Fortschritte der Physik / Progress of Physics: Band 29, Heft 3 [Reprint 2022 ed.] 9783112655887

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FORTSCHRITTE DER PHYSIK HERAUSGEGEBEN IM AUFTRAGE DER PHYSIKALISCHEN GESELLSCHAFT DER DEUTSCHEN DEMOKRATISCHEN R E P U B L I K VON F. KASCHLUHN, A. LÖSCHE, R. RITSCHL UND R. ROMPE

H E F T 3 • 1981 . B A N D 29

A K A D E M I E - V E R L A G

EVP 1 0 , - M 31728

•

B E R L I N

ISSN 0015 • 8208

BEZUGSMÖGLICHKEITEN Bestellungen sind zu richten — in der DDR an eine Buchhandlung oder an den AKADEMIE-VERLAG, D D R - 1080 Berlin, Leipziger Straße 3 - 4 — im sozialistischen Ausland an eine Buchhandlung f ü r fremdsprachige Literatur oder an den zuständigen Postzeitungsvertrieb — in der BRD und Berlin(West) an eine Buchhandlung oder an die Auslieferungsstelle KUNST UND WISSEN, Erich Bieber OHG, 7000 Stuttgart 1, Wilhelmstraßc 4—6 — in Österreich an den Globus-Buclivertrieb, 1201 Wien, Höchstädtplatz 3 — in den übrigen westeuropäischen Ländern an eine Buchhandlung oder an die Auslieferungsstelle KUNST UND WISSEN, Erich Bieber GmbH, CH - 8008 Zürich/Schweiz, Dufourstraße 51 — im übrigen Ausland an den Internationalen Buch- und Zeitschriftenhandel; den Buchexport, Volkseigener Außenhandelsbetrieb der Deutschen Demokratischen Republik, D D R - 7010 Leipzig, Postfach 160; oder an den AKADEMIE-VERLAG, DDR - 1080 Berlin, Leipziger Straße 3—4

Zeitschrift „Fortschritte der P h y s i k " Herausgeber: Prof. Dr. Frank Kaschluhn, Prof. Dr. Artur Lösche, Prof. Dr. Rudolf Ritsehl, Prof. Dr. Robert Rompe, im Auftrag der Physikalischen Gesellschaft der Deutschen Demokratischen Republik. Verlag: Akademie-Verlag, D D R - 1080 Berlin, Leipziger Straße 3 - 4 ; Fernruf: 2236221 und 2236229; Telex-Nr. 114420; B a n k : Staatsbank der D D R , Berlin, Konto-Nr. 6836-26-20712. Chefredakteur: Dr. Lutz Rotlikirch. Anschrift der Redaktion: Sektion Physik der Humboldt-Universität.zu Berlin, D D R - 1040 Berlin, Hessische Straße 2. Veröffentlicht unter der Lizenznummer 1324 des Presseamtes beim Vorsitzenden des Ministerrates der Deutschen Demokratischen Republik. Gesamtherstellung: V E B Druckhaus „Maxim Gorki", D D R - 7400 Altenburg, Carl-von-Ossietzky-Straße 30/31. Erscheinungsweise: Die Zeitschrift „Fortschritte der Physik** erscheint monatlich. Die 12 Hefte eines Jahres bilden einen Band. Bezugspreis je Band 180,— M zuzüglich Versandspesen (Preis für die D D R : 120,— M). Preis je Heft 15,— M (Preis für die D D R : 1 0 , - M). Bestellnummer dieses Heftes: 1027/29/3. © 1981 b y Akademie-Verlag Berlin. Printed in the German Democratic Rcpublic. AN (EDV) 57618

Fortschritte der Physik 29, 9 5 - 1 3 4 (1981)

Gamma Ray Astronomy and the Origin of Cosmic Rays REINHARD SCHLÏCKEISER

Max-Planck-Institut

für Radioastronomie,

Bonn,

BRD1)

Summary The classical production processes for high energy gamma radiation in the interstellar medium are discussed and confronted with recent satellite observations. I t is shown that gamma ray data in their present form do not provide definite conclusions on the origin of cosmic rays. Some current ideas on the nature of gamma ray point sources are presented.

I. Introduction

Cosmic gamma rays are believed to result mainly from interactions of cosmic rays with interstellar matter and radiation fields. Hence the question of the origin of cosmic gamma rays is closely related to the origin of cosmic rays which is a long standing problem of high energy astrophysics. Regarding the electron component of cosmic rays, the discovery of the universal 2.7 K microwave blockbody radiation (PENZIAS and WILSON, 1965) has made a galactic origin probable. Relativistic electrons in the metagalaxy with the same intensity as measured near the solar system would produce much more X-ray background radiation by inverse Compton interactions with the microwave photons than is observed (FAZIO et al., 1966). Regarding the nucleon component of cosmic rays, conclusions on their origin are expected from gamma ray and neutrino astronomy. It is one purpose of this work to discuss carefully the ability of gamma ray astronomy with respect to this question. T h e s a t e l l i t e e x p e r i m e n t s S A S - 2 (FICHTEL et al., 1975) a n d C O S - B (SCARSI et al., 1977)

— both of which use spark chamber telescopes — have measured the celestial distribution of gamma rays with energies larger than 50 MeV with reasonable accuracy. We, therefore, restrict our discussion of gamma ray astronomy to this energy range and neglect low-energy gamma ray production by thermal bremsstrahlung emission models as well as gamma ray lines (see RAMATY et al., 1979).

We start by discussing the classical production processes for high energy gamma radiation and their effect on the radiating particles (§ 2). In § 3 we confront gamma ray observations with the predictions of diffuse gamma ray astronomy and summarize the present status of the discussion. § 4 is devoted to some current ideas on the discovered phenomenon of point sources in the gamma ray sky. !) Auf dem Hügel 69, 5300 Bonn 1 1

Zeitschrift „Fortschritte'der Physik", Bd: 29, Heft «

96

REINHAKD SOHLIOKEISEB

II. Cosmic Rays as Part of the Universe 1. Our present view of the structure of the universe In order to discuss relevant production processes of high energy gamma rays we have to recall astronomical observations concerning the structure and the content of our galaxy and the universe. All with the human eye visible stars are members of a large stellar system — called Milky Way — which has the form of a flat disk with a radius of 30 kpc1) and a height of 0.5 kpc. The solar system lies nearly in the middle of the disk roughly 10 kpc away from the galactic center. With respect to the center the galactic coordinates (I11, 6 n ) have been introduced. These are the angles of a spherical coordinate system with the solar system as origin; the direction (ln = 0°, bn = 0°) points to the galactic center. A galactocentric cylindrical coordinate system (R, 12 kpc) molecular hydrogen is practically absent. In the vicinity of young, bright 0 - and B-stars, the interstellar gas is ionized by the intense ultraviolet radiation of these stars. These regions are called H II-regions. Their radial density distribution is similar to that of molecular hydrogen. !) 1 kpc = 103 pc = 3.086 • 1021 cm.

Gamma Ray Astronomy and the Origin of Cosmic Rays

97

Besides this "classical" picture of the interstellar medium, observations of the soft X-ray background ( T A N A K A and B L E E K E E , 1977) and the interstellar O - V I absorption line ( J E N K I N S , 1978) have revealed the existence of a further component of the interstellar g a s : a dilute (n < 10~3 cm- 3 ), hot (T 105 — 106 K ) corona type plasma which may occupy up to 80 percent of the volume of the interstellar gas. This component is believed to result from intersecting supernova remnants (Cox and S M I T H , 1974). Outgoing supernova shocks sweep up interstellar material, leaving behind a dilute hot gas. Model simulations (SMITH, 1977) indicate that there is a certain probability that a new outgoing U.5 rm ~3

u.o n(H!°2n(H2l

i-n(HI)

3.5

3.0

2.5

2.0

i 1.5

L n ,

1.0

0.5 ~n(H ) 2

r - 4 £ H

nlHII-

0.0 0

8

10

12

1U

-L. 16

18

R (kpc) Fig. 1. Radial density distribution of atomic (HI) and molecular (HJ) hydrogen in the galaxy (GORDON and BTJKTON 1976). Courtesy of The Astrophysical Journal.

supernova shock crosses an old existing supernova remnant, so that the shock propagates preferentially in this medium giving rise to a network of dilute, hot plasraa in the matter disk. Depending on initial conditions we may end up with two typical configurations: (i) a matter disk interwoven with a network of hot dilute "tunnels" (Cox and SMITH, 1974), (ii) interstellar matter clumped in clouds embedded in a bath of hot, dilute plasma ( M C K E E and O S T R I K E R , 1977). Theoretical problems concerning the stability and the structure of this new component are still unresolved. The space between the stars is further populated by a cosmic photon and particle radiation. Figure 2 shows the estimated energy densities of the electromagnetic radiation in different frequency ranges. Shown are galactic and extragalactic values. The latter are derived by comparing corresponding intensities from the galactic poles (b11 = ± 9 0 ° ) with those from the galactic plane (6 n = 0°). The metagalactic component dominates only in the microwave and X-ray frequency range. 1*

98

REINHARD SCHLICKEISEK

Kg. 2. Estimated energy densities of electromagnetic radiation in different frequency ranges in galactic and metagalactic space (LR,: Long-wavelength radio waves, M : microwaves, 121: infrared, O: optical photons, UV: ultraviolet radiation, S X I I : soft X-rays, X31: X-rays, YIY-rays). Reproduced from SILK (1970) by Courtesy of Space Science Reviews.

The interstellar electromagnetic radiation field in the vicinity of the solar system may be represented by the sum of diluted blackbody distributions plus the isotropic microwave background ( P I C C I N O T T I and B I G N A M I , 1 9 7 7 ) . Table 1 gives the parameters temperature T ; and energy density w{ of the various components. The parameters of the infrared component are very uncertain ( P I C C I N O T T I and B I G N A M I , 1 9 7 7 ) . The energy density of the ionizing UV-radiation (wavelength ). < 912 Â) near the galactic plane, W U V = 1 . 2 5 • 1 0 - 3 eV c m - 3 ( A L L E N , 1 9 7 3 ) , is much smaller and therefore neglected. Table 1 E l e c t r o m a g n e t i c r a d i a t i o n field in t h e v i c i n i t y of t h e s o l a r s y s t e m i

TilK

wJeV

1 2 3 4

20000 5000 30 2.7

0.09 0.3 0.2 0.25

cm-3

comment spectral type B spectral type G — K Infrared Microwaves

The cosmic particle radiation consists predominantly of protons and «-particles. Roughly 1 percent of the cosmic rays are electrons, positrons e + as well as negatrons e~. Due to the multiple deflection of these charged particles in cosmic magnetic fields, the observed intensity is practically isotropic for energies smaller than 10 16 eV. Figure 3 shows the observed intensities of cosmic protons and «-particles near the solar system. Below 1 GeV/nuc the spectra are influenced by solar modulation. At higher energies the spectra may be represented by a single power law 1(E) = K • E~p with K and p constant and positive. Due to the influence of cosmic magnetic fields, it is impossible to observe directly the intensity of cosmic rays with energies smaller than 10 1 6 eV in other regionsof the universe. This can be done only indirectly by measuring the electromagnetic radiation that results from interactions of cosmic rays with other components of the interstellar medium. In the following we therefore discuss the radiation and interaction processes of cosmic ray electrons and nucleons.

Gamma Ray Astronomy and the Origin of Cosmic Rays 10

.•••

. 1

Ì

c: \

v

Hydrogen

\

/

\

99

0.1

2».

Cb

J

Helium

b.

A

^

\

V\ \\

i. 10-3

50 MHz one may neglect the absorption of synchrotron radiation by free-free transitions with the thermal electrons of the interstellar gas and the influence of the ionized medium (Razin-effect) ( R A M A T Y , 1974). Integrating the volume emissivity along the line of sight results into the specific synchrotron intensity from the direction (I", bu) : 00

¿synch(v; lu, bu) = f dxe(v;ln, o

b11, x) erg s" 1 cm"2 ster" 1 Hz' 1 .

(9)

b) Inverse Compton scattering The inverse Compton effect of high-energy electrons traversing a photon gas has been originally discussed as an energy loss process of cosmic electrons by F O L L I N ( 1 9 4 7 ) as well as F E E N B E R G and P R I M A K O F F ( 1 9 4 8 ) . SAVEDOFF ( 1 9 5 9 ) and F E L T E N and M O R R I SON ( 1 9 6 3 ) have pointed out that this interaction may be an important source of cosmic high-energy photons. In this interaction the photon receives part of the kinetic energy of the relativistic electron and is scattered into a higher frequency range.

Gamma R a y Astronomy and the Origin of Cosmic Rays

101

Assuming that target photons and relativistic electrons are spatially isotropic distributed — for anisotropic problems see JONES et al. (1974) — the differential cross section for this process is given by the Klein-Nishina formula (e.g. JAUCH and ROHRLICH, 1955) averaged over initial photon polarizations and summed over final photon polariz a t i o n s (GINZBURG a n d SYROVATSKII, 1 9 6 4 ; JONES, 1970):

o(Er, e, E) =

3