Are cosmic rays galactic Are cosmic rays galactic or extragalactic?or extragalactic?

A.A. Petrukhin, S.Yu. MatveevMoscow Engineering Physics Institute

Vulcano 08

Contents

1. Introduction: cosmic ray origin problem.2. Gamma-flux of SMC – a key for this problem

solution.3. Calculations of gamma-flux from SMC.4. Discussion and conclusion.

Introduction

The main questions of cosmic ray origin are:When? How? Where?

The first question has the simplest answer: permanently.

The second question has many answers, the number of which is approximately equal to the number of astrophysics theorists.

The third question has three general answers: Sun, Galaxy, Universe.

Solar, galactic, extragalactic cosmic rays

• For energy >1 GeV the contribution of solar particles in total cosmic ray flux is very small

• The main argument in favor of galactic origin is very large total energy of extragalactic cosmic ray flux in the Universe if their density is the same as near Earth.

• The main argument in favor of extragalactic origin is isotropy of cosmic rays.

How to choose between galactic and extragalactic models?

• There are very little experimental possibilities to do this choice.

• One of them (and the main) is measurement of diffuse -flux from other galaxies.

• If cosmic rays are extragalactic their intensity must be approximately the same in any galaxy.

• Therefore we can evaluate -flux from nearest galaxies (the best of them is Small Magellanic Cloud) taking into account CR flux near the Earth and compare it with experimental data.

Sources of diffuse gamma flux

Three basic processes of diffuse gamma-flux generation exist:

decays of neutral pions produced in pp-interactions

bremsstrahlung of cosmic ray electrons

Compton scattering of relic photons on electrons

Usually it is assumed that the first process is the main.

0 0 0... ; 2 .p p p p

e p e p

;e e E E

Comparison of contribution of various processes (F.W. Stecker)

Calculations and measurementsThe first calculations of gamma flux from Small Magellanic Cloud were done about 40 years ago and gave the value

2·10-7 photons / cm2 s

The experimental results (EGRET data) were obtained about 15 years ago and gave upper limit of the flux

< 4·10-8 photons / cm2 s

After that some authors announced that “cosmic rays are not metagalactic”. But last calculations (2001) gave the value

(1 - 2)·10-8 photons / cm2 s

Taking into account that results of AGILE and GLAST will be obtained very soon, we recalculated this value.

Gamma-rays from neutral pion decay

• Jp(Tp) - proton differential spectrum

• σπ(Tp) - cross section for π0 production

• ς(Tp) – multiplicity of neutral pions

• ω (>E,Tp,Tπ) - probability of gamma ray formation with energy more than E,

0

( ) ( ) ( ) ( , , ) ( )p p p p p pJ E T T E T T J T dT

Proton differential spectrum 3 2 2.73( ) 10 (1 ( ) ) ( )p p p

p pp p p

m T mJ T

T m m

Approximation of experimental data for neutral pion production in p-p reaction

( )1 exp( ( ))p

p c

aT

k T T

Probability of gamma ray formation

• The probability ω(E ,T ) is constant between minimum and maximum energy of gammas

• The probability ω(T ,Tp) can be represented as:

• Parameters Tc and w depend on kinetic proton energy:

( , , ) ( , ) ( , )p pE T T E T T T

2

2

( )1exp( 2 )

/ 2cd T T

dT ww

( ) 0.45 94.7c pT MeV T

( ) 0.26 28.3pw MeV T

Results

Discussion-1

What is the reason of such big difference (~ 2 times)?

We calculated the contributions of π0 production for each channel separately (one pion, two pions etc.).

In the calculations based on the works of F. Stecker total cross section of π0 production multiplied by pion multiplicity was used.

We compared these functions in both calculations.

Comparison of used cross section approximations

• Approximation by F.W. Stecker:

0 0

25 7.642

27 0.53

10 ,0.4 0.7( ) ( )[ ]

8.4 10 , 0.7

T T GeVT T cm

T T GeV

Discussion-21. Apparently this difference is the main reason of discrepancy between two calculations.

Other functions and in the first turn the behavior of proton energy spectrum at small energies don’t give essential contribution in this discrepancy.

2. If our calculations are correct we can evaluate the expected flux from SMC as 0.6·10-8 photons/cm2 s approximately two time less than previous value.

Such decrease of the flux from π0 production can drastically change the situation with solution of problem of cosmic ray origin by using defuse γ-flux from SMC.

Changes of contribution of various processes in defuse gamma flux

Conclusion

The main purpose of my talk is not to give the final value of gamma flux from SMC since for that recalculations of contribution of all processes must be done, but to give

warningthat situation is not so clear and more detail analysis is required before future AGILE and GLAST data will be possible to use for the solution of CR origin problem.

Thank you !