The Galactic diffuse emission

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• The Galactic diffuse emission

Sabrina Casanova, MPIK Heidelberg
XXth RENCONTRES DE BLOIS 18th - 23rd May
2008, Blois
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Outline

• Motivations Sources of cosmic rays and galactic
  diffuse gamma emission
• GeV excess measured by EGRET
• Pinpointing the sources of cosmic rays
• TeV observations with HESS and Milagro
• Truly diffuse emission and unresolved sources
• Goals of future observations and theoretical
  speculations

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Do g-rays answer the open questions
concerning the origin of cosmic rays ?

• What are the cosmic ray accelerators and the
  primary spectra ?
• g-rays are produced by cosmic-ray interactions in
  their sources and
• point back to the production locations
• How high in energy can galactic sources produce
  particles?
• The highest energy particles produce
  highest energy g-rays.
• Are the accelerators of hadrons different from
  electrons?
• Hadronic and leptonic mechanisms produce
  different energy spectra and g-ray time
  variability
• How do cosmic rays propagate in the Galaxy ? What
  is the rate of production of relativistic
  particles ?
• Gamma ray spectrum and spatial
  distribution provide spectra and density of
  hadrons and leptons in different regions of the
  Galaxy

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Production mechanisms Hadronic Processes

• p p -gt po -gt g
• p p -gt p -gt e n

p
p
p
p
e
p
n
µ
n
?
p
p0
n
p
p
?
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Electromagnetic Processes

• Synchrotron Losses ( not important as g emission
  mechanism but important for the electron cooling
  )
• E g a (Ee/mec2)2 B
• Inverse Compton Scattering
• (dominant leptonic emission mechanism at
  GeV-TeV)
• E f (Ee/mec2)2 E I
• Bremsstrahlung (important emission mechanism at
  MeV energies)
• E g 0.5 E

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Exotic Gamma-Ray Production

• Particle-Antiparticle Annihilation
• WIMP called neutralino, c, is postulated by SUSY
  
• 50 GeVlt mclt few TeV
• Primordial Black Hole Evaporation
• As mass decreases due to Hawking radiation,
  temperature increases causing the mass to
  evaporate faster
• Eventually temperature is high enough to create a
  quark-gluon plasma and hence a flash of gamma-rays

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The conventional model for the Galactic
diffuse g ray flux.
Electron and proton flux measured locally
Matter distribution
Low energy photon density
Electron flux measured locally
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GeV excess measured by EGRET
Hard nucleon injection spectrum (Gralewicz et al.
1997 Aharonian Atoyan, 2000 )? Hard
electron injection spectrum (Porter Protheroe
1997, Strong Moskalenko, 2000 )? Physics
of ?0 production ( Kamae et al, 2004 )?
Unresolved ??- ray sources Exotic dark matter
(DeBoer et al, 2005) Instrumental EGRET
response (Stecker et al, 2007 Moskalenko et al,
2007)?
GeV Excess
Hunter et al. ApJ (1997)?
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Model of cosmic-ray production propagation in
the Galaxy optimized GALPROP model

• Uses antiproton gamma data
• to fix the nucleon and electron spectra
• Uses antiprotons to fix
• the intensity of CR nucleons _at_ HE
• Uses gammas to adjust
• the nucleon spectrum at LE
• the intensity of the CR electrons
• Uses EGRET data up to 100 GeV

EGRET
COMPTEL
?o
inverse Compton
TOTAL
brems- strahlung
extragalactic background
Strong,Moskalenko Reimer,2004
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• CONVENTIONAL MODEL the electron and proton
  spectra locally measured are representative of
  the Galactic cosmic ray spectrum everywhere in
  the Galaxy (Bertsch et al, 1993).
• OPTIMIZED GALPROP MODEL the proton and electron
  densities are allowed to vary roughly of a factor
  2 and 4 in order to match the EGRET data
  (Strong, Moskalenko Strong, 2004).
• Cosmic ray injection is a stochastic process
• The cosmic ray spectra close to injection sources
  vary in both spectral index and normalization
  with respect to the so called sea of cosmic
  rays due to energy dependent diffusion processes
  .
• The cosmic ray spectra close to sources are time
  dependent due to injection and diffusion history
  .

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The cosmic ray flux close to a source varies in
spectral index and intensity.
Aharonian Atoyan, 1996
CR sea
1 102yr
2 103yr
3104yr
4105yr
Do 1026 cm2/s
Do 1028 cm2/s at 10 GeV
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Detection of Passive clouds

• Maybe some of EGRET unidentified sources
• At energies lt 1 GeV GLAST can detect close
  clouds if M5 /d2 gt 0.1
• At energies gtgt 1 GeV GLAST can detect clouds only
  if
• M5 /d2 gt10

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Detection of clouds with an accelerator
Typical CLOUD n 130 cm-3 , radius 20 pc,
mass 105 solar masses
Impulsive source
Continuous source
102
105
103
104
103
104
105
102
Agile sensitivity at 1 GeV 4 X 10-8 GeV
cm-2 s-1
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Looking for pevatrons the emission from a SNR
and from a cloud close to the SNR
Gabici Aharonian 07
at 1 Kpc
400 yr
2000 yr
8000 yr
(104 solar masses)?
2000 yr
8000 yr
32000 yr
CR spectrum inside the SNR shell extends to PeV
energies mainly during the Sedov phase
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SNR stochasticity and electron spectrum
Swordy, ICRC 2003
100 TeV Electron
1 GeV Electron
B 3mG and CMB photons for 100 TeV electrons
te 103 years Rdiff 100 pc
Bremsstrahlung
E(dE/dt)-1,yr
Ionization
Coulomb
IC, synchrotron
107 yr
106 yr
Ekin, GeV
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TeV observations of diffuse sources
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TeV Diffuse Emission from the Galactic Center as
a Probe for Cosmic Ray Sources
Spectral index 2.29 0.07 0.20 implies
harder CR spectrum than in solar neighborhood ?
Proximity of accelerator and target
(Aharonian et al, 2006)?
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Correlation with molecular clouds
-0.2 lt b lt 0.2
molecular clouds
150 pc
at 8 kpc, 0.2 30 pc
at 8 kpc, 0.2 30 pc

• ? Interaction of CRs with molecular cloud material

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Milagro Galactic Longitude Profile
Inner Galaxy
(Abdo et al, 2008)?
-2 ltblt2
Optimized GALPROP model
Cygnus Region
The Cygnus Region shows an excess with respect
to the optimized GALPROP model. The emission
from the inner Galaxy is consistent with the
GALPROP optimized model.
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Column densities from Milagro inner Galaxy and
from the Cygnus Region.
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Milagro inner Galaxy
65
Cygnus Region
85
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Galactic Latitude Profile of Milagro Observations
(Abdo et al, 2008)?
total
IC
p0
The narrow data distribution seems to favour a
hadronic mechanism b00
and s 0.9 (for the inner Galaxy) and 2.0 (for
the Cygnus region)
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TeV Diffuse Emission from the Cygnus Region probe
the cosmic ray distribution
Abdo et al, 2007
100 pc
Cygnus Region Matter Density Contours overlaying
Milagro Obs.
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Truly diffuse emission or unresolved
sources ?
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Milagro emission from the inner Galaxy

• CR spectrum 1
• CR spectrum 2 hard spectrum due to a population
  of CR sources

E2 dN/dE TeV cm-2 s-1sr-1
1
2
TeV
Consequences for diffuse neutrino fluxes for
km3net
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Population of unresolved sources?
Aharonian et al., ApJ 636, 2006
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Number-intensity relation for HESS source
population
Diffuse emission due to unresolved sources

• 11 of 15 new HESS sources detected above 6 per
  cent Crab flux are included in the logN-logS plot
  
• TeV sources (PWNe and SNRs) distributed like
  radio pulsars in the Galaxy
• A significant part of the Milagro diffuse
  emission is due to unresolved sources

Casanova Dingus, 2008
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• Cosmic ray injection is a stochastic process
• The cosmic ray spectra close to injection sources
  vary in both spectral index and normalization
  with respect to the so called sea of cosmic
  rays due to energy dependent diffusion processes
  .
• The cosmic ray spectra close to sources are time
  dependent due to injection and diffusion history .

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Goals of Observations of Diffuse Sources

• Image spectrum spatial distribution of large
  scale Galactic diffuse emission
• Determine level of small scale emission that is
  clumpy (clouds)?
• Compare morphology of diffuse emission at the
  resolution of H2 and H1 survey
• Compare images spectra with those from other
  wavelengths
• Observe all possible photons energy fluxes

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Fluctuations in the cosmic ray flux produce
significant fluctuations in the gamma ray flux if
the region around the cosmic ray source contains
enough target material !
CR sea
Aharonian Atoyan, 1996
CR sea
Impulsive source
Continuous source