Covering Solar-Wind Charge-Exchange
from Every Angle with Chandra
Brad Wargelin
Chandra X-Ray Center
Smithsonian Astrophysical Observatory
CXC
AAS/Chandra12
Heliospheric Solar Wind Charge Exchange
CXC
AAS/Chandra12
Charge Exchange:
The Forgotten Atomic Physics
CXC
AAS/Chandra12
Outline
Astrophysical charge exchange
• Solar wind charge exchange
Charge exchange X-ray emission
Solar Wind Charge Exchange (SWCX) X-rays
•
•
•
•
ROSAT
Geocoronal CX
Heliospheric CX
Soft X-Ray Background and SWCX
SWCX in the Chandra Deep Field-South
Future observations
CXC
AAS/Chandra12
Charge Exchange
Charge exchange (CX) is the radiationless transfer of one (or more)
electrons from a neutral atom or molecule to an ion.
• Molecular cloud chemistry:
O+ + H  O + H+ (13.618,13.599 eV)
• Solar wind proton-H CX:
H+ + H  H + H+
Some SW protons (tied to B field) CX with neutral H from the ISM,
particularly between the heliopause and bowshock, creating hot H
atoms, or Energetic Neutral Atoms (ENAs).
CXC
AAS/Chandra12
Local Interstellar Cloud (partially neutral, 26 km/s)
Hydrogen
Wall
CXC
AAS/Chandra12
IBEX and ENA Imaging
Energetic Neutral Atoms (ENAs)
no longer tied to B field. These
can be “seen” by the Interstellar
Boundary Explorer (IBEX) to
image the CX interaction region.
CXC
AAS/Chandra12
Astrospheres and Mass Loss Rates
Excess blue-shifted/broadened
Lyman-α absorption due to the hot
atoms in the Hydrogen wall can be
used to determine mass loss rates
of other stars surrounded by
partially neutral ISM (Wood, et al.
2001-2005).
Top line = intrinsic stellar profile (modeled)
Dashed = after ISM absorption
Hashed = excess absorption vs other lines
a Cen B Lya; Linsky & Wood (1996)
CXC
AAS/Chandra12
Ionization balance of metals in
photoionized nebulae—CX cross
sections are large and a tiny fraction of
neutral H is all it takes to make CX
more important than RR and DR (e.g.,
Dalgarno 1985, Ferland et al. 1997).
Emission of 511-keV annihilation line in
the GC via positronium formation
(nearly 100%). Roughly half of the
positronium is formed via CX with the
rest from RR. (2g emission with opposite
spins, 3g with same spin.)
Churazov et al. (2005, 2011)
CXC
AAS/Chandra12
Charge Exchange X-Rays
O8+ + H  *O7+ + H+ O7+ + H + hn
Highly charged ions capture
electrons into high-n levels
(nmax ~ q 3/4) that emit X-rays.
CX is a semi-resonant process.
During a collision, energy
levels distort and overlap at
“curve crossings.”
Cross sections are large (few
× 10-15 cm2).
(13.6 eV)
CXC
(35 eV)
AAS/Chandra12
CX X-Rays in Olden Times
Idea dates back to 1970s:
• Galactic Ridge X-rays from
cosmic rays? (Silk &
Steigman 1969)
ASCA; Tanaka (2002)
S
Ar
Ca
Fe
ASCA spectra of GC, Sgr,
Sct with identical model
curves (other than norm,
NH, 6.4-keV line).
CXC
AAS/Chandra12
CX X-Rays in Olden Times
Idea dates back to 1970s:
• Galactic Ridge X-rays from
cosmic rays?
No.
Watson (1976),
Bussard et al. (1978)…
Revnivtsev et al. (2006,
2007)
CXC
AAS/Chandra12
CX X-Rays in Olden Times
Idea dates back to 1970s:
• Galactic Ridge X-rays from
cosmic rays?
• X-ray lasers: population
inversion in “hollow ions”
CXC
AAS/Chandra12
CX X-Rays in Olden Times
Idea dates back to 1970s:
• Galactic Ridge X-rays from
cosmic rays?
• X-ray lasers: population
inversion in “hollow ions”
• Tokamaks: plasma edges
and neutral beam heating
(e.g., Rice et al. 1986)
CXC
Wargelin et al. (1998)
AAS/Chandra12
CX X-Rays in Olden Times
Idea dates back to 1970s:
• Galactic Ridge X-rays from
cosmic rays?
• X-ray lasers: population
inversion in “hollow ions”
• Tokamaks: plasma edges
and neutral beam heating
• Supernova remnants (Wise
& Sarazin 1989)
CXC
AAS/Chandra12
CX X-Rays in Olden Times
Idea dates back to 1970s:
• Galactic Ridge X-rays from
cosmic rays?
• X-ray lasers: population
inversion in “hollow ions”
• Tokamaks: plasma edges
and neutral beam heating
• Supernova remnants?
Cygnus Loop; Katsuda et al. (2011)
CXC
AAS/Chandra12
CX X-Rays in Olden Times
Idea dates back to 1970s:
• Galactic Ridge X-rays from
cosmic rays?
• X-ray lasers: population
inversion in “hollow ions”
• Tokamaks: plasma edges
and neutral beam heating
• Supernova remnants?
CXC
CX X-ray emission was known
AAS/Chandra12
Discovery of Solar Wind CX X-Rays
Key events:
•
Comet Hyakutake, ROSAT
(Lisse et al. 1996)
•
SWCX explanation (Cravens
1997):
Highly charged ions in SW
+ neutral H2O, CO, CO2
CXC
AAS/Chandra12
SWCX X-ray Spectrum (for Slow Wind)
Model CX spectrum (C,N,O) with 6 eV resolution
CXC
Wargelin et al. (2004)
AAS/Chandra12
Discovery of Solar Wind CX X-Rays
LINEAR S4
Key events:
•
•
•
C/1999 S4 (LINEAR)
Chandra/Lisse 2000
Comet Hyakutake, ROSAT
(Lisse et al. 1996)
SWCX explanation (Cravens
1997)
First CCD spectrum of comet,
by Chandra (Lisse et al. 2000)
Beiersdorfer et al. (2003)
CXC
AAS/Chandra12
Discovery of Solar Wind CX X-Rays
LINEAR S4
Key events:
•
•
•
C/1999 S4 (LINEAR)
Chandra/Lisse 2000
Comet Hyakutake, ROSAT
(Lisse et al. 1996)
SWCX explanation (Cravens
1997)
First CCD spectrum of comet,
by Chandra (Lisse et al. 2000)
Two dozen comets and several
planets to date by ROSAT,
EUVE, BeppoSAX, Chandra,
XMM, Swift, Suzaku.
Meanwhile……
Beiersdorfer et al. (2003)
CXC
AAS/Chandra12
LTEs & Discovery of Geocoronal and Heliospheric Emission
ROSAT All Sky Survey
(RASS; 1990) revealed
multi-orbit (“Long Term”)
enhancements in the
SXRB.
Cravens, Robertson, &
Snowden (2001):
temporal correlations
between counting rate
and SW flux
 LTEs are from
geo/helio SWCX
fluctuations.
CXC
¼-keV band, Galactic coords;
Snowden et al.
(2009)
AAS/Chandra12
Geocoronal Emission
Geocoronal emission
= SWCX in Earth’s
exosphere, outside
the magnetosphere
(R > 10RE).
CXC
AAS/Chandra12
Geocoronal Emission
Geocoronal emission
= SWCX in Earth’s
exosphere, outside
the magnetosphere
(R > 10RE).
X-ray missions
generally look out
through the flanks.
CXC
Robertson et al. (2006)
AAS/Chandra12
Lunar X-Rays (on the Dark Side) are Geocoronal
ROSAT, Schmitt 1990
CXC
Chandra, Wargelin et al. 2004
AAS/Chandra12
Moon, Chandra; Wargelin et al. (2004)
CXC
HDF-N, XMM; SnowdenAAS/Chandra12
et al. (2004)
Heliospheric Charge Exchange
Solar wind + H/He from
ISM  100-AU halo
Heliospheric CX ~ 10x
geocoronal CX
CXC
Model heliospheric emission from CX with H. Axis units in AU.
LIC is moving to the right. Robertson et al. AIP Proc. 719 (2004).
AAS/Chandra12
Heliospheric Emission--looking down on ecliptic plane
(AU)
More neutral H
upwind
He focussing
cone downwind
(AU)
Pepino et al. (2004)
CXC
AAS/Chandra12
Slow vs Fast Solar Wind
At solar max, wind is
a mix of slow and fast
at ~all latitudes.
At solar min, wind is
stratified, with slow
wind near the ecliptic.
The fast wind is much
less ionized and
produces less CX
X-ray emission.
CXC
AAS/Chandra12
CX Emission at Solar Min--view from Ecliptic Plane
Stratified wind:
slow and highly
ionized near
ecliptic  higher
CX emissivity.
Little emission in
fast wind.
CXC
AU
Pepino et al. (2004)
AAS/Chandra12
The Soft X-Ray Background (SXRB)
SXRB emission components:
•
•
•
•
Absorbed extragalactic (~power law)
Absorbed thermal Galactic Halo emission
Unabsorbed thermal from Local Bubble
Heliospheric and geocoronal SWCX
How much emission is from CX vs the Local Bubble? The
answer strongly affects our models of the LB.
CXC
AAS/Chandra12
Modeling CX Emission
Need to know
• H and He distributions
• SW composition and density all along line of sight (LOS)
• State-specific CX cross sections for all ions (and neutrals) as f(v)
• Radiative decay paths and line yields
Local SW measured by ACE
CXC
AAS/Chandra12
Living in a Fog
We can try to observationally separate SWCX emission from
cosmic components with differential measurements:




CXC
Spatially (using dark clouds to block distant emission)
Spectrally (some day, with high-resolution nondispersive detectors)
Temporal changes (periods of hours to Solar Cycle)
Observation geometry
AAS/Chandra12
The Chandra Deep Field-South
4 Msec of observations:
• 3 in Oct, Nov 1999 (-110 C)
• 9 in May, Jun, Dec 2000
• 12 in Sep-Nov 2007
• 31 in Mar-Aug 2010
RA,Dec
Gal l,b
Ecl lat,lon
3:32:28, -27:48:30
223.6, -54.4
41.1, -45.2
The CDFS is the only X-ray
deep field conducted during
Solar Max and Min and it has
the greatest orbital coverage.
CXC
AAS/Chandra12
LOS is 45 down,
into page.
2000
0.8 Ms in
1.0 Ms in
2.0 Ms in
CXC
(solar max)
(solar min)
(solar min)
J. Slavin
AAS/Chandra12
In 2000, there is slow wind
all along the LOS and most
observations are from
within the He cone.
From within the He cone,
CX intensity is higher and
more of the emission is
from nearby, where SW
conditions are measured.
CXC
AAS/Chandra12
In 2007, SW is stratified,
LOS through fast wind,
observations all outside He
cone, looking downwind.
CXC
AAS/Chandra12
Expectations for observed SXRB in 2000 vs 2007:
• Higher baseline level
• More variability
• Closer correlation with ACE-measured SW ion flux
CXC
AAS/Chandra12
CXC
AAS/Chandra12
CXC
AAS/Chandra12
Compare O emission vs average ACE/SWICS O7+ flux
CXC
Many thanks to the ACE/SWICS team for their public data!
AAS/Chandra12
2000:
Higher SXRB
Variable
Correlated with
local SW
2007:
Lower SXRB
Nearly constant
Little correlation
with SW
CXC
AAS/Chandra12
The goal is to
accurately model
and remove
SWCX emission
and obtain the
true cosmic
background.
?
CXC
AAS/Chandra12
The Future
High-resolution spectra from microcalorimeters will help immensely.
Astro-H launch in 2014 (E ~ 5 eV) .
• CX spectra differ from collisional: enhanced high-n Lyman, He-like f...
• Explore the 1/4-keV band (where ROSAT LTEs are strongest)
• 500 km/sec  E=1 eV at 600 eV
CXC
AAS/Chandra12
100 s of SXRB from XQC rocket flight vs thermal model (McCammon et al. 2002)
CXC
AAS/Chandra12
CX Spectra
High-n levels are preferentially populated.
CXC
AAS/Chandra12
CX Spectra
High-n levels are preferentially populated.
• H-like: enhanced high-n
H-like Fe (with N2 in EBIT)
CXC
AAS/Chandra12
CX Spectra
High-n levels are preferentially populated.
• H-like: enhanced high-n
• He-like: enhanced triplet f and i
EIE at 15 keV
He-like Fe
CX with N2
10 eV/amu
Wargelin et al. (2008)
CXC
AAS/Chandra12
Astrospheric Charge Exchange
CX must also occur around other
stars with highly ionized winds
(G,K,M) residing inside clouds
with neutral gas (LIC, G).
Imaging + spectra yields:
• Mass-loss rate
• Local nneutral
• Wind velocity and composition
• Astrosphere geometry
Coronal emission is ~104x brighter, though.
Need very large collecting area, good spatial and spectra resolution.
CXC
AAS/Chandra12
Reviews
“Charge Transfer Reactions”
Dennerl, Space Sci. Rev. (2010)
Astrophysical examples and broad historical review
“EBIT charge-exchange measurements and astrophysical applications,”
Wargelin et al., Canadian J. Physics (2008)
Astrophysical examples and lab spectra/atomic physics
CXC
AAS/Chandra12
CXC
AAS/Chandra12
Geocoronal
emission
responds to
SW much
faster than
heliospheric.
CXC
AAS/Chandra12
Talk amongst yourselves….
Things to think about:
• Roughly 10% of RASS SXRB is from unresolved Galactic point sources
• RASS was conducted at solar max
• R12 (1/4-keV) rate ~4 x R45 (3/4-keV) rate
• LTEs most prominent in R12 band
• No models or data for CX in 1/4-keV band
CXC
AAS/Chandra12
More detailed
correlations:
Short-term X-ray
variability vs ACE
O7+ flux  fractional
contribution of
geocoronal CX
emission
Account for Chandra
orbital geometry
CXC
AAS/Chandra12
More detailed
correlations:
Short-term X-ray
variability vs ACE
O7+ flux  fractional
contribution of
geocoronal CX
emission
Account for Chandra
orbital geometry
CXC
AAS/Chandra12
Diffuse X-Ray Spectrometer (DXS)
SXRB 150-300 eV. Sanders et al. (2001)
CXC
AAS/Chandra12
Where does the observed SWCX emission originate?
From within the He
cone, CX intensity is
higher and more of
the emission comes
from nearby, where
SW conditions can
be measured.
CXC
AAS/Chandra12
CX X-Rays in Olden Times
Idea dates back to 1970s:
• Galactic Ridge X-rays from
cosmic rays?
• X-ray lasers: population
inversion in “hollow ions”
• Tokamaks: plasma edges
and neutral beam heating
• Supernova remnants (Wise
& Sarazin 1989)
Cygnus Loop; Katsuda et al. (2011)
CXC
AAS/Chandra12
CXC
AAS/Chandra12
CX Spectra
High-n levels are preferentially populated.
• H-like: enhanced high-n
• He-like: enhanced triplet f and i
He-like O (with CO2).
Beiersdorfer et al. (2003)
CXC
AAS/Chandra12
Solar Max vs Solar Min
At solar max, wind is
a mix of slow and
fast at ~all latitudes.
At solar min, wind is
stratified, with slow
between latitude -20
and +20.
The fast wind is
much less ionized
and produces less
CX emission.
CXC
Ulysses data. E.J. Smith et al., Science 2003
AAS/Chandra12
Living in a Fog
Observed geo/helio SWCX emission depends on CX emissivity all
along the LOS, which means that it depends on:




SW flux for each X-ray emitting ion as f(t,x) (only measured locally)
Neutral gas density as f(t,x) (modeled)
Observation direction
Observation location
We can try to separate SWCX emission from cosmic components
with differential measurements:




CXC
Spatially (using dark clouds to block distant emission)
Spectrally (some day, with high-resolution nondispersive detectors)
Temporal changes
Observation geometry
AAS/Chandra12
Expectations for observed SXRB
in 2000 vs 2007:
• Higher baseline level
• More variability
• Closer correlation with ACE-measured
SW ion flux
CXC
AAS/Chandra12
CX X-Rays in Olden Times
Idea dates back to 1970s:
• Galactic Ridge X-rays from
cosmic rays?
• X-ray lasers: population
inversion in “hollow ions”
• Tokamaks: plasma edges
and neutral beam heating
• Supernova remnants?
Cygnus Loop; Katsuda et al. (2011)
CXC
AAS/Chandra12
CXC
AAS/Chandra12
CXC
AAS/Chandra12
Compare O emission (500-700 eV) vs…
Model CX spectrum with 6 eV resolution
CXC
AAS/Chandra12
Conducted at solar maximum.
CXC
AAS/Chandra12
Merge 52
observations
for source
detection
CXC
AAS/Chandra12
Remove 400+
sources
CXC
AAS/Chandra12
Trim radius to
common FOV
with radius 7.7’.
CXC
AAS/Chandra12
Prox Cen M5.5 V
EV Lac M3.5 V
x Boo G8 V + K4 V
= III,IV
l And, DK Uma
are marginal
detections.
Wood et al. (2005)
CXC
AAS/Chandra12