energy spectrum and primary composition from direct measurements

I l l I I l I I l I I I q

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ELSEVIER Nuclear Physics B (Prec. Suppl.) 75A (1999) 2 2 - 2 7

Energy spectrum and primary composition from direct measurements T. Shibata ~

~Department of Physics, Aoyama Gakuin University, 6-16-1 Chitosedai Setagaya-ku, Tokyo, 157-8572, a apan

The goal of direct observation of the galactic cosmic rays (GCR) is to understand : 1. what is the origin of cosmic rays ?, 2. how are they accelerated to so high energies ?, and 3. how have they propagated in our galaxy before they arrive to Earth ?

In addition to these three questions, it is also a fundamental question whether there exist an- tiparticles (~, e +) originated in some novel source, such as evaporating primordial black holes [1], an- nihilation of neutralino dark mat ter [2] and super- conducting cosmic strings [3], well enhanced from those produced secondarily through the propagation of cosmic rays in our galaxy.

Recently the experimental techniques for both antiproton and positron observations have devel- oped remarkably, for instance those by BESS[4] and HEAT[5] groups respectively. In this short report, I would like to give an overview of cosmic ray spectrum and composition, including rare elements such as isotopes, 15 and primary electron.

Nowadays, it is well accepted that the supernova and its associated shock are the most likely scenario for the above first two questions, origin and acceleration, although several critical questions still remain. For instance, i) if the origin of GCR is either direct injecta from the supernova itself, or the hot component of the interstella gas; ii) we are also interested in the period of acceleration, i.e., if it is early in the lifetime of supernova remnant (fresh materials ?), or delayed after the size of shock has expanded big enough that in- dividual shocks from nearby SN may be overlapping; iii) if acceleration occurs only in the source (one shot acceleration) or if there are further re

acceleration in ISM (continuous acceleration); iv) what is the theory explaining the so called "knee" phenomena, giving a soft energy spectrum with index 3.1 above the "knee" (,-, a few PeV) ?, or one might ask if the cosmic ray composition observed above the "knee" is the same as that observed below.

In fig. 1, ! show the present status of cosmic- ray direct observations as a function of rigidity. The rigidity range obtained by the direct mea- surement nowadays covers 7 ~ 8 orders of magni- tude, extending to PV, though the statististics is yet very poor in the region, > a few tens TV for proton and helium, and > a few TV for heavier elements. I would like to discuss several points concerning this figure.

I think everybody agrees on the following results, shown in the top line of the figure :

1) the most effective rigidity to store cosmic rays in our galaxy is a few GV, and the mate- rial thickness they have traversed is as large as 10 g/cm2;

2) the index of proton energy spectrum is significantly softer than those of heavier ones, the former giving --~ 2.75 and the latters with ,-- 2.60;

3) a break exists at ,-~ PV in the all-particle spectrum, the so called "knee", the spectral index giving ,-~ 2.7 below the "knee" and ,-~3.1 above.

The first result has been confirmed through the observation of B/C and sub-Fe/Fe ratio. This problem is closely connected with a reacceleration problem and also with the second result. In fact, Simon et M. [6] pointed out that particles inter-

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T. ShibatalNuclear Physics B (Proc. Suppl.) 75,4 (1999) 22-27 23

I Pt~sent _~tus of co~Jc-rmy ~ obser~wtion I B ~r*i** ~ l .TS

,1. ~ 1 0 g / c m ~ Bh,av u ~ 1 . 6 " / ~ T l e e "

I I I M V G V T V P V Rig id i ty

- - ,

I . U l t r a - h e a v y "~* ' ' - - I ~ .. . . . . . I f o r p ~ Fe I _- i_soto_pe_ _ j I S u b - E e / F e I ( I - r y comp . ) l

. . . . . I { . . . . . .

. . . . . . .

I ionizati~)n

I -,.,(Eo I - - - - ~ ¢ ~ ~ ,e¢o,d ~t,ge ace.? (Rm= x ~ IOOTV ) ex t ra -ga lac t i c?

V o y a g e r HEAO 3 G r i g o r o v e t .a l . CRRES CRN SOKOL

Experiment: ULYSSES IMAX JACEE N i t BESS RUNJOB ISEE 3 BETS

Figure 1. Overview of the present status of cosmic-ray direct observation

acting with turbulent magnetic fields should gain energy due to a stochastic second-order Fermi- type of acceleration, and consequently the escape length, ), ..... becomes a power-law in rigidity vari- able with an exponent o f - l / 3 , giving a rather fa- vorable result for an anisotropy observed at a few TV, while a simple leaky box model without the rea.cceleration gives a too high anisotropy.

The second result is not independent of the first result. For instance, if Acs~ oc R -1 /3 , then the source spectrum of proton is given by R-'-,.75+() 33 - /~-~ 42. The difference of spectral index between proton and heavy elements would be explained simply by the difference of collision mean free path in ISM, say, ~-0 60 g / cm 2 for proton and ~-- 2.5 g / c m 2 for iron, if the spectral indexes of all elements are the same at the source. While t.his is very likely in the rigidity region from a few GV to ~ TV, the situation is not so simple in the highest rigidity region. As discussed later again, the recent da ta obtained by SOKOL [7] and JA(IEE [8] show that the difference seems to increase as the energy gets higher (enhancement of heavy elements at higher energies), though the statistics is not yet enough.

Another problem we remark here is that of the

difference in spectral index between proton and helium. While SOKOL and JACEE show that the helium spectrum is significantly harder than the proton spectrum in the energy region from a few TeV to ~ 100 TeV, RUNJOB [9] gives nearly the same exponent with ,-~ 2.8. This problem is very important in relation with a non-linear acceleration process. Ellison [10] points out that the proton and helium would give a different spectral index, A~ = flp,.oto,~ - flheu,~m ~ 0.1, due to the non-linear acceleration process, coming from different Z / A value in these two elements.

The third result is related to the m a x i m u m energy expected from shock acceleration in SNR, due to a finite lifetime of the shock, while it works very well below the "knee". Though this problem is out of my subject, I would like to touch it shortly as it may be connect to the cosmic ray propagation as well as its origin and acceleration below the "knee". On the energy spectrum from PV to EV, a plausible theory is not yet confirmed, mainly due to poor experimental data, particu- larly in the composition.

I think there are two possible standpoints to understand the physics above the "knee" with spectrum shape R - a l , i.e., one is that the source

24 T. Shibata/Nuclear Physics B (Proc. Suppl.) 75A (1999) 22-27

above the "knee" is the same as that below the "knee" and the other is that of the existence of a new source (including an extra-galactic one). A typical model based on the former standpoint is proposed by Axford [11], assuming some second stage acceleration due to multiple shock interac- t, ions in our galaxy. In this case, the composition doesn't change so drastically between below and above the "knee", and no significant bump might appear.

An alternative is to postulate some new source, as for example proposed by Protheroe [12], assuming a proton dominance coming from AGN. In this case, the composition changes drastically from below and above the "knee", and a promi- nent bump might appears.

Now let us look again at fig. 1, and view the experimental data. recently obtained. Due to tech- nical diffÉculties as well as to limited flux, 1) the observation of ultra-heavy and isotopes is limited to lower rigidity < a few hundred MV; 2) the data on B /C and sub-Fe/Fe ratios are available for rigidities < a few hundred GV, and 3) the energy spectrum and composition of proton to iron covers the rigidity < a few tens TV for proton and helium, < a few TV for heavier elements.

In order to convert the cosmic ray spectra observed at Earth to those at the source, we face many troublesome effects, the solar modulation, the ionization loss and the energy-dependent spallation cross section, in addition to the escape rate from our galaxy. Among them, it is very complicated and difficult to get reliable data on the spallation cross section with use of machine beam; this often causes confusion in converting the observed data to those at the source. These effects are naturally critical in the lowest rigidity range, but they are negligible in the highest rigidity region, say > 10 GV, so that experimental data with higher rigidity are necessarily required, even if the accuracy in both charge and energy deter- ruination is somewhat reduced.

In the bot tom space of Fig. 1, I wrote the names of experiments in correspondence to the rigidity they have covered. Of course I have no space to discuss each result in this short sum- mary, but I will touch several recently obtained important ones.

Recent spacecraft measurements on isotopes, by ISEE-3 [13], Voyager [14] and Ulysses [15], have brought us quite important results. Firstly, all three spagerafts show the absence of significant ~gNi in cosmic rays. Since the lifetime of 59Ni decay into ~9Co is 7.5 × 104 year, the nucleosynthesis must have occurred more than 105year before the particles were accelerated. This time delay, > 105year, is long enough that the shock expands to a radius > 50~100pc and may be overlapping with those from other nearby SN.

Secondly, Voyager data indicate that ~ 25% of 49V and 51Cr isotopes, ahnost secondary products in the galaxy, have decayed during the propagation in the galaxy. Since the decay rate strongly depends on the energy of isotope, we can esti- mate the contribution of re-acceleration, and the data show that its effect is rather small. The reacceleration problem is discussed later again in reports of anti-proton, electron, and sub-Fe/Fe ratio, all of which seem to support a very small contribution of the reacceleration.

Thirdly, Voyager and Ulysses data confirmed that the cos/nic ray composition at the source is quite similar to the solar composition other than for four isotopes, 4He, 13C, 14N and 22Ne. Al- though these measurements have been performed by various authors for many years, they have fluctuated too much to get a definite speculation. Now the data of these two missions will change our view on the isotope composition of GCR. An interesting speculation from the Voy- ager and Ulysses data has recently been summa- rized by Webber [16].

Now let us go to the antiproton problem. The antiproton measurements have been performed by many authors until now, covering a few hundred MeV to a few GeV as shown in fig. 1. Among them, BESS data is remarkable in its statistics as well as in the technique of p identification, using a solenoidal magnet, drift chamber and TO F plas- tic scintillation hodoscopes. Most recently, the BESS group reported on low-energy cosmic-ray antiprotons at solar minimum ('95), measuring the energy range 0.28 to 1.4 GeV. They have un- ambiguously observed 43 events and the shape of energy spectrum seems to be rather flat be-

T Shibata/Nuclear Physics B (Proc. Suppl.) 754 (1999) 22-27 25

Low 1 GeV, while the p flux of BESS'93 data [17] drops significantly with lower energy below 1 GeV, giving a good agreement with a stan- dard leaky box model (SLB). Though they don' t exclude the possibility of secondary antiprotons produced during the passage of protons in our galaxy, they point out that BESS'95 da ta sug- gest an admixture of primary antiprotons coming from some novel process such as evaporating primordial black holes. So, we have to wait a little bit more with great interest to get a firm conclusion and in fact they are planning to extend the flux value as well as the energy range much fllrther in near several years.

In addition to these problems, Mitsui [18], a member of BESS, has studied the contribution of re-acceleration process, based on BESS data as well as on HEAT e + data. Both da ta are consistent with SLB and/or diffusion model without the re-acceleration process. This result is matched with the analysis of 49V and ~lCr isotopes obtained by Voyager mentioned before.

Next. I would like to briefly touch the analysis of pr imary electrons recently performed by. Nishimura et al. [19]. As it is well know, the behavionr of the electron component is quite different from that of the nuclear one, namely the former strongly loses its energy due to the syncrotron radiation and the inverse compton scat- tering. The energy region of pr imary electron spectrum effective in reacceleration process is 100 MeV ~ 10 GeV, where the three processes, the escape, bremsstrahlung and syncrotron, give nearly the same contribution to the propagation, making the analytical calculation somewhat complicated. They conclude that if the contribution of reacceleration is as small as a few % per g /cm 2 and the path length decreases as R-1/3, the electron spec- t rum gives an unnatural bump somewhere around 2 ~ 3 (]eV, not consistent with their own data. So, the pr imary electron da ta again seems to support no reacceleration, while Nishimura agrees that it is rather natural to assume that a process of reacceleration exists, since cosmic rays must be very often scattered by magnetic cloud in our galaxy.

I would like to present one more example sup-

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rigidity R ( G V )

Figure 2. Rigidity dependence of abundance ratio, sub-iron(Z=21-23)/iron.

porting the no reacceleration conclusion. It is a sub-Fe/Fe ratio in the rigidity range a few GV ,-- TV obatined by our balloon-borne emulsion chamber. I show it in Fig. 2, together with the da tg compiled by Grove et al.[20]. Though the statistics is very poor in > 100GV, it is obvious that typical calculations[16,21,22] including the reacceleration process show a ratio much higher than our data, and that the SLB model without reacceleration is closer to the data.

So, all recent experimental da ta seem to support the SLB model without reacceleration. But as Nishimura comments , I also feel these results are not understandable, as the interaction between cosmic-rays and magnetic cloud must hap- pen in our galaxy. This is now a new open question.

Now, I go to a subject, cosmic ray spectrum and composition obtained by the most recent experiment, RUN JOB, together with typical oth- ers where the reacceleration process is no more important , but the rigidity dependance of the escape length is closely connected. I skip the details of RUN JOB-program and straightforwardly show its result.

In fig. 3, proton and helium components are presented. One clearly notes that the absolute value of proton flux is consistent among RUN-

: ' ' ' ;);r;ton"~';;? "h'eiiu';i '~:~;ect'rU ! : ' : ' i i : I It s ~ : :1 ÷ ~ : N~u)r(l~ ct. al. • /~ ; Kawainuta el. a[. ?[

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: • ,~ : sOKnL o . : RUN.)O~ '~S :

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II1 [ If' I I I ~ ] 0 ~ ] ()a 1 0 ~ 1 O"

k i n e t i c e n e r g y Eo ( G e V / n )

Figure 3. Differential energy spectra for proton and helium components, where intensity is mul- t.iplied by E~ 5 to emphasize the spectral feature.

1 0 ]

- ~ 10"

10 -~

::= I 0 :

JOB, JACEE and SOKOL, but the absolute flux of helium flux of RUN JOB is of approximately ~t factor of two less than that of JACEE and SOKOL, though the statistics of RUN JOB is not enough. We have to wait a little bit more to get a definite conclusion for the helium component.

In fig. 4, tlhe heavy components are presented together with those obtained by previous experiments. While one sees that the absolute iron flux is consistent among all groups with exponent of,~ 2.6, those of (',NO and NeMgSi by RUN JOB are slightly less than those of JACEE and SOKOL.

In fig. 5, I show the all-particle spectrum obtained from the fbur groups, where the grey area is estimated from the summation of previous di- reet observations. Though the statistics around PeV is poor, the spectrum decreases monotoni- cally from hundred GeV/part icle to PeV/particle. In fig. 6, I present the average of primary mass < In A > v.s. primary energy E p . Both JACEE and RUN JOB show that the average mass be- conies gradually heavier as energy gets higher, > 100 TeV/particle.

Anyway, we need more statistics in the region > 100 ToV/particle. 1 feel any balloon experiment is now over the limit to cover such high energy region with enough statistics, and maybe we have to wait the next century experiments, which will

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1 0' 1 02 1 0" 1 0' 1 0 s 1 0 ~

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Figure 4. Differential energy spectra for three kinds of heavy element, CNO-group, NeMgSi- group and iron-group.

I I1 +

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l O'

p r i m a r y e n e r g y El , ( G e V / p a r t i c l e )

2 6 T Shibata/Nuclear Physics B (Proc. Suppl.) 75,4 (1999) 22-27

Figure 5. All-particle spectrum obtained by four groups.

T Shibata/Nuclear Physics B (Proc. Suppl.) 75.4 (1999) 22-27 27

I O: 111~ I 1)" 10 ' 1 IP I O'

i)rimary energy E I) (GeV/particle)

Figure 6. Average mass of primary cosmic rays, < In A >. v.s. primary energy, Ep.

use the international space station (ISS), now in progress.

I greatly thank the Organizing Committee for giving me the oppotunity to talk on cosmic ray spectrum and composition as an invited speaker and for their warm hospitality during my stay in Gran Sasso.

R E F E R E N C E S

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gary), 2 (1993) 17. 8. T. Tominaga et al., Proc. 23rd ICRC (Cal-

gary), 2 (1993) 21. 9. A.V. Apanasenko et al., ISAS-report No.37

(Institute of Space and Astronautical Sceience, March t998).

10. D.C.E.11ison, Proc. 23rd ICRC (Calgary), 2 (1993) 763.

11. W.I. Axford, Proc. ICRR (Institute of Cosmic Ray Research) Int. Symp. Kofu, (November 1990) 406.

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13. R.A. Leske, Astrophys. J. 405 (1993)567. 14. A. Lukasiak et al., Adv. Space Res. 19 (1997)

747. 15. J.J. Connen and J.A. Simpson, Astrophys. J.

475 (1997) L61. 16. W.R. Webber, Space Sci. Rev. 81 (1997) 107. 17. A. Moiseev et al., Astrophys. J. 474 (1997)

479. 18. T. Mitsui, Doctor Thesis (University of

Tokyo, 1996). 19. J. Nishimura et al., ICRR-report 116-96-1

(October 1996) 1. 20. J.E. Grove et al., Astrophys. J. 377 (1991)

680. 21. U. Heinbach and M. Simon, Astrophys. J. 441

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energy spectrum and primary composition from direct measurements

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