Interactions SNR/MC

Report
Supernova remnants and
molecular clouds
Armand Fiasson
LAPP - Annecy-le-vieux
Where CRs originate?
• The usual suspects up to the knee are
supernova remnants (SNRs)
– Diffusive shock acceleration
– Conversion efficiency into CR ~10-30 % needed and
expected
• TeV γ-ray emitters (RX J1713, Vela Jr …)
=> particle acceleration up to 100 TeV
• But…
– Hadronic or leptonic?
– PeV accelerators?
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Hadronic or leptonic?
Tycho - Fermi
RX J1713.7-3946 - Fermi
Giordano et al. 2012
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Abdo et al. 2011
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Hadronic or leptonic?
RX J1713.7-3946
Tycho
Giordano et al. ApJ 744 L2 (2012)
Both leptonic and hadronic interpretation come out
from these observations
Does RXJ1713.7-3946 accelerate hadrons?
Not enough matter to detect them?
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Abdo et al. ApJ 734 28 (2011)
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The key point: matter
• When a leptonic scenario is favoured: no hadrons or not
enough target?
• A solution: dense matter concentration surrounding the
remnant = molecular clouds
– Enough matter to detect hadrons in the remnant when interacting
with the cloud
Drury, Aharonian & Voelk 1994
– Clouds in the vicinity of the remnant may help detecting the highest
energy particle that escaped the remant
Aharonian & Atoyan 1996
Gabici et al. 2009
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Molecular clouds
• The molecular clouds are major components of the ISM
– Cold matter => T~10-20 K
– Neutral molecular matter: mainly H2
– Density above a few 10 atoms/cm-3 up to 106 atoms/cm-3
 When integrated over 10 pc scale = very massive target!!
• Visibility in the range radio – millimetric wavelength
The H2 has no electric dipole => no rotationnal line
=> detection through other lines CO, CS…
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SNR/MC associations
Already suggested as potential gamma-ray emitters in 1979
Montmerle, 1979
•
•
•
•
•
Massive stars originate in dense regions => dense molecular clouds
These stars end rapidly in SNRs
These SNRs evolves in the vicinity of the parent cloud
The SNRs accelerate CRs
The CRs interact into the cloud and produce γ-rays
 Such associations should be frequent
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OH masers
• The distance to SNRs is not well known
 association with MC difficult to assess
• Physical interactions can be traced
– Line broadening, OH masers …
• OH masers are produced via collisions
 shock propagating through a dense cloud
Wardle, 2002
 >10% of the known SNRs are OH emitting
• Correlation between OH emission and gammaray emission
Hewitt et al., 2009
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List of known associations
34 confirmed
+ 19 probable
+ 11 possible
> 20 coincident
γ-ray sources
Jiang et al. 2010
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Caveats
•
SNR-MC associations seem to be good targets to search for CR acceleration but …
•
Some caveats to keep in mind:
– OH masers indicate physical SNR-MC association
The shock propagation is affected by the dense matter
What about the CR acceleration efficiency?
– Unusual SNR morphology due to the inhomogeneous medium
Probably not the best candidates for comparison with theoretical
predictions
– Maser emitting SNRs appear to be rather evolved SNRs with age ~ 10 kyr
Most energetic particles have escaped from the remnant
Not the best candidates to study the highest energy in the shock region
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4 years ago…
MAGIC
VERITAS
HESS
A few TeV sources with strong indication of
hadronic origin
W28, IC443, CTB37A, G359.1-0.5, W51
Fermi & AGILE observations were in
progress
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The picture now
AGILE
Fermi-LAT
MAGIC
VERITAS
HESS
All the TeV sources have been confirmed at GeV energies
by Fermi or AGILE
+ other candidates have been discovered (GeV and/or TeV)
…
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W28, IC443, CTB37A, G359.1-0.5, W51
W44, W30, Kes41, G349.7+0.2, W41, W49, 3C391 …
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W28
HESS – Aharonian et al. 2008
AGILE – Giuliani et al. 2010
Fermi – Abdo et al. 2010
• Several clouds detected by HESS at TeV energies
 Molecular clouds in the vicinity of the shock (OH masers)
 Plus giant clouds distant from the shock
• Confirmation at lower energies by AGILE and Fermi
… but 2 hotspots over 4 visible
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W28 - CR Diffusion
AGILE – Giuliani et al. 2010
Fermi – Abdo et al. 2010
• Hadronic origin of the γ-rays favoured
• The brightest spot in TeV is the fainter in
GeV
– Different SNR-MC distance
 Escaped CR from the shell having
diffused toward the clouds
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IC443
Ackermann et al. 2013
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W44
Ackermann et al. 2013
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GeV-TeV overview
•
Hadronic origin of the γ-rays
favored
•
Bright GeV sources
•
Faint TeV sources + soft
spectra
•
Break in the CR spectra at
GeV energies
 Diffusion effects probably
Ackermann et al. 2013
Li & Chen 2012
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Neutrino from these clouds?
Qiang et al. 2011
• Soft hadron spectra above a few GeV
 low neutrino flux expected above 1 TeV
 ~0.1-0.5 neutrino/year
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CR and cloud ionisation
• Molecular clouds are composed mainly of
neutral H2
 Ionised by the presence of CRs
 Modified chemistery
HESS-W51
• The abundance ratio of several species is
proportionnal to the ionisation rate
• GHz observations have been led towards IC443
and W51
 Over-ionisation in the cloud associated with
IC443 and W51
 ~100 times the typical value
MAGIC - IC443
Indriolo et al. 2010
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Ceccarelli et al. 2011
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What to expect from CTA?
HESS 500h
CTA 500h
Hermann 2008
• Improved sensitivity above a few 10 GeV by a factor ~10
• Improved angular resolution by a factor ~5
 Increased number of sources at TeV energies
 Increased sensitivity to the highest energies and
propagation effects
Acero et al. 2012
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Summary
SNR/MC association: a class of sources in rapid expansion
• A lot of new candidates at TeV energies (HESS MAGIC VERITAS)
• A lot of detection at GeV energies and precise spectral measurement by
Fermi & Agile
 Point to a hadronic origin of gamma-rays
 Common spectral features
• Propagation effects visible (W28, IC443…)
• Interesting class of sources for various community
– CRs & ionisation into molecular clouds and their chemistery
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