iron k spectra from l-shell ions in photoionized plasmas work in progress duane liedahl physics and...
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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
€
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
€
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