Charge Exchange Models for X-ray Spectroscopy with AtomDB v2.0

1. Charge Exchange Models for X-ray Spectroscopy with AtomDB v2.0 Randall Smith, Adam Foster & Nancy Brickhouse Smithsonian Astrophysical Observatory

2. Building a CX modelfor X-ray Astrophysics Back to basics! The total emissivity from a cubic centimeter is just given by the densities, velocities, and cross sections of various processes involved: Considering recombination only, we get: Radiative Recombination Charge Exchange Dielectronic Recombination

3. Building a CX modelfor X-ray Astrophysics In 1969, Allen & Dupree pointed out that dielectronic recombination dominated collisional ionization at high temperatures, although radiative recombination picks up at low temperatures. (there are exceptions…)

4. Building a CX modelfor X-ray Astrophysics Regardless of the exact value, CX swamps other processes by factors of 100× – 1000× If neutral H or He mixes with highly-ionized material, CX dominates (until the H, He is ionized)

5. Building a CX model for X-ray Astrophysics Question: Why recombination only? Answer: If electron excitation is even remotely possible, it will tend to dominate the x-ray emission.

6. Building a CX modelfor X-ray Astrophysics • Upshot: An astrophysical plasma dominated by CX will have: • ionized metals • neutral hydrogen • a rapidtransition region Typical circumstances might involve a stellar wind CX’ing into the surrounding ISM, or shocked SNe ejecta (unshocked is cold) hitting a molecular cloud.

7. Highly-ionized plasmas don’t wait • Consider an ion Z+q created in a stellar wind or a supernova shock, travelling at 300 km/s. • At a minimum, it has q electrons nearby to ensure charge neutrality. • At a density of 0.1 cm-3 it will travel <100 pc before recombining and reaching equilibrium Smith & Hughes 2010

8. Identifying Recombining Plasmas – Broad-band, low-resolution features Ozawa+ 2009 Some supernova remnants show strong Lya, Lyb lines plus Radiative Recombination Continua (RRC) features that are unmistakable signatures of electron recombination – mixed with bremsstrahlung and line emission from hot electrons.

9. Identifying Recombining Plasmas –High-resolution spectra In the lab… In the sky…

10. Distinguishing Charge Exchange from Electron Recombination Similarities: • Both lead to large (F+I)/R ratios • Both can give large Lyb/Lyaratios and other high-n shell ratios (though they may differ in detail) Differences: • Electronic recombination leads to RRCs, which are not present in CX plasmas

11. ACX – AtomDB Charge Exchange • Assumes CX 100% efficient for all ions • Begins with a thermal equilibrium ion balance • Assumes all CX into one n shell: Janev & Winter (1985) “State-selective electron capture in atom-highly charged ion collisions”

12. ACX – AtomDB Charge Exchange • l-distribution not as straightforward; we consider multiple approaches: Janev & Winter (1985)

13. ACX – AtomDB Charge Exchange After populating the relevant n’ shell, ACX follows all the radiative decay transitions, using existing AtomDB atomic structure files combined with new atomic data created via the atomic code ‘AutoStructure’

14. ACX: A first look • An ACX model folded through the Suzaku BI response curve. • The results depend strongly on the initial ionization balance; at lower temperatures, the carbon and nitrogen lines are much stronger. Foster et al 2012

15. ACX: A first look • An ACX model folded through the Astro-H SXS response curve. • The strength of the forbidden line of the O VII triplet clearly evident. • Little sign of lower energy transitions, at least at this temperature.

16. ACX: A first look • An ACX model folded through the Astro-H SXS response curve. • The strength of the forbidden line of the O VII triplet clearly evident. • Little sign of lower energy transitions, at least at this temperature.

17. The Diffuse X-ray Spectrometer • Bragg-crystal spectrometer that flew on Space Shuttle Endeavour in January 1993 as part of STS-54. • Observed diffuse sky from 44-84Å (e.g the ¼ keV band) obtaining ~25 ksec of ‘good’ data. Sanders et al. (2001)

18. Pure Thermal Model Fits to DXS • The best-fit MeKaL model to the Diffuse X-ray Spectrometer data, with variable Mg, Si & Fe. • Note that the strong lines are misplaced, and the overall fit is poor. Sanders et al. (2001), Fig 14

19. CHIPS – The diffuse EUV sky • Launched 2003 to observe the diffuse EUV from 90-265Å • in 13 Msec, detected only one diffuse line of Fe IX at 171Å; tight constraints of ~6 LU on others Hurwitz, Sasseen & Sirk (2005)

20. ACX & DXS • Three component model including thermal plasma plus Fast and Slow SWCX folded through the Diffuse X-ray Spectrometer response curve.

21. ACX & DXS

22. ACX & Astro-H

23. Conclusions • Both charge exchange and electron recombination create forbidden-dominated spectra and large Lyb/Lya • Electronic recombination also creates continua that will not be seen in charge exchange spectra • A new approximate model ‘ACX’ will soon be available that produced a (scaled) CX spectrum suitable for fitting in the absence of detailed knowledge of the underlying ion density and velocity distribution.

24. Backup

25. Building a CX modelfor X-ray Astrophysics • At Te > 4×104 K, hydrogen is 99.9% ionized. • At Te~106 K, electrons outnumber neutral hydrogen by more than a million to one. • In thermal equilibrium ve ~ 43 × vH If the electrons are in a Boltzmann distribution and the plasma is ionized, then and , dominating the fact that :

26. Are we Boltzmann yet?– or –What are the relevant time constants? For CX, scaled to v=300 km/s, s = 10-15 cm2: Electrons & protons reach a Boltzmann distribution and Hydrogen ionizes in: Summers+ 2006 (where kTe= 35 eV thermal equivalent of 100 km/s velocity)

27. What about s? • Do proper cross sections matter? • Not that much, unless you have detailed information about the distribution of the ion and neutral components.* • We do know that the CX cross section is sufficiently large that when ions and neutrals are able to CX, they will. • If CX can happen, it will. * Relative rates do matter

28. Recombining plasmas are inefficient • Creating a 0.654 keV O VIII photon via: • Electronexcitation: requires 0.654 keV of electron energy, O7+ ion remains • Recombination: requires 0 keV electron energy, but destroys the O8+ ion that required 0.87 keV to create. But most importantly: • Electronexcitation and recombination can co-exist in a plasma; Electron excitation and CX cannot.