Download - The Galactic diffuse emission
The Galactic diffuse emission
Sabrina Casanova, MPIK Heidelberg
XXth RENCONTRES DE BLOIS 18th - 23rd May 2008, Blois
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
What are the cosmic ray accelerators and the primary spectra ?-rays are produced by cosmic-ray interactions in their sources
andpoint back to the production locations
How high in energy can galactic sources produce particles?
The highest energy particles produce highest energy -rays. Are the accelerators of hadrons different from electrons?
Hadronic and leptonic mechanisms produce different energy spectra and -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
Do -rays answer the open questions concerning the origin of cosmic rays ?
p + p o +… +… p + p ± +… e ± + +…
Production mechanisms :Hadronic Processes
p
p
p
p
π0
γ
γp
p
n
p
π+
μ+
e+
• Synchrotron Losses ( not important as emission mechanism but important for the electron cooling )– E (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 ~ 0.5 E
Electromagnetic Processes :
“Exotic” Gamma-Ray Production
Particle-Antiparticle Annihilation • WIMP called neutralino, is postulated by SUSY • 50 GeV< m< 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
q
qor or Z
lines?
The conventional model for the Galactic diffuse ray flux.
)( )(
),( 4
)()()(
)( ),(
)(),(
4
,
02
2
,,
02
max
max
rudE
EEdrEdE
rdV
rnrnrn
dEEEd
rEdE
EEdrEdE
rdV
dtdEdAddn
kICkek
r
HIIHHI
kbremsske
kHadrkpk
r
Electron and proton flux measured locally
Electron flux measured locally
Matter distribution
Low energy photon density
Hunter et al. ApJ (1997)
GeV Excess
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 measured by EGRET
EGRET COMPTEL
extragalacticbackground
inverseCompton
brems-strahlung
o
TOTAL
Model of cosmic-ray production & propagation in the Galaxy: optimized GALPROP model
Uses antiproton & gamma dataUses antiproton & gamma data
to fix the nucleon and electron spectrato fix the nucleon and electron spectra
Uses Uses antiprotonsantiprotons to fix to fix
the the intensityintensity of CR nucleons @ HE of CR nucleons @ HE
Uses Uses gammasgammas to adjust to adjust the nucleon spectrum at LEthe nucleon spectrum at LE the the intensity intensity of the CR electrons of the CR electrons
Uses EGRET data Uses EGRET data up to 100 GeVup to 100 GeV
Strong,Moskalenko & Reimer,2004
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 .
The cosmic ray flux close to a source varies in spectral index and intensity. Aharonian & Atoyan, 1996
1= 102yr2 =103yr3=104yr4=105yr
CR sea
Do = 1028 cm2/s at 10 GeV Do = 1026 cm2/s
Detection of Passive clouds
s cmph
TeV 110 x 1.5 Flux 22
51.75
13-
kpcdME
Maybe some of EGRET unidentified sources
At energies < 1 GeV GLAST can detect close clouds if M5 /d2 > 0.1
At energies >> 1 GeV GLAST can detect clouds only if M5 /d2 >10
Detection of clouds with an accelerator
Impulsive source Continuous source
Agile sensitivity at 1 GeV : 4 X 10-8 GeV cm-2 s-1
102
103
104105
102
103
104
105
Typical CLOUD : n = 130 cm-3 , radius = 20 pc, mass = 105 solar masses
Looking for pevatrons: the emission from a SNR and from a cloud close to the SNR
Gabici & Aharonian 07
400 yr
2000 yr
8000 yr
32000 yr
(104 solar masses)
at 1 Kpc
8000 yr
2000 yr
CR spectrum inside the SNR shell extends to PeV energies mainly during the Sedov phase
SNR stochasticity and electron spectrum
107 yr
106 yr
BremsstrahlungBremsstrahlungIonizationIonization
CoulombCoulomb IC, synchrotronIC, synchrotron
Ekin, GeVEkin, GeV
E(dE
/dt)
E(dE
/dt)-
1-1,y
r,y
r
B = 3G and CMB photons
for 100 TeV electrons
te = 103 years Rdiff= 100 pc
1 GeV Electron 100 TeV Electron
Swordy, ICRC 2003
TeV observations of diffuse sources
(Aharonian et al, 2006)
Spectral index2.29 ± 0.07 ± 0.20 implies harderCR spectrum than insolar neighborhood Proximity of accelerator and target
TeV Diffuse Emission from the Galactic Center as a Probe for Cosmic Ray Sources
Correlation with molecular clouds
Interaction of CRs with molecular cloud material
150 pc
molecular clouds
-0.2° < b < 0.2°
at 8 kpc,0.2° ~ 30 pc
at 8 kpc,0.2° ~ 30 pc
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.
Milagro Galactic Longitude Profile
2 x
GP
Cygnus Region
Optimized GALPROP model
Inner Galaxy (Abdo et al, 2008)
-2 <b<2
Column densities from Milagro inner Galaxy and from the Cygnus Region.
85°
65°
30°
Milagro inner Galaxy
Cygnus Region
(Abdo et al, 2008)
Galactic Latitude Profile of Milagro Observations
The narrow data distribution seems to favour a hadronic mechanism
b0=0 and = 0.9 (for the inner Galaxy) and 2.0 (for the Cygnus region)
2σ ~
20bb
eFlux
IC total
TeV Diffuse Emission from the Cygnus Region probe the cosmic ray distribution
Cygnus Region: Matter Density Contours overlaying Milagro Obs.
Strong & Moskalenko
GALPROP model of Cygnus Region
standard
optimized
Inverse Compton
Pin
bremsstrahlung
Abdo et al, 2007
100 pc
“Truly” diffuse emission or unresolved sources ?
Milagro emission from the inner Galaxy
TeV
E2 d
N/d
ET
eV c
m-2
s-1sr-
1
CR spectrum 1:
CR spectrum 2 : hard spectrum due to a population of CR sources
PeVEe
GeVEE 1/
5.0
2
801
1
2
Consequences for diffuse neutrino fluxes for km3net
Population of unresolved sources?
Aharonian et al., ApJ 636, 2006Aharonian et al., ApJ 636, 2006
Number-intensity relation for HESS source population
• 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
Diffuse emission due to unresolved sources
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 .
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
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 !
Impulsive source Continuous source
2
5)(dMEEF
Aharonian & Atoyan, 1996CR seaCR sea