BACKGROUND RADIATION, GAMMA RAY

Carl E. Fitchel

It is now known that the diffuse celestial radiation extends well into the gamma ray region, at least to approximately 200 MeV, and that it is isotropic at least on a coarse scale. The degree of isotropy that has already been shown to exist, taken together with the spectrum being different from that of the galactic diffuse radiation, strongly supports this diffuse radiation being extragalactic in origin. The intensity of the radiation is rather weak and its level has already caused the rejection of the steady-state theory of the universe wherein matter and antimatter are continuously created in equal amounts in an expanding universe. For this theory to have been correct, the radiation would have had to be several factors of 10 more intense.

The intensity, energy spectrum, and the degree of isotropy that has been established have, in fact, eliminated many theories that have either involved a diffuse celestial gamma radiation or been designed to explain it. Among the still possible explanations are that it is the sum of radiation from active galaxies or it is from the interaction of matter and antimatter at the boundaries of superclusters of galaxies. Future measurements may very well be able to separate these two alternatives. It is also possible that part of the diffuse emission, but probably not all because of the spectral shape, may be the result of the quantum-mechanical decay of small black holes created by inhomogeneities in the early universe. These theories and the details of the experimental tests that can be made will be considered after a brief review of the current status of the knowledge of this radiation.

THE EXISTING OBSERVATIONAL PICTURE

The first indication that the diffuse radiation extended above the x-ray region into at least the low-energy gamma ray region came from instruments flown in the Ranger 3 and 5 Moon probes. Subsequently, there have been a large number of measurements in the low-energy (approximately 0.1-20 MeV) region, establishing its general nature. In the high-energy gamma ray realm, the first measurements came from Explorer 11. Although they gave only an upper limit, they provided the refutation of the steady-state theory mentioned previously. The first suggestion of a high-energy component came from a telescope on OSO-3, and the results from the SAS-2 gamma ray telescope provided measurements on the isotropy and the energy spectrum from 35 MeV to about 200 Mev, above which energy the intensity falls below the galactic diffuse radiation even well away from the galactic plane.

Although the intensity of the measured gamma radiation in the low-energy region is stronger compared to the galactic radiation than the high-energy gamma radiation, there is the considerable problem of background radiation created in the surrounding material and even in the instrument itself. The difficulty is sufficiently severe that early results consisted principally of upper limits and uncertain reported positive results. The background radiation is now better understood, although still high. The Apollo 15 and 16 gamma ray telescopes were on booms of variable lengths allowing the background radiation to be carefully measured as a function of distance from the spacecraft. The final results from these missions are now generally accepted as being a good general representation of the radiation level. There remains, however, some question about the exact shape in the 1/2-10-MeV region.

In the high-energy region, the nature of the instrumentation and the physical interactions of both gamma rays and cosmic particles causes the background radiation to be very low relative to the measured radiation. However, the separation of the extragalactic diffuse radiation from the galactic diffuse emission is a challenge. Several different types of analyses have been made, starting with the most simple one of studying the intensity and energy spectrum as a function of galactic latitude and continuing with careful comparisons to the matter column density and even galaxy counts. All of the studies give approximately the same result for the magnitude and energy spectrum in this portion of the energy range.

Figure 1 shows the energy spectrum of the diffuse radiation. Because of the large number of experimental points, only a few of the results are included to avoid confusion. References to the considerable number of other results, including the early ones, may be found in the articles listed at the end of this entry. The level of the galactic radiation is also shown for comparison. Notice that there is a general trend toward a steepening as the energy increases from the low-energy x-ray region into the gamma ray region. However, in addition, there appears to be a ``bump'' in the region around 1 MeV. This feature, should it be real, is important in trying to interpret the nature of this radiation. The concern, in spite of the now much-improved instrument design and background radiation subtraction techniques, is that this is just the region where a significant background is known to exist. Early experiments indeed saw a very large background in this region.

Figure 1. The diffuse extragalactic gamma radiation. The high-energy high-latitude galactic emission is shown for comparison. The dashed line is shown only to guide the eye. [From Trombka and Fichtel (1983).]

In the first concept, the proposed baryon nonconserving forces in the grand unified field theory revived interest in considering a baryon symmetric universe, containing superclusters of matter and others of antimatter. The low level of the extragalactic diffuse radiation, itself, has effectively eliminated regions of baryons and regions of antibaryons on a smaller scale, because in that case the gamma ray intensity would be much larger than observed. Although the nucleon-antinucleon annihilation spectrum has a maximum at about 70 MeV, when it is integrated over cosmological time and the redshifts are properly considered, a spectrum similar to the one in Fig. 1 is found with a bump at about 1 MeV corresponding to the largest appropriate redshift. In this model the x-ray emission, including the hard x-rays, is presumed to be due to a different source. The intensity level is also consistent with reasonable choices for the parameters.

On the largest scale, this theory predicts a smooth distribution over the sky; however, a crucial test of this theory (in addition to a precise measure of the energy spectrum) would be the detection of enhancements of the radiation in the direction of boundaries between close superclusters of galaxies. The excess diffuse radiation associated with these boundaries would be at the higher energies. Calculations suggest that with future high-sensitivity gamma ray telescopes, there is hope of seeing these ridges if they exist. Because there are very few ways to test whether our universe is baryon symmetric on this scale, the search for these ridges is of considerable interest for cosmological theory.

Although the information on gamma ray emission from active galaxies is very limited at present, a Seyfert galaxy, a quasar, and a radio galaxy have all been seen. It is, of course, very speculative to assume that they are typical. However, if this assumption is made or even that they are a bit exceptional, and if x-ray data and gamma ray upper limits on other galaxies are used as support of the spectral shape that is seen, then a cosmological integration using parameters developed from other areas of astronomy shows that active galaxies could explain the diffuse radiation. An important test of this theory will be whether enough additional active galaxies are seen and their spectra are appropriate as the increased sensitivity of future gamma ray telescopes permit the detection of weaker sources. The appeal of this model is that active galaxies are certainly expected to produce gamma rays, and the intensity level is not unreasonable; so the only question that remains is whether the spectra and level are compatible with the observed diffuse intensity.

The third possible origin to be mentioned here is that associated with the death of primordial black holes. It has been postulated that these primordial black holes can evaporate by the tunneling process at an accelerating rate that ends in an explosive gamma ray burst. These events are predicted to be significantly harder in energy spectrum than the low-energy gamma ray bursts that have been observed. The primordial black hole bursts when integrated over cosmological time might produce the diffuse gamma ray background, or at least part of it. The observed spectral shape may create a difficulty, but the most significant test of this model would be the observation of high-energy gamma ray bursts of the appropriate type. Attempts to detect them so far have been unsuccessful; however, in view of the uncertain mass spectrum and other features, further searches are certainly appropriate.

SUMMARY

An extragalactic diffuse gamma radiation appears to exist with reasonably well-defined characteristics. There are at least three possible explanations for this radiation, each of which has a specific experimental test. In view of the desire to know whether the universe as a whole is baryon symmetric, to understand the nature of active galaxies, and to know if primordial black holes exist and what their properties are if they do, there is certainly considerable fundamental astrophysics involved with the resolution of the matters associated with the understanding of the diffuse extragalactic radiation. Hopefully, this future scientific work will add to the important contributions to science that the study of the extragalactic diffuse radiation has already made.

1. Bignami, G.F., Fichtel, C.E., Hartman, R.C., and Thompson, D.J. (1979). Ap. J. 232 649.
2. Fichtel, C.E. and Trombka, J.I. (1981). Gamma Ray Astrophysics, New Insight into the Universe. NASA SP-453.
3. Kinzer, R.L., Johnson, W. N., and Kurfess, J.D. (1978). Ap. J. 222 370.
4. Marshall, F., Boldt, E., Holt, S., Miller, R., Mushotzky, R., Rose, L., Rothschild, R., and Serlemitsos (1980). Ap. J. 235 4.
5. Page, D.H. and Hawking, S.W. (1976). Ap. J. 206 1.
6. Ramaty, R. and Lingenfelter, R.E. (1982). Ann. Rev. Nucl. Part. Sci. 32 235.
7. Schonfelder, V., Graml, F., and Penningsfeld, F.P. (1980). Ap. J. 240 350.
8. Stecker, F.W., Morgan, D.L., and Bredekamp, J. (1971). Phys. Rev. Lett. 27 1469.
9. Thompson, D.J. and Fichtel, C.E. (1982). Astron. Ap. 109 352.
10. Trombka, J.I., Dyer, C.S., Evans, L.G., Bielefeld, M.J., Seltzer, S.M., and Metzger, A.E. (1977). Ap. J. 212 925.
11. Trombka, J.I. and Fichtel, C.E. (1983). Phys. Rep. 97 175.
12. White, R.S. (1987). Enc. Phys. Sci. Tech. 763.
13. Wolfendale, A.W. (1983). Ouart. J. Roy. Soc. Astron. Soc. 24 226.
14. See also Antimatter in Astrophysics; Gamma Ray Astronomy, Space Missions.