Are cosmic rays galactic or extragalactic?

1
Are cosmic rays galactic or extragalactic?

• A.A. Petrukhin, S.Yu. Matveev
• Moscow Engineering Physics Institute
• 
  Vulcano 08
• Contents
• Introduction cosmic ray origin problem.
• Gamma-flux of SMC a key for this problem
  solution.
• Calculations of gamma-flux from SMC.
• Discussion and conclusion.

2
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.

3
Solar, galactic, extragalactic cosmic rays

• For energy gt1 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.

4
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.

5
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.

6
Comparison of contribution of various processes
(F.W. Stecker)
7
Calculations and measurements
The first calculations of gamma flux from Small
Magellanic Cloud were done about 40 years ago and
gave the value 210-7 photons / cm2 s The
experimental results (EGRET data) were obtained
about 15 years ago and gave upper limit of the
flux lt 410-8 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.
8
Gamma-rays from neutral pion decay

• Jp(Tp) - proton differential spectrum
• sp(Tp) - cross section for p0 production
• ?(Tp) multiplicity of neutral pions
• ?? (gtE?,Tp,Tp) - probability of gamma ray
  formation with energy more than E?,

9
Proton differential spectrum
10
Approximation of experimental data for neutral
pion production in p-p reaction
11
Probability of gamma ray formation

• The probability ??(E ? ,T? ) is constant between
  minimum and maximum energy of gammas
• The probability ??(T? ,Tp) can be represented as
• Parameters Tc and w depend on kinetic proton
  energy

12
Results
13
Discussion-1
What is the reason of such big difference ( 2
times)? We calculated the contributions of p0
production for each channel separately (one pion,
two pions etc.). In the calculations based on the
works of F. Stecker total cross section of p0
production multiplied by pion multiplicity was
used. We compared these functions in both
calculations.
14
Comparison of used cross section approximations

• Approximation by F.W. Stecker

15
Discussion-2
1. 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 dont
give essential contribution in this
discrepancy. 2. If our calculations are correct
we can evaluate the expected flux from SMC as
0.610-8 photons/cm2 s approximately two time
less than previous value. Such decrease of the
flux from p0 production can drastically change
the situation with solution of problem of cosmic
ray origin by using defuse ?-flux from SMC.
16
Changes of contribution of various processes in
defuse gamma flux
17
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
• warning
• that 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.

18
Thank you !