MSSL
Emission lines all around us:
Charge eXchange near and far
G. Branduardi-Raymont
Mullard Space Science Laboratory
University College London
(reviewing lots of other people’s work)
‘Clusters of galaxies and hot baryons’
MSSL, 6 – 8 October 2015
MSSL
X-ray production by Charge eXchange (CX)
• Charge eXchange (CX)
Solar Wind Charge eXchange (SWCX)
Highly ionised ions collide with neutrals/molecules
electron capture (‘charge exchange’)
de-excitation with X-ray line emission
e.g. H2 + O7+
H2+ + O6+ + hν
ν
• Studied since origin of atomic physics
• X-ray production efficiency
recognised ~20 years ago: Jupiter &
cometary emissions
Cravens et al. 1995 & 1997, Dennerl 2010
(~10-15 cm2)
• Very high CX cross-section
so works well with low density target gas
7+
O
O7+
6+*
O
O6+*
OO6+6+
Dennerl 2009
• Scattering of solar X-rays has cross-sections <10-18 cm2, so requires
target of high density to be effective
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Comet C/2000 WM1, 2001 Dec. 13 – 14
Optical
XMM-Newton
Dennerl et al. 2003
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Comet C/2000 WM1: combined RGS + EPIC pn spectrum
residuals
counts s 1 keV 1
EPIC pn
RGS 1
RGS 2
0.2
0.3
0.5
instrumental energy [keV]
1.0
Dennerl et al., priv. comm.
MSSL
Jupiter – XMM-Newton, 2003: EPIC
OVII (0.55–0.60 keV)
MgXI (1.30–1.40 keV)
FeXVII (0.70–0.75, 0.80–0.85)
5 – 10 keV
B-R et al. 2007
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Jupiter – XMM-Newton, 2003: EPIC
Jupiter’s auroral and disk spectra
C or S?
OVII by CX
North
0.2 – 2 keV: ~ 0.5 GW
2 – 7 keV: 90 – 40 MW
Disk coronal spectrum
South
Electron
bremsstrahlung
B-R et al. 2007
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Jupiter – XMM-Newton, 2003: RGS
OVIII
FeXVII
OVII
• RGS clearly resolves auroral CX from disk soft X-ray
emission lines (but not the C/S ambiguity)
• Width of OVII and OVIII lines corresponds to energies of
few MeV for O ions
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Jupiter – Athena X-IFU simulation
XMM-Newton RGS – 210 ks
Athena X-IFU – 20 ks
- Extended wavelength range
- 2 orders of magnitude higher effective area
- Non-dispersive spectroscopy
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Mars disk and exosphere (halo): XMM-Newton RGS
• Fluorescent scattering of solar X-rays in CO2 atmosphere
• Solar wind charge exchange (SWCX) in the exosphere
+/- 50”
+/- 10”
- 50” to - 15” and + 15” to + 50”
Dennerl et al. 2006
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X-rays from Venus at solar maximum (2001)
• Fluorescent scattering of solar X-rays in upper atmosphere
• O-Kα, C-Kα (and N-Kα ?) detected; also CO/CO2 signature
Chandra ACIS/LETGS
Dennerl et al. 2002
• Venus exosphere
more condensed than Mars
SWCX radiation closer to limb
Easier to detect at
solar minimum (2006)
Dennerl et al. 2012
SWCX
Fluorescence
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X-rays from Venus at solar minimum (2006)
Chandra ACIS
• First evidence of exosphere
X-rays from SWCX at the
sunward limb
0.29 – 0.51 keV
• O-Kα fluorescence of solar
X-rays uniformly distributed over
illuminated disk
0.51 – 0.55 keV
C, N
O-Kα
α
0.55 – 0.79 keV
• Spectra from the two regions are
indeed different
Dennerl et al. 2008
OVII
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The Earth’s geocorona
• LTE of the ROSAT All Sky Survey ¼ keV background
Snowden et al. 1995
• Time variable O emission lines
on the dark side of the Moon
Chandra ACIS
Wargelin
et al. 2004
Correlation with solar wind
flux
SWCX in Earth’s geocorona
• Suzaku observations of the NEP:
Increase in soft X-ray lines correlated
with solar wind proton flux
• Systematic study with XMM-Newton
Carter et al. 2008, 2010 (CME), 2011
Fujimoto et al. 2007
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A global view of the Earth’s magnetosphere
• SWCX expected to produce a large increase in X-ray line flux
during solar wind storms
• SWCX X-rays can be used to
image boundaries of the Earth’s
dayside magnetosphere
• Validate models and reach better
understanding of the Earth’s response
to the impact of the variable solar wind
• SMILE joint ESA and CAS mission
currently under study: soft X-ray telescope,
highly elliptical polar orbit
B-R, Wang et al. 2015
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The Earth’s magnetosphere on 17th March 2015
T. Sun, 2015
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Solar system CX X-ray emission as a ‘background’
• The heliosphere is also shining in SWCX X-rays by SW encounter
Cox 1998
with inflowing ISM stream
• Solar system CX (geocorona and heliosphere) now treated as b/g:
can be separated by variability, e.g. XMM-Newton HDFN,
Chandra MBM12 molecular cloud, Suzaku NEP observations
Snowden et al. 2004, Smith et al. 2005, Fujimoto et al. 2007, Carter et al. 2011
• Controversial claim of detection of WHIM emission, with possible
SWCX OVII possibility
Kaastra et al. 2003, Bregman Lloyd-Davies 2006, Nevalainen et al. 2007
• Local Hot Bubble affected by solar system CX (~60 pc cavity
around Sun, 106 K emission, now to be shared with geocoronal
and heliosphere CX): a large part, if not all, could be due to CX
Koutroumpa et al. 2007, 2008, 2009
• Can be resolved via line ratios with high resolution spectrometers
MSSL
High resolution spectroscopy of CX X-ray emission
• CX X-ray emissions have no continuum, only lines and different
line ratios from cooling hot gas
• Cross-sections for collisional (hot) plasmas strongly favour
n= 2 1 >> 3 1 >> 4 1 etc. …
• Not so for CX: High-n lines enhanced at low collision energies,
more for H-like
than for He-like
spectra
• For Heand H-like
Wargelin et al. 2008
spectra …
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Hot gas – cool clouds interfaces
• Interfaces between hot gas and cool clouds in ISM expected to
be likely locations of CX emissions
• For H atom in a hot plasma mean free path for CX ~ same as
mean free path for collisional ionization
Lallement 2004
MSSL
Extra-solar CX X-ray emission scenarios
Expectation/evidence for CX in
•
•
•
•
•
•
•
Local ISM: North Polar Spur
SNRs
High velocity clouds in Galactic halo
Galactic Ridge
Stellar winds
Galactic winds
Clusters of galaxies
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Local ISM: North Polar Spur
Emission in unresolved OVII and NeIX triplets
shifted to lower energies by 10 – 15 eV w.r.t.
OVIII and NeX
Lallement 2009
Miller et al. 2008
Willingale et al. 2003
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Supernova Remnants
• SNR also contain interfaces between hot gas and cool clouds
• Early CX suggestions
Chamberlain 1956, Serlemitsos et al. 1973
• Detailed study of expected CX emission in SNR from neutral H
and highly ionised heavy ions at periphery of fast shocks vs
collisional ionisation, recombination, excitation
CX no more
than 10% (unless gas mixed)
Wise & Sarazin 1989
• While CX emission from interface regions is negligible, it could
be enhanced by limb brightening
Lallement 2004
• Cygnus Loop: Suzaku spectra, OVII triplet enhanced forbidden
to resonance line throughout the rim; correlation with Hα
filaments
Roberts & Wang 2015
•
Need to model CX contribution!
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The Galactic halo
• Galactic halo itself may have significant CX: MHD simulation of
SN-driven ISM (hot gas flow from disk to halo) under-predicts
0.5 – 2 keV surface brightness
Henley et al. 2015
High velocity clouds in Galactic halo
• ROSAT flux from high velocity clouds is ~ same order expected
for CX from clouds in a hot halo
Kerp et al. 1999, Lallement 2004
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Galactic Ridge
• CX has been considered as possible origin of X-ray emission
from Galactic Ridge (disk-like region around GC, radius several
kpc, scale height ~ 100 pc)
• Early investigations
Watson 1975, Blint et al. 1976, Christensen et al. 1977
• CX by low energy CR heavy ions with ISM neutral H (ASCA)
Tanaka et al. 1999, 2002
• Not supported by
Suzaku observations
(higher resolution,
OVII …)
Ebisawa et al. 2008
• However, more recent Chandra observations do not support CX
hypothesis, rather superposition of individual sources
Revnivtsev et al. 2009
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Stellar winds: late-type stars
• SWCX with ISM neutrals entering the heliosphere
winds in
late-type stars
Wargelin & Drake 2001
• Stringent constraints on mass loss
from Proxima Centauri from
absence of CX emission
Wargelin & Drake 2002
• Exoplanets: Stellar wind –
Hot Jupiter CX can produce
1022 erg s-1 X-ray emission
(106 x Jupiter aurora)
Kislyakova et al. 2015
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Stellar winds: The Orion Nebula
• XMM-Newton EPIC spectra show that hot plasma at ~ 2 MK
pervades the SW extension of the Nebula and must be flowing
into the adjacent ISM
Guedel et al. 2008
• Hot gas channelled by cooler structures
• Outflow phenomenon (and CX) may
be common to massive star-forming
regions
likely CX production
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Galactic winds: M82 & M31
• Starburst galaxy winds have been shown to interact with cool
halo clouds
CX X-ray emission could be non-negligible
Lallement 2004
• Hot wind drives a shock wave into cool clouds and heats them,
clouds recombine and emit Hα, X-rays from shocked wind + CX
• Possible detection of a CVI transition (n = 4) at 0.46 keV in M82
Tsuru et al. 2007
• M82 OVII triplet line ratios consistent
with those expected for CX
Ranalli et al. 2008
• Similar OVII line
ratio implications
for M31
Liu et al. 2010
ratio
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Galactic winds: M82 & M51
• Detailed analysis of 104 ks XMM-Newton observation of M82
Zhang et al. 2014
with CX & thermal plasma model
• CX largely responsible for
enhanced forbidden lines of
various He-like ion triplets and
good fractions of Lyα transitions
of CVI, OVIII and NVII
• M51: Thermal spectrum but OVII
triplet is forbidden line dominated
• OVII triplet flux located at faint
regions near edges
CX?
Liu & Mao 2015
Power law – black
Thermal plasma – orange
CX – purple
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Clusters of galaxies
• X-ray emission from central regions of clusters so bright to imply
cooling of intracluster gas on timescales shorter than cluster’s
lifetime
• Spectra (e.g. XMM-Newton RGS) of cooling gas cannot be fitted
by cooling flow models: expected coolest gas is missing
Peterson et al. 2003
• Optical (Hα) filaments correlated with X-ray
emission suggest interfaces formed in mixing
layers
CX at interfaces producing X-rays
• Estimates suggest that CX contribution non
negligible, could mimic emission from hot gas
and lead to overestimate the cooling if pure
cooling flow model applied
Lallement 2004
MSSL
In summary:
CX is really ubiquitous in the Universe,
so we are better to get used to including it
in our spectral modelling!
Thank you