Iron K Spectra from L-Shell Ions in Photoionized Plasmas

Work in Progress

Duane Liedahl

Physics and Advanced TechnologiesLawrence Livermore National Laboratory

Motivations: Si K fluorescence from L-shell ions in HMXBsspectroscopic diagnostic potentialhigh spectral resolution at Fe K provided by Astro-E 2

Focus: resonant Auger destruction

Applications: stellar winds in HMXBsaccretion disks in AGN and X-ray binaries

ASCA data from Vela X-1 motivated a wind model based on spherically symmetric mass loss

See Sako et al., ApJ, 1999

contours of log ionization parameter =L/nr2

• Model accounts for emission from H-like and He-like ions of several elements• Does not account for fluorescence lines that were observed

Si charge state distribution

From wind model we can predict high-ionization component ofhigh-resolution Chandra spectrum

Chandra spectrum

Vela X-1data shows entire Si L-shell K spectrum

Several charge states are separated, although each ionic component is a blend

Mg-F

ONCB

Be

Same features are observed in other X-ray binaries

Broad range of silicon charge states suggests thatfluorescence from iron L-shell ions should be observed

Fabian, et al., 2000, PASP

Iron K lines are used to probe black hole accretion disks

Nandra, et al.,1999, ApJ

Models used to fit relativistic Fe K line involve 6.4 keV “near-neutral line” or H-like and He-like lines – no K from Fe L

Where are the K lines from Fe L-shell ions?

It was suggested that resonant Auger destruction is responsible.(Ross, Fabian, & Brandt 1996, MNRAS; see also Band et al. 1990, ApJ)

How does silicon observed in HMXBs respond to this process?

Resonant Auger destruction is simply line scattering with a high destruction probability per scatter

Since destruction probability is high, a good approximation is to zero out all K linesfrom Fe L-shell ions – right?

mechanism operates for F-like to Li-like ions

(for K need vacancy in n=2 shell)

€

Pesc(τlc)=1π

du0

∞

∫ H (a,u)E2 τlcH (a,u)H (a,0)

⎡

⎣ ⎢

⎤

⎦ ⎥

We use an escape probability method to model resonant Auger destruction

geometric setup Pesc vs. line center optical depth

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τlu =NHAzFionplπe2

mcflu φ(ν)

line optical depth depends on fractional population of lower level

Calculation of line optical depths requires knowledge of level population distribution appropriate to local plasma conditions

Many iron K transitions terminate on excited levelsExample: Be-like Fe XXIII

Calculations performed with the HULLAC atomic physics package

This line terminates on ground

Fe XXIII illustrates the selective action of resonant Auger destruction

four lines to levels 6, 7, 8

models folded through Efwhm = 6 eV gaussian resolution kernel

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YKα =Kα photons produced s-1

K −shell holes produced s-1×(RAD modifications)lines

∑

Define an effective fluorescent yield to account both for atomic physicsand resonant Auger destruction

A better assessment of resonant Auger destruction accounts for level population distributions

We have demonstrated the effect of Fe RAD for the “nebular case,”that is, only ground levels are significantly populated

Should be adequate for most HMXB environments, not so for disks

Consistent treatment requires population kinetics modelfor pre-ionization charge state (cf., Jacobs et al. 1989, Phys Rev A)

Photoionization out of excited levels provides access to different autoionizing levels, resulting in a different K spectrum

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Yul =Aul

Aujrad

j∑ + Auk

auto

k∑

line fluorescence yieldhigh yield lines

low yield lines

We include a bright EUV radiation field to drive the level populations

schematic of ion layer in anaccretion disk atmosphere

level populations for 9 lowest Be-like levelsfor kT = 80 eV Planckian

low yield lines

high yield lines

Level population distribution can make a big difference in both fluorescence line spectra and ion fluorescence yield

Comparison of Fe XXIV spectra in “zero-D” showing effect of different level population distributions

Resonant Auger destruction modifies the outgoing spectrum but does not entirely quench emission

Disk environment leads to enhancement of effective fluorescent yield for this ion – even with resonant Auger destruction

Li-like iron is not an exception – similar results are found for three other charge states

Calculation by Mario Jimenez-Garate; figure provided by Chris Mauche

Vertical disk structure calculations show that column densities of Fe L ionsare each a few times 1018 cm-2 (see also Nayakshin & Kallman 2001, ApJ)

K emission from Fe L boosts theoretical “Fe line”equivalent widths for expected accretion disk parameters

ionic equivalent widthsLi-like through F-like Fe summed equivalent width

suggested range of column density found in accretion disk atmosphere

AGN models predicting relativistic O VIII emission shouldinclude Fe L K calculations for consistency

Summary and Comments

Focus of this work is on assessing spectroscopic effects of resonant Auger destruction

Conclude that it is not valid to “turn off” K lines from L-shell ions when modeling disks

That L-shell ions are not required in fits to AGN spectra remains puzzlingbut will be better tested with Astro-E 2

Fe K lines from L-shell ions should be observed in some HMXBs

Connection between HMXBs and AGN in this context? HMXB spectra can be used to exercise spectral models by providingconstraints based on observations – feed back into disk models

Resonant Auger destruction can be turned into a new classof plasma diagnostic