Origin of Cosmic Rays

An atternpt is made to diraw a systematic view on the origin of cosmic rays. On the basis of the composition of primary cosmic rays and the galactic radio emission. arguments are presented that the galactic cosmic rays are stored in the galactic halo of spherical shape for the mean lifetime of about 10S years. The local sources of cosmic rays consist ef the following two kinds; one is supernovae at which about 10'G of ejected particles are accelerated to cosmic ray energy and the other is supergiant and red giant stars at which the above ratio seems to be 10'S-10r'. The chemical composition of cosmic rays from the latter is equal to that Qf the interstellar matter, while the former sources are responsible for the overabundance of heavy nuclei in cosmic rays, Our general view is summarlzed in gl and detailed discussions on individual problems are made in gg2-5, leaving out more specific problems to Apps,I-VIII, Each section is prepared as independently as passible. and its centents are summarized at the beginning of the section. Sl. General view g2, Cosmic rays in the Galaxy 2. 1. Classification of the components of primary cosmic rays 2.2. Charge spectrum of primary nuclei 2,3. Energy spectrum 2.4. Contaminating cemponents in the primary radiation g3. Supernova origin 3, 1. Energetics in the Crab nebula 3.2. Electromagnetic radiation from the Crab nebula 3. 3. Acceleration mechanism and nuclear particles in the Crab nebula g4. Cosmic rays associated with the matter ejection 4. 1. Cesmic ray outbursts at so}ar flares 4.2. Cosmic ray production at active stars g5. Extragalactic origin Appendices I. Yields of nuclides due to spallation processes 1l, Frag, mentation prebabilitiee II-Electronic Mbrary Publication Office, Progress of Theoretical Physics NII-Electronic Library Service ublicationOffice,Progressof heoretical hysics 2 S, Hayakawa, K. Ito and Y. Terashima III. Radioactive energy source in Type I supernovae IV. Dynamics of supernova explosion V. Mechanism of acceleration VI. Synchrotron radiation VII. Diffusion problems VIII. Intensity of $olar cosmic rays gl. General view Since the discovery of the cosmic radiation various attempts have been made for explaining how and where it is generated (Ha 50). However, they had been more or less speculative until we knew some important features of primary cosmic rays, such as the presence of heavy nuclei (Fr 48), the practical absence of electrons (Hu 48) and the power energy spectrum extending to very high energies (Co 49). Increasing knowledge about primary cosmic rays has been supplemented by various astrophysical considerations, among which the galactic magnetic fields responsible for stirring and accelerating cosmic ray particles (Fe 49) and the synchrotren radiation of electrons in the magnetic fields (Gi 51) may be regarded as e £ primary importance. The 'latter has provided a basis fer interpreting the strong radio emission from discrete sources, particularly the Crab nebula, and supernovae may be regarded as candidates for cosmic ray sources (Sh 53a). The supernova origin is supported also by the fact that heavy nuclei are overabundant in cosmic rays (Ha 56a). Throughout these studies*) effOrt is devoted rnainly for constructing a reasonable model on the origin of cosmic rays, leaving out the mechanism of acceleration as a forthcoming problem (Mo 57). The most comprehensive work along this line seems to be due to Ginzburg (Gi 56, 57), with whom we share almost the same opinions, in spite of that his and our works have been carried out nearly independently. ・ In the present paper we attempt a more detailed analysis of the model concerning the galactic origin of cesmic rays, extending our previous werk (Ha 56a). Although our model is similar to that adopted by Ginzburg (Gi 57), our analysis is made as quantitative as possible within the Iimited knowledge about cosmic rays as well as astronomy, and there exist some differences particularly in the following respects. The composition of primary cosmic rays is considered in relation to the evolution of stars and the formation of elements (Ta 56, Bu 57a, b) and the mean thickness of the interstellar matter traversed by cosmic rays is deduced as a few g cm-2 in g2. This leads us to assume that the mean lifetime of cosmic rays in the Galaxy is deterrnined essentially by the escape out of the Galaxy, while *) Not all of important works are cited in the present paper, and references are limited only to those directly related to our discussions. Publication Office, Progress of Theoretical Physics NII-Electronic Library Service ublicationOffice,Progress f heoretical hysics Origin qf (:bsrnic Rars 3 Ginzburg assumed it is mainly due to nuclear collisions. An analogous difference is seen with relativistic particles in supernova remnants, such as the Crab nebula; the relativistic electrons leak out of the nebulae in our model, as seen in S3. In addition to the supernova origin, which is the sole origin assumed by Ginzburg, other possible sources are considered in g4. On the basis of the solar outburst of cosmic rays associated with violent fiares, the eMciency for accelerating particles caused by the activity at stellar surfaces is estimated, and the contribution of the stellar activity, which is likely to be quite frequent at supergiants and red giants, to galactic cosmic rays is discussed. The responsibility of the extragaiactic origin is considered in S5, but nothing definite can be concluded at the present stage of our knowledge. Summarizing the discussions in g2.4, we shall describe the general view of eur model in what follows. Our model is based on the following assumptions which are convincing in many respects; (i) tbe galactic magnetic fields tir and store cosmic rays inside the Galaxy, while they may or may not be responsible for acceleration, (ii) the galactic radio emi$sion is mainly due to relativistic electrons in the magnetic fields, (iii) the charge spectrum of primary cosmic rays reflects the relative abundances of elements at their sources, and (iv) cosrnic rays are stationary during the nearly entire history of the Galaxy, say, for last 10i7 sec. The assumption (i) allows us to introduce the total number of cosmic ray particles, N, and the mean lifetime for them to survive, T, in the Galaxy. These are related to each other through the overali production rate of cosmic rays, Q, by N-:-QT, (1.I) provided that the production rate is constant in time, according to the assumption (iv). N and Q are expressed in terms of the density n(r) and the production rate per unit volume q(r), respectively, as iV=-Sn(r)dr, Q-=Sq(r)dr. (i・2) The assumption (ii) facilitates to know the spatial distribution of cesmic rays from the intensity contour of general galactic radio waves. The latter indicates that the radio sources extend to the galactic・halo and their spatial distribution is approximately expressed by 1/r fer rSIR=5 × 1022cm. Consequently, the spatial distribution of cosmic rays can be assumed as n(r)-・-noCa/r) for rSR, (1.3) where a==・3 × 1022cm is the distance of the earth from the galactic center Publication Office, Progress of Theoretical Physics NII-Electronic Library Service ublicationOffice,Progress f heoretical hysics 4 S, rrayakawa, K. Ito and Y, Terashima and ne=1 × 10-iOcmr3 is the density of cosmic rays in the vicinity ef the earth. The assumption (iii) is important in determining the mean thickness of interstellar matter traversed by cosmic rays. An appreciable abundance of Li, Be and B in primary cosmic rays in comparison with their negligible relative abundance in cosmic elements suggests that these nuclei in oosmic rays are produced as a consequence of the fragmentation of heavier nuc]ei colliding with the interstellar gas. This interpretation allows us to deduce the mean thickness of interstellar matter traversed by cosmic rays as X'-"m3gcm-2. (1.4) This is also consistent with other evidences on the charge spectrum. The overabundances of heavy nuclei in cosmic rays compared with cosmic abundances, particularly of iron, indicate that important part of cosmic rays are produced in the last stage of the stellar evolution, probably in supernovae; in their cores heavy elements such as iron can be synthesized under a very high temperature and then these elements can be scattered in the interstellar space due to violent explosions. Indeed, the Crab nebula, a supernova remnant, provides an evidence foy the generation of cosmic rays at a rate of q, !!1 × 10`e sec-i, (1.5) as wiII be shown in g3. The assumption (iv) is based on such a view that the stellar evolution is essentially stationary for major part of the history of our Galaxy, while observations of natural radio activity can tell us the cosmic ray intensity to be substantially constant at least for last one million years (Pe 57). If the essentially stationary nature is taken for granted, we are able to define the overall production rate through (1.1) with the aid of the mean lifetime T £ ! 3× 10 i5 sec (1 .6)