Charge Exchange Models for
X-ray Spectroscopy with
AtomDB v2.0
Randall Smith, Adam Foster &
Nancy Brickhouse
Smithsonian Astrophysical Observatory
Building a CX model for 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:
Dielectronic Recombination
Radiative Recombination
Charge Exchange
Building a CX model for 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…)
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.
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
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.
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 Lyb/Lya 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 Xray Spectrometer
response curve.
ACX & DXS
ACX & Astro-H
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.
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, 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)
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
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.