Creating and Characterizing an X-ray Photoionized Nebula in the Laboratory

1. Creating and Characterizing an X-ray Photoionized Nebula in the Laboratory David H. Cohen1,2, Joseph J. MacFarlane1, Duane Liedahl3, James E. Bailey4 1Prism Computational Sciences, Madison, Wisconsin 2Bartol Research Institute, University of Delaware 3Livermore National Laboratory 4Sandia National Laboratory Presented at the NIF Science Workshop, Pleasanton CA, 5 October 1999

2. We are entering a new era of astrophysical X-ray spectroscopy (Chandra)

3. The first Chandra spectra are available At a resolution approaching l/Dl = 1000 (300 km s-1), line complexes such as the helium-like triplets can be separated… However, line profile analysis is not possible in most (but not all) cases

4. Historically, the most-studied cosmic X-ray sources have been coronal-- primarily the Sun • However, many interesting X-ray sources are not coronal/collisional, but rather are photoionized • These tend to involve compact objects, which are a source of hard, continuum X-rays -- AGG/Quasars, X-ray Binaries,…

5. ...but any environment where relatively cool plasma coexists with a strong source of X-rays will require photoionization modeling to understand • X-ray emission characteristics of photoionized plasmas: • radiative recombination continua • recombination cascade emission lines • fluorescent emission • inner-shell absorption

6. Photoionized plasmas Both plasmas in the two models shown have similar ionization levels (the XPN model is not hot, but is photoionized), however a very different group of lines is present in each Spectra of X-ray photoionized plasmas are quite different from collisional plasmas • Characterized by the ionization parameter, x=L/nR2 typically 1 < logx < 4 in photoionized astrophysical sources • value of x controls ionization level • if ne is low enough, then level populations are set by photoion./photoexcit./2-body recombination/spont. emission (upper levels are populated by recombination cascades, not collisions) collisional photoionized

7. High quality data from XRBs, AGNs are coming Current state of the art is ASCA(until Chandra) In this low-resolution spectrum of the XRB Cyg X-3 the only overt sign of photoionization’s dominance are the radiative recombination continua - note that they are narrow because of the low plasma temperature, which is so low that the high ionization states we see cannot be produced by collisions

8. The accretion disk in this object is seen in absorption against the central engine -- the ionization/excitation conditions in the disk are controlled by the radiation field Soon to have high resolution X-ray spectra of cosmic sources Detailed absorption spectroscopy of photoionized sources such as this micro-quasar (ASTRO-E simulation) Also will resolve individual emission lines from radiative recombination cascades and fluorescence in sources like Cyg X-3

9. XSTAR, CLOUDY do exist as off-the-shelf photoionization codes for astrophysicists, but much work is required if they are to be used as X-ray spectral modeling codes Collisional plasmas have been studied extensively (e.g. in tokamaks and solar spectra) and detailed spectral models exist (MEKAL, Raymond-Smith)  but much less work has been done with photoionized plasmas, both observational/experimental benchmarking and modeling collisional vs. radiative plasmas high vs. low density In the laboratory, radiation interaction with high-density (optically thick) plasmas leads to shock waves and gradients Radiation from left onto solid KCl X-rays tend to over-ionize optically thin plasmas (they don’t get a plasma nearly to the temperature that would be required to achieve the same ionization state via collisionally equilibrium) -- ions tend to be photoionized by photons having energies close to threshold, so most of the photon’s energy goes into overcoming the electron’s potential well, and little goes into the kinetic energy of the liberated electron But, X-rays are absorbed volumetrically in low density gases  potential for relatively gradient-free plasmas

10. You can isolate a single element in lab, whereas astrophysical spectra have a (uncertain) mixture of elements How can the NIF contribute to our understanding of photoionization-dominated astrophysical plasmas? Schematic of argon gas experiment The absorption spectroscopy determines the ionization balance The emission spectroscopy determines the temperature Emission spectroscopy Converter (laser to X-rays) Gas target Fluence of 1010 ergs with high-Z foil on NOVA 1012 ergs on NIF, and if n=1018 cm-3 and R=1 cm then x=103 Absorption pectroscopy (also, direct measurement of the converter) X-rays Laser beams • Spectra will be a guide to identifying features in astrophysical data • Spectra will be used to benchmark, and drive the improvement of, codes

11. The radiation is again incident from the left. However, due to its optical thinness, the radiation is deposited evenly throughout the volume, and the plasma is gradient-free. Hydrodynamical simulation of gas photoionization experiment ro~10-5 g cm-3 (1% normal density) (TR=250 eV subtending 1 ster)

12. Normal density (ro~10-5 g cm-3) minor opacity effects, gradients (and numerical noise)

13. Predicted Spectra(5 ns, normal density) For the low-density case (1018 cm-3) the lines are not optically thick absorption spectroscopy (K-shell) = ionization balance emission spectroscopy (L-shell) = temperature

14. Issues to address Note: laboratory densities will be somewhat higher than most astrophysical sources, but will still be instructive (show many of the same features, be useful for benchmarking) -- can attain desired ionization parameter • X-ray converter -- foil or under-dense radiator? • Target -- gas jet or gas bag? (how low in density can we go?) • Diagnostics -- spectroscopy of : • ionizing source • recombination emission from gas • backlit absorption from gas (for ionization balance) - using converter? Separate backlighter? • Other - DANTE-type absolutely calibrated measurement • Equilibration times

15. Why NIF? • More X-rays to go for higher ionization parameter, x • Harder X-rays to ionize directly out of the K-shell and to look at higher-Z plasmas • Longer duration (using multiple, staggered beams) to get closer to ionization equilibrium