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