ELSEVIER Nuclear Physics B (Proc. Suppl.) 60B (1998) 33-36

I ~ | I I I W'-Ya 'd -" i'&~ [4&1 L1

P R O C E E D I N G S S U P P L E M E N T S

Gamma ray astronomy and the origin of cosmic rays

A.W. Wolfendale

Physics Department, University of Durham, Durham, DH1 3LE, United Kingdom

A brief survey is made of the r61e of Gamma Ray Astronomy in elucidating the origin of cosmic, rays. Topics considered include gamma rays from the Magellanic Clouds, the radial gradient of cosmic rays in the Galaxy, evidence for SNR acceleration and the presence of a Giant Galactic Halo.

1. ~ T R O D U C T I O N

The problem of the origin of cosmic rays (CR), by which we mean the predominant particle - component, arises - as is well known - from the presence of the tangled Galactic magnetic field which causes the paths of the vast majority of CR to bear no relationship to the directions to their sources. Help has come, however, from the gamma ray component, or at least those gamma rays which can be identified as the interaction products of CR on gas nuclei (and photons) in the ISM, and elsewhere. Thus, if the target gas is known and the gamma rays can be measured, then the CR intensity can be inferred at places remote from the earth.

2. THE M A G E L L A N I C CLOUDS

2.1. The Large MageUanic Cloud

There are two reasons for interest in the flux of gamma rays from the Large Magellanic Cloud, both relating to the intensity of the initiating cosmic rays in that Galaxy and thus to their origin. The first is to do with the so-called Ginzburg test (Ginzburg, 1972) in which the inferred mean CR intensity in the LMC is compared with that locally, at earth. If the former is significantly smaller than the latter then an extragalactic origin for all the local particles can be ruled out. The second concerns the extent to which there are mechanisms in the LMC which themselves generate CR particles.

New data from the EGRET instrument on the Compton Gamma Ray Observatory (CGRO) (see references) allowed A1-Dargazelli et al. (1996a) (to be referred to as ref. I) to examine these problems in

some detail and a brief summary of their results will be given here.

Before continuing it can be remarked that there have been previous studies of the problem. Thus, Houston and Wolfendale (1982) and Fichtel et al.

(1991) had made predictions of the flux of gamma rays to be expected from the LMC before CGRO observations were available and, since CGRO, there have been analyses by Chi and Wolfendale (1993) and Sreekumar et al. (1992). The results of the last two analyses differed sharply, with Chi and Wolfendale arguing that the average CR intensity in the LMC is much less than that locally and Sreekumar e t al. estimating an LMC CR intensity very similar to that locally. The reasons for this remarkable discrepancy were considered in I and it was concluded that the average CR intensity in the LMC is indeed much less than that locally. Specifically, the average CR intensity in the LMC (by 'CR' here we mean mainly protons in the range 3-30 GeV and electrons of energy 0.1-1 GeV) is (21 + 8)% of that locally.

A further feature of considerable interest reported in I was the gradual displacement of the peak position of the gamma ray contours with increasing energy. Figure 1 shows the situation. It will be noted that the peak moves from being near the centre of the gas distribution to being near the centre of the gas distribution to being near that of the photon fields. The reason advanced in I was that Inverse Compton (IC) interactions play an important r61e in generating the gamma rays and that, because of the somewhat flatter IC spectrum, they play a near- dominant r61e at the highest energies.

0920-5632/98/$19.00 © 1998 Elsevier Science B.V. All fights reserved. PI! S0920-5632(97)00499-4

34 .4. W. Wolfendale/Nuclear Physics B (Proc. Suppl.) 60B (1998) 33-36

-30" , ,

-31" Hp 0

-32" "HC / o

-33" z~

YC -3/," ta UP

_35"1 , , 281"

30 Don

\

Os~

o UC

~ B

. !

280" 279" 278" 277" = i

I

Figure I. Peak positions, (denoted by P) and Centroids (denoted by C) for the LMC. Key H: gas (HI and H2); R: radio, 408 MHz; U: ultra-violet, Y: yellow light; B: 'bright stars'; 3°Dor: 30 Doradus region of very violent stellar activity; SX-X-rays from SNR (centroid). --- peak of gamma ray intensity from ref. I, the arrow denotes increasing gamma ray energy. See ref. II for references to the data.

2.2. The Small Magellanic Cloud

In the early work, by Sreekumar et al. (1992) and by Chi and Wolfendale (1993), only upper limits to the T-ray flux from the SMC were reported. However, AI-Dargazelli et aL (1996b) (denoted, ref. II) claimed a detection for gamma rays of energy above 300 MeV. The ratio of fluxes from SMC to LMC (above 300 MeV) was reported as 0.39 + 0.17.

2.3. Conclusions from the Magellanic Cloud Analyses

The measurements indicate that the average CR intensity in the MC is much less than that locally (by a factor -- 5) showing that the local CR are not extragalactic in origin, for which equality is required. The earlier claims, by Dodds et aL (1975), for a 'cosmic ray gradient' in the Galaxy, and thus a Galactic origin for most CR detected at earth, are thus confirmed.

The nature of the sources of CR in the MC which presumably produce the majority of the CR in the Clouds, is not known. The ratio of the inferred T-ray

emissivities for the Clouds, SMC/LMC is 0.57 + 0.25, a figure that is remarkably high in comparison with other ratios for the Clouds (e.g. stellar mass, 0.2; radio, 0.12; SNR, 0.19). A further complication is that the 'source' of y-rays in the SMC appears to be small compared with the gas contours, and the similar contours in the LMC. It is possible that a discrete 7- ray source is responsible for many of the SMC y-rays.

3. THE RADIAL GRADIENT

The rt le of studies of the radial gradient in elucidating cosmic ray origin has already been mentioned. It is now quite certain that the gradient exists (e.g. Strong and Mattox, 1996) despite earlier claims for its absence at y-ray energies above 300 MeV.

10

aiR)

1-0

0-1

\ \ ' \ /~ ' .. . . . . . .

H~

/ \ \ )

SUN

R(kpc}

Figure 2. Surface densities of various entities versus Galactocentric distance (the sun is at R = 8.5 kpc).

P1 - pulsars after Lyne et al. (1985) S1 - SNR after Kodaira (1974) $2 - SNR after Li et al. (1991), with ro = 7 kpc OB - OB associations for e : 90°-180°-270 ° M - mass distribution for stellar material after Bahcall and Soneira (1980).

A.W. Wolfendale/Nuclear Physics B (Proc. Suppl.) 60B (1998) 33-36 35

The heavy line is the mean CR intensity (for E > 100 MeV) vs R normalized to unity at R = 8.5 kpc.

It is very evident that the CR gradient is much smaller than that of astronomical entities which might be expected to have relevance to CR acceleration (SNR, OB associations, etc.). Figure 2 shows the situation.

Fatemi and Wolfendale (1996) and Erlykin et al. (1997) have suggested that the reason for the disparity is the presence of a large Galactic Halo. Such a Halo, of linear dimension = 100 kpc, was, in fact, put forward by Wdowczyk and Wolfendale (1995) and by A1-Dargazelli et al. (1996c) in connection with the trapping of CR of very high energies (iron nuclei up to 1018 - 1019 e V ) . There is astronomical evidence, too, for such a Giant Halo - as summarised in the latter paper just mentioned.

There is also interest in what might be called the 'fine-structure' of the gradient as a means of pressing the search for acceleration sites - and propagation characteristics - further.

A number of workers (Grenier et al. (199?, Erlykin et al., 1997) have drawn attention to a longitude-dependence of the gradient. Specifically, the gradient is steeper in the 2nd Quadrant than in the 3rd. This is an interesting result in that the number of SNR in the 3rd Quadrant is smaller than that in the 2nd. Taken together with Figure 2 it seems difficult to avoid the conclusion that the Galaxy does, indeed, have a Giant Halo.

LIII LI

,o- @. 4 -

1180 120 60 0 --60 --120 - 1 8 0

¢-3 i

I(n ¢~

b -

I180

LIII LII LI

I ; J I _1120 120 6 0 - 6 0 - 1 8 0

Goloctlc Longltude(degrees)

Figure 3. CR emissivity (q/4n) from COS B data for

Er: 300-5000 MeV for two latitude ranges: b+ (+10°/+20 °) and b- (-10°/-20°). The positions of the SNR Loop edges are indicated. A rough lower envelope is indicated - the Loop edges are seen to be the sources of enhanced emissivity, viz, the sites of CR acceleration (after Wolfendale and Zhang, 1994).

5. CONCLUSIONS - AND THE WAY AHEAD

4. SPECIFIC COSMIC RAY SOURCES

That most CR seen at earth come from Galactic sources is now quite clear but the nature of the sources is less so. The present author feels that the case for SNR shocks - for energies less than about 1016 e V - is quite strong but overwhelming evidence is lacking. The gamma ray technique has been used in a search for excesses associated with the edges of the large local SNR with apparently positive results.

Figure 3 shows recent work in this area. The evidence looks good for SNR shocks accelerating the parents of 0.1-1 GeV y-rays, at least.

There is also work in similar vein in which distant SNR have been identified in y-rays above 100 MeV (Sturner and Dermer, 1995).

The y-ray technique has had a number of successes in our efforts to find out where low energy CR are coming from.

In order of diminishing certainty these are; (i) the demonstration that most (> 80%) of local CR of low energy (nuclei with E: 1-30 GeV and electrons with E: 0.1-I GeV) are derived from Galactic sources. (ii) the low energy CR are accelerated by SNR shocks. (iii) the Galaxy has a 'Giant Halo' extending to some 100 kpc from the sun.

For the future, more precise measurements of y- ray fluxes, and the associated astronomical parameters: gas mass, radiation fields, SNR shocks, pulsar characteristics and positions, etc., should allow this list to be confirmed, or otherwise, and real progress to be made.

36 A.W. Wolfendale/Nuclear Physics B (Proc. Suppl.) 60B (1998) 33-36

ACKNOWLEDGEMENTS

The author is most grateful to Professor Jean-Noel Capdevielle and his colleagues for organizing such as excellent Symposium.

References

1. AI-Dargazelli, S.S., Wolfendale, A.W. and Zhang, L., J. Phys. G., 22, 1097 (1996a).

2. A1-Dargazelli, S.S., et al., J. Phys. G., 22, 1057 (1996b); 22, 1825 (1996c).

3. Bahcall, J.N. and Soneira, R.M., Astrophys. J. (Lett.), 238, LI7 (1980).

4. Chi, X. and Wolfenale, A.W., J. Phys. G., 19, 795 (1993).

5. Dodds, D., Strong, A.W. and Wolfendale, A.W., Mon. Not. R. Astr. Soc., 171569 (1975).

6. Erlykin, A.D., Smialkowski, A. and Wolfendale, A.W., to be published (1997).

7. EGRET data obtained through the Compton Observatory Science Support Center GOF account provided by the NASA Goddard Space Flight Center.

8. Fatemi, S.J. and Wolfendale, A.W., J. Phys. G., 22, 1089 (1996).

9. Fichtel, C.E. et al., Astrophys. J., 374, 134 (1991).

10. Houston, B.P. and Wolfendale, A.W., Vistas Astron., 26, 107 (1982).

11. Kodaira, K., PASJ, 26, 211 (1974). 12. Li, Z., Wheeler, J.C., Bash, F.N. and Jeffreys,

W.H., Astrophys. J., 378, 93 (1991), 13. Lyne, A.G., Manchester, R.N. and Taylor, J.K.,

Mon. Not. R. Astr. Soc., 213, 613 (1985). 14. Ginzburg, V.L., Nature, 239, 8 (1972). 15. Grenier, I.A. et aL, Proc. Int. Cosmic Ray Conf.

(Rome), 2, 178 (1985), 16. Sreekumar, P. et al., Astrophys. J., 400, L67

(1992). 17. Strong, A.W. and Mattox, J.R., Astron.

Astrophys., 308, L21 (1996). 18. Sturner, S.J. and Dermer, C.D., Astron.

Astrophys., 293, L17 (1995). 19. Wdowczyk, J. and Wolfendale, A.W., Proc. Int.

Cosmic Ray Conf., (Rome), 3, 360 (1995). 20. Wolfendale, A.W. and Zhang, L., J. Phys. G.,

20, 935 (1994).