axw ore ellian,

• • • • • The ionization balance is determined by the shape and strength In a balance The gas temperature adjusts so that the heating and cooling The gas cools by radiation electrons (mostly) and heat the gas The photoelectrons created in this way collide with ambient The photons ionize the in gas. gas? What happens when an external photon source illuminates the photoionized gas the temperature is not a free parameter of the radiation field P hotoionization and

• • • • • • • O Compton scattering Collisional Collisional Dielectronic recombination (+cooling) Recom Photoionization thers (m bination (+cooling) ore later) excitation(cooling) ionization (cooling) (+heating) Processes

=> All results depend on the ratio photon flux/gas density or For the gas as a w For each ion: "ionization param ~photon flux ~electron density Heating = cooling ~photon flux ~electron density Ionization = recombination Ionization hole eter" and Therm al B alance

• • • • • • Observed spectrum differs differ Microphysical processes, such as Ionization balance is more 'democratic' Temperature depends on global shape of spectrum Temperature is not a free parameter Temperature lower for same ionization than coronal, T~0.1 Et • T=/4k tem At high ionization parameter, the gas is fully ionized, and perature is determ Consequences ined by C of Photoionization dielectronic recombination, om pton scattering and inverse h /k • • • • • • shells to exist. where the ionization state is low enough to allow ions with filled Inner shell ionization and fluorescence is also important in gases efficiently than it would be in a coronal gas diverse ions such as N VII, O VIII, D features is directly proportional to temperature cold electrons directly to the ground state. The width of these A Excitation is often by recombination cascade condition. In a In coronal gas, need ue to the dem lso get recom photoionized O bserved bination continua (R ocratic ionization balance, it is m gas there are few kTe~ S D pectrum E to collisionally Si er lines w R C XIV can coexist and emit : s Em ) due to recombination by ission excite lines. hich satisfy this ore likely that

Helium-like ion level diagram ( Density dependence of He-like lines Porquet and Coronal photoionized Dubau 1998) (Shulz Chandra HETGS spectrum of Vela X-1 et al. 2002) (C Chandra HETGS spectrum of anizares et al. 2000) Capella • • • Chandra HETGS X-ray spectra appear to confirm this prediction directly 'hole' of the doughnut, even though we don't see nucleus If so, we should see emission from ring of cold material. which our direct line of sight to the nucleus is blocked by a thick NGC 1068 is the prototype of Seyfert Em ission spectrum photoionized material in the

of 2 galaxies, i.e. AGN in N G C

1068

• • • • the accretion disk and on range of radii where emission occurs. redden the line profile, and shape depends on inclination of The com broadening in Seyfert As discovered by ASCA, this line shows evidence for relativistic photoionized Are likely to probe the hottest and most highly ionized regions of (photoionized) X-ray sources. relative abundance of iron.observed from all classes shell fluorescence process, and expected to be bright due the Widely Emitted by all stages of iron, due to the efficiency K bined effects of special and general relativity broaden gases, due to the high galaxies and som Iron K

Lines e black hole candidates. ic num ber of iron. Line Broadening by Black Hole Disk Emission Fabian et al. 2000 Iron K Line from Seyfert Galaxy MCG-6-30-15 Tanaka et al., 1995 • • • • • partial ionization of the gas Detection of such edges are indicative abundance anomalies or absorption are not expected at the thresholds. So the net cross section scales as E approximately ~Z In addition, the cosmic abundances of elements decrease energy, and that the threshold energy scales ~Z hydrogenic ion is that the cross section ~Z A crude approxim absorption is uniquely associated with A bsorption by interstellar m -4 ation for the above carbon Absorption aterial is in every spectrum photoabsorption cross section of a -3 , and large jumps in photoionized -2 at the threshold 2 . sources. , but elements vs. wavelength ( Cross section for photoionization Zombeck for abundant ) Interstellar absorption (Morrison and

McCammon; Zombeck ) • • • • The first Chandra state. Photons absorbed in photoionizing absorption by O VII and VIII ASCA also discovered features which were interpreted as of this phenomenon. from this source, and it remains one of the most extreme examples A MCG-6-30-15 is a relatively bright Seyfert SCA Ex am discovered the first p Seyfert le 4 :

spectra failed to find the same features. Photoabsorption

1 galaxy relativistically photoionization MCG-6-30-15 these ions from the ground

broadened iron K lines spectrum 1 galaxy 'edges', i.e.

of the Chandra HETG Spectrum of MCG-6-30-15 R elativistically broadened O

V II Em ission Simple Absorption (O VII, O VIII)l Including O VII 1s-np absorption Iron n=2-3

UTAs (F e II-V ) F e I L shell photoionization B est fit: OVII + Fe UTA • • • Sum requiring an exotic explanation. iron + n=2 photoabsorption appear to provide a good fit without Com the O VIII One possible explanation is relativistically not fit with simple photoelectric absorption Chandra The spectrum in the 15-20 A (0.6-0.8 • A (739 The O VII absorption edge is not at the energy expected, 16.8 bined effects of the 1s- m and XMM gratings contains complex features which do ary: Lalpha line (and N eV) Absorption np absorption + n=2-3 transitions of V spectra II). keV of ) band observed with broadened emission in MCG-6-30-15 • • • • sources, and is m Absorption is (essentially) unique to photoionized features which have use as diagnostics. The spectra emitted by photoionized distribution. significant w Photoionization spectrum XRB, CV), and the physics of astrophysical situations, including AGN, galactic binaries (BHT, 'Photoionization' . ays, i.e. Low equilibrium differs from coronal in is likely im ore im portant than was thought 5 yrs ago S er tem um portant in a w photoabsorption m perature, m ary plasmas contain characteristic ide range of ore dem is in every ocratic ion • • • • Multi-component N Radiative Time dependence: • • on-therm Gas average time we average transfer see ionizing al gases may spectrum be spectrum transiently W may h at’s not ionized be M the issin same due to, as g eg the ., gas response flow to the time • • Books Tools • • • • • • Mihalas Osterbrock APEC Photoion Xstar Cloudy ‘ Stellar ‘Astrophysics Whe Atmospheres re of

to Gaseous ’ go from Nebulae he ’ ( Ferland) re