The electromagnetic spectrum of the Crab Nebula has been determined experimentally in the radio, optical, and X-ray regions [1], in which it follows a power law of the type S(v) = Av−α, where S(v) is the power (in watts/m2 sec Hz), A and α are constants, and v is the frequency in Hz. Recent measurements [2–5], however, show a deviation from a power law in the microwave region (see Figure 1). In this paper, we investigate the origin of this deviation and calculate the γ-Ray spectrum due to this increase in the microwave photons via the Compton scattering from high-energy electrons.

The 10-m optical reflector at Mt. Hopkins, Arizona, has been used to search for cosmic γ-Rays by the detection of atmospheric Čerenkov radiation from energetic particle showers. Approximately 100 drift scans of the Crab Nebula during 1968–69 have yielded no positive evidence of a γ-Ray flux. The upper limit to the flux at 1.7 × 1011 eV is 2.0 × 10−10 photons/cm2 sec. Assuming γ-Rays of this energy are produced by Compton scattering, a lower limit on the average magnetic field in the Crab Nebula is 1.5 × 10−4 gauss. This experiment also verifies previous evidence that the high-energy electrons in the Crab Nebula are not the secondary products of high-energy proton interactions but must have been accelerated from lower energies.

Recent observations of radio emission from Sco X-1 at centimeter wavelengths by Andrew and Purton (1968) and by Ables (1969) have added an important new dimension to the study of galactic X-ray sources. We have considered two alternative interpretations of the radio emission from this object. First, we discuss whether it is possible to attribute the radio emission of Sco X-1 to thermal bremsstrahlung from an optically thin plasma at a temperature of 15 to 20 keV. This interpretation would have the merit of relating the radio properties to the existence of a hot, flaring plasma component whose presence has been invoked to explain the observations of hard X-rays (≳ 35 keV) from this object (Peterson and Jacobson, 1966, Buselli et al., 1968, Riegler et al., 1968, Riegler, 1969, Agrawal et al., 1969a, McCracken, 1969). That the 6-cm radio emission and the hard X-radiation should be found to vary by similar amounts over similar time scales lends this hypothesis a certain appeal. However, the measurement at 11 cm of flux density amounting to ≈0.07 flux units (1 flux unit ≡ ≡ 10−23 erg cm−2 sec−1 Hz−1) reported at this Symposium by McCracken makes it virtually untenable to regard the radio emission of Sco X-1 as arising from thermal origins. Excessive optical and infrared continuum radiation would be produced; moreover, the shape of the radio spectrum is inconsistent with any conceivable thermal model. An explanation for the radio emission of Sco X-1 must therefore be sought in terms of some non-thermal radiation mechanism.

The present status of observations of the diffuse cosmic X-rays is discussed. The energy spectrum in the energy range 1–100 keV has been well established. The flux around 0.25 keV appears to be rather high. The basis of the classical argument that the integration of normal galaxies in the universe is not sufficient to explain the diffuse X-ray flux is re-examined. Recent observations around 0.25 keV are discussed and results are compiled.

Recent observations in the X- and γ-Ray region of the electromagnetic spectrum have given strong evidence for the existence of an extragalactic intensity with a slowly steepening power law spectrum in the region 103 to 108 eV. Further data from the OSO-III high energy γ-Ray detector are in agreement with earlier published reports, and suggest that the γ-Rays from high galactic latitudes have a softer spectrum than those from the galactic plane.

It is widely believed that the diffuse X-ray background, observed on several occasions over the energy range from 0.25 keV to above 1 MeV has an extragalactic origin. Evidence for this comes from the generally reported isotropy above several keV [1, 2, 3] and the observed galactic latitude dependence at 0.25 keV, believed to result from the interstellar attenuation of these low energy photons in passage through the Galaxy [4, 5].

In this paper we present observations of the diffuse background X-rays in the energy range 20–120 keV, based on two balloon experiments carried out from Hyderabad (latitude 17.6°N, longitude 78.5°E), India. The flights were made on April 28, 1968 and December 22, 1968. The detector used was a NaI(Tl) crystal of effective area 97.3 cm2 and thickness 4 mm. The crystal was surrounded both by active and passive collimators. The passive collimator was a cylindrical graded shield of lead, tin, and copper, and the active collimator was a plastic scintillator surrounding the shield. The FWHM of the telescope was 18.6° and the geometrical factor for isotropic radiation 13.2 cm2 sr. The pulses from the NaI crystal were sorted into ten contiguous channels extending from 17 to 124 keV. An Am241 source came into the field of view of the telescope periodically and provided in-flight calibration of the detector. All the information was recorded on photographic film.

A cosmic ray detector, sensitive to γ-Rays with energies greater than 500 MeV is being flown on board the OGO-5 satellite. The spacecraft was launched into a highly eccentric orbit, apogee 145000 km, on March 4, 1968. γ-Ray observations are restricted to altitudes higher than 80000 km, thereby excluding interference from the radiation belts and reducing the influence from the earth albedo flux. A description of the instrument is published in the literature (Rogowski et al., 1969).

A γ-Ray telescope incorporating an acoustic spark chamber is included in the payload of the OGO-5 spacecraft. The performance of the instrument, which is sensitive to photons of energy 25 to 100 MeV, is discussed.

Observations are limited to a portion of the sky near Cygnus, but the first month's data indicate a variation of intensity showing a maximum in the direction of the galactic plane. If this plane contains a line source of radiation, its intensity is found to be (9 ± 5) × 10−4 photons cm−2 sec−1 rad−1 above an energy of 40 MeV.

Recent measurements of the low energy diffuse cosmic X-radiation are reported. The results indicate that the large flux observed below 1 keV is inconsistent with an extrapolation of the power law spectrum observed above 2 keV. While the radiation between 2 and 10 keV appears largely isotropic, at least to the limits of resolution of the detector, the 0.27 keV radiation shows significant asymmetry with respect to galactic latitude.

The diffuse background of 2–20 keV X-rays over a band of the sky extending from Scorpius to the North galactic pole is found to be isotropic to within 5%, with a spectrum given by where n = (1.35 ±0.100.07).

A comparison with spectra at higher energies indicates that the lower energy spectrum is flatter, corresponding to an apparent unit change in spectral index within the band 20–80 keV. A spectral break in this energy region has been discussed in connection with the collisional energy loss lifetime for metagalactic protons that radiate X-rays via inverse bremsstrahlung collisions with the ambient electrons of the intergalactic medium (Boldt and Serlemitsos, 1969; Hayakawa, 1970).

The significance of high energy (≳ 30 MeV) γ-Ray astronomy and its relationship to cosmic rays and many of the high energy processes of the universe has been realized for over a decade. Through the last six to eight years, searches for point sources, mostly from balloon experiments, but also Explorers 11 and OSO-3, have been unsuccessful in clearly establishing the existence of any point source. Recently on OSO-3, Clark et al. (1968) have obtained positive evidence for a celestial γ-Ray flux which is anisotropic with a higher intensity in the direction of the galactic center region. In this talk, I wish to summarize our balloon results relating to both of these questions and indicate what we hope to accomplish with our new large γ-Ray detector over the coming months.

An apparatus to study the cosmic X-ray background was launched aboard an ESRO Skylark rocket on May 22, 1967 from the Perdas de Fogu range in Sardinia.

After many years of fruitless research on primary γ-Rays, the results obtained by Clark et al. [1] with the OSO-3 satellite, proved for the first time the existence of a primary γ-Ray flux. The study of the distribution of this radiation showed a strong anisotropy in the direction of the galactic disk and, more precisely, in the direction of the galactic center. Now the production of γ-Rays in the spatial medium is related to high energy processes and to the presence of relativistic electrons. The high energy processes bring about an emission of γ-Rays essentially by decay of the π° mesons created for instance, by the interaction of cosmic radiation with interstellar matter or by matter-antimatter annihilation. The relativistic electrons can lead to an emission of γ-Rays by various processes: in particular, by bremsstrahlung of the electrons in the interstellar matter or by the Inverse Compton effect with the photons of the stellar light or of the infrared radiation background.

In this paper, we wish to report new data obtained on the total, or omnidirectional, cosmic γ-Ray flux in the 250 keV to 6 MeV range. Flux in this energy range up to 2 MeV was first detected on the Ranger 3 (Metzger et al., 1964) and has been confirmed up to 0.5 MeV and extended to lower energies by a series of rocket (Gorenstein et al., 1969; Seward et al., 1967) and balloon measurements (Bleeker et al., 1966; Rocchia et al., 1967). Observation of the spectrum at higher energies has become of considerable importance since the recent successful detection of galactic 100 MeV γ-Rays by the OSO-3 (Clark et al., 1968).

Measurements of the soft X-ray background intensity show that the intensity is largest in that part of the sky where the columnar atomic hydrogen density is smallest. In the direction bII =-60°, lII = 10° the intensity is found to be 195 ± 20 photons cm−2 sec−1 ster−1 keV−1 near 0.26 keV and 20 ± 3 photons cm−1 sec−1 ster−1 keV−1 near 0.9 keV. The intensity in the galactic plane is unexpectedly large.

In this paper we review the theories which have been proposed to account for the extra-galactic X-ray background. Although there is still no detailed theory, one may devise reasonable models which account in a natural way both for the intensity and the spectral shape over the whole energy band, provided that cosmological evolutionary effects are included. A model based on Compton scattering of cosmic black body photons by relativistic electrons in radio sources at large redshifts (z ≳ 4) seems to give the most satisfactory explanation. However, the data are not yet good enough to discriminate against alternative models.

A discussion of the recent observations in the soft X-ray region (< 1 keV), and their relevance to the physics of interstellar and intergalactic gas, is given. The available data are somewhat confusing, but it seems that this part of the spectrum may still be consistent with a simple extrapolation of the non-thermal spectrum at higher energies, though various workers have claimed the detection of a new component probably due to hot intergalactic gas. If this interpretation is correct one may deduce interesting conclusions about the state of ionization and composition of the intergalactic gas, because of the importance of the absorption effects in this energy band.

Also it appears that the Galaxy is more transparent than one would deduce from 21-cm observations. However, due to the lack of observational data, no firm conclusions can be reached.

It has been found difficult to explain both the absolute intensity and energy spectrum of the background component of cosmic X-rays in terms of a superposition of X-ray sources in distant galaxies [1] and of the inverse Compton collisions of metagalactic electrons with microwave photons [2]. Although the absolute intensity could be obtained by choosing suitable values of parameters which are not well known as yet, the gradual bending of the X-ray spectrum around 30 keV provides a critical test of theories. In this respect innerbremsstrahlung of intergalactic protons colliding with electrons is a candidate of background X-rays, since the X-ray spectrum is closely related to the proton spectrum and the latter usually bends in the non-relativistic region.

In a recent observation Clark et al. [1] have found that the intensity of γ-Rays of energies around 100 MeV is strong in the direction of the galactic plane; the observed intensity of the order of 10−4 cm−2 sec−1 rad−1 is greater by an order of magnitude than that predicted on the neutral pion decay hypothesis [2]. It should, however, be remarked that the theory is based on the assumption that cosmic rays are uniformly distributed in our galaxy. The assumption seems doubtful in view of the experimental result that the γ-Ray intensity depends rather strongly on the galactic longitude.

The implications of the existence of a dense, hot, intergalactic plasma are discussed for friedman universes with a zero cosmological constant. The points of interest are the temperature of the intergalactic gas and the degree of ionization of its constituents. The soft X-ray emission of this dense intergalactic gas must be in agreement with the actual observations of isotropic X-ray background around 1–3 keV and 0.27 keV. Moreover the model cannot be in contradiction with the lack of Lyman α absorption in the emission spectra of the quasi-stellar objects.