Charge Exchange Models for X-ray Spectroscopy with

AtomDB v2.0

Randall Smith, Adam Foster & Nancy Brickhouse

Smithsonian Astrophysical Observatory

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:

Building a CX model for X-ray Astrophysics

Dielectronic Recombination Radiative Recombination Charge Exchange

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…)

Building a CX model for 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)

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.

Building a CX model for X-ray Astrophysics

Upshot: An astrophysical plasma dominated by CX will have:•ionized metals•neutral hydrogen•a rapid transition 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.

Building a CX model for X-ray Astrophysics

Highly-ionized plasmas don’t wait

Smith & Hughes 2010

• 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

Identifying Recombining Plasmas – Broad-band, low-resolution features

Some supernova remnants show strong Ly, Ly lines plus Radiative Recombination Continua (RRC) features that are unmistakable signatures of electron recombination – mixed with bremsstrahlung and line emission from hot electrons.

Ozawa+ 2009

Identifying Recombining Plasmas –High-resolution spectra

In the lab…

In the sky…

Distinguishing Charge Exchange from Electron Recombination

Similarities:

•Both lead to large (F+I)/R ratios

•Both can give large Ly/Ly ratios 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

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”

ACX – AtomDB Charge Exchange

• l-distribution not as straightforward; we consider multiple approaches:

Janev & Winter (1985)

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’

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

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.

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.

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)

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

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)

ACX & DXS

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

ACX & DXS

ACX & Astro-H

Conclusions

• Both charge exchange and electron recombination create forbidden-dominated spectra and large Ly/Ly

• 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.

Backup

Building a CX model for 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 :

Are we Boltzmann yet?– or –

What are the relevant time constants?For CX, scaled to v=300 km/s, = 10-15 cm2:

Electrons & protons reach a Boltzmann distribution and Hydrogen ionizes in:

(where kTe= 35 eV thermal equivalent of 100 km/s velocity)

Summers+ 2006

What about

• 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

Recombining plasmas are inefficient

• Creating a 0.654 keV O VIII photon via:– Electron excitation: 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:

• Electron excitation and recombination can co-exist in a plasma; Electron excitation and CX cannot.