ESA Science & Technology - Publications Archive

The Second Small Astronomy Satellite (SAS-2) high-energy (in excess of 35 MeV) gamma-ray telescope has detected pulsed gamma-ray emission at the radio period from PSR 0833-45, the Vela pulsar, as well as an unpulsed flux from the Vela region. The pulsed emission consists of two peaks following the single radio peak by about 13 ms and 48 ms. The luminosity of the pulsed emission above 100 MeV from Vela is about 0.1 that of the pulsar NP 0532 in the Crab nebula, whereas the pulsed emission from Vela at optical wavelengths is less than 0.0002 that from the Crab. The relatively high intensity of the pulsed gamma-ray emission, and the double peak structure, compared with the single pulse in the radio emission, suggest that the high-energy gamma-ray pulsar emission may be produced under different conditions from those at lower energies.

The second ESA Bulletin was devoted to a report on the Cos-B mission and was published just a few weeks after the launch of the spacecraft. it contains a detailed review of the project through design and development and on to launch and the first detection.

Data from the COS-B mission and H I and CO surveys of the entire galactic plane are used to explore the correlation between diffuse galactic gamma-rays and gas tracers. An average ratio of H2 column density to integrated CO temperature of about 2.3 + or - 0.3 x 10²⁰ molecules cm^-2 (K km/s)^-1 is obtained. It is suggested that the previously noted soft gamma-ray spectrum toward the inner Galaxy may be due to a soft gamma-ray spectrum of the emission which is distributed like molecular gas.

Energetic (exceeding 35 MeV) gamma-rays have been observed from the direction of Cygnus X-3 with the SAS-2 gamma-ray telescope. The statistical significance of the excess above the galactic and diffuse radiation is approximately 4.5 sigma. In addition, the gamma-ray flux is modulated at the 4.8-hr period observed in the X-ray and infrared regions, and within the statistical error is in phase with this emission. The flux above 100 MeV has an average value of about 4.4 x 10^-6 photons cm^-2 s^-1. If the distance to Cygnus X-3 is 10 kpc, this flux implies a luminosity of more than 10³⁷ erg/s if the radiation is isotropic and about 10³⁶ erg/s if the radiation is restricted to a cone of 1 steradian, as it might be in a pulsar. Upper limits are presented for the gamma-ray flux from other known or suspected periodic X-ray sources.

A list of 25 high-energy (greater than 100 MeV) gamma-ray sources detected by COS B is presented. Only four sources are identified with well-known objects. Of the remaining sources, 20 are at low galactic latitude, and they may represent a new galactic population. Their luminosity is estimated to be in the range of (0.4-5) x 10³⁶ erg s^-1. It is seen that several hundred such sources may exist in the galaxy. Their nature is not understood.

Present knowledge on gamma radiation that originates from cosmic-ray interactions with gas and photons in the ISM is reviewed. Basic principles of such radiation and the main observational results are described. The significance of the findings for molecular gas studies and cosmic ray studies is discussed. Present understanding of radiation from gamma-ray point sources is briefly addressed, emphasizing the possible diffuse nature of these sources.

New results on the temporal and spectral characteristics of the high energy gamma ray emission from the Vela pulsar are presented. A new sensitive analysis method using maximum likelihood techniques has been applied to the final COS-B dataset. The whole pulsed flux is found to exhibit long term variability. The energy spectrum of this total emission cannot be described by a single power law and the data show strong evidence for a spectral break at approximately 300 MeV. Five discrete emission regions within the pulsar light-curve have been identified, with the spectral characteristics and long term behavior being different. These results support the idea that various source regions simultaneously exist in the pulsar magnetosphere and that the physical processes generating the gamma rays in these sites differ with the location.

COS-B viewed the Cyg X-3 region seven times between November, 1975, and February, 1982; a search for steady gamma-ray emission pulsed at the characteristic 4.8-hour period did not reveal its source. Leiden-MIT balloon experiment observations of Cyg X-3 in May, 1979 show the 4.8-hour modulation with sinusoidal light curve and modulation depth of 0.30, for energies of up to about 140 keV. The strong variability of Cyg X-3 over more than one order of magnitude at energies below 20 keV does not emerge in the data collected at hard X-ray energies.

The gamma-ray emissions from the Crab pulsar and nebula are analyzed. The data were collected from 1975-1982 in the Galactic anticenter region by COS-B in the energy range 50-3000 MeV. The temporal behavior of the pulsed gamma-ray flux from PSR 0531 + 21 and the unpulsed gamma-ray emissions from the Crab region are examined. It is determined that the spectrum of the pulsar emission is represented by a single power law with an index of 2.00 + or - 0.10 over the energy range of the instrument, and no systematic variation of spectral index with pulsar phase is noted; the spectrum of the unpulsed gamma-ray emission is represented by a single power law with a spectral index of 2.7 + or - 0.3.

This is one in a series of papers prepared in association with the release of the final COS-B database. Various aspects of the performance of the COS-B gamma-ray telescope are studied using in-flight data. First, the time-variation of the sensitivity and background over the 6.7 years of the mission is investigated. COS-B observed many regions of the sky repeatedly over the mission lifetime, allowing the study of the temporal behavior of the instrument. Using a multivariate optimization algorithm, the relative sensitivity for the 65 observation periods is determined to an accuracy generally better than 10 percent. The temporal variations of the instrumental background, assumed to be related to a cosmic-ray scaler aboard the satellite, are simultaneously determined. Second, the dependence of the instrumental background on the inclination to the telescope axis is analyzed using observations of high galactic latitude areas. The resulting corrections to the instrumental parameters are a necessary prerequisite for the combination of the data into large-scale maps for scientific analysis. The implementation of the corrections to give such maps is outlined.

The cosmic ray (CR) distribution in the Galaxy inside the solar circle is determined by using the kinematics of both H I and CO to ascertain the spatial distribution of the interstellar gas. The conversion factor between integrated CO line intensity and the molecular hydrogen column density is found, and possible large-scale variations of this relationship throughout the Galaxy are investigated. The galactocentric CR distributions are shown to be described satisfactorily by exponential distributions for radii larger than about three, with a radial scale length of 4-11 kpc for electrons and a scale length greater than 18 kpc for nuclei.

In the present derivation of the radial distribution of high energy gamma-ray emissivity in the outer Milky Way, the kinematics of H I are used to construct column-density maps in various galactocentric distance ranges that are then used in combination with COS-B spacecraft gamma-ray data to determine emissivities in these distance ranges. The results obtained imply a hardening of the gamma-ray spectrum with increasing distance, and include an energy-dependent decrease that is interpreted as a steep gradient in the cosmic-ray electron density and a near constancy of the nuclear component. The galactic origin of electrons with energies of up to several hundreds of MeVs is confirmed.

Pulsed gamma-ray emission from radio pulsars has been sought in a study of pulsar parameters for 145 radio observations, contemporary with gamma-ray observations, by the COS-B spacecraft. No signal has been detected in the 50 MeV-2 GeV energy range of the study, either globally or from individual pulsars. These results indicate that the average product of the conversion efficiency from the pulsar braking power into gamma-rays, and the pulsar moment of inertia, cannot exceed 6 x 10⁴³ g cm², for pulsar ages greater than 40,000 years. This implies that the contribution of old pulsars to the observed galactic gamma-ray emission is less than 5 percent for gamma-ray energies greater than 50 MeV.

A search through COS-B data for gamma-radiation from a number of extragalactic objects reveals no strong evidence for any positive identification except, as already reported, for the region around the quasar 3C273. Upper limits to the photon fluxes from active extragalactic objects are evaluated using a likelihood method. They reinforce the conclusion reached by Bignami et al. (1979) from the SAS-2 data that the flux upper limits lie substantially below the extrapolation of spectra measured at X-ray energies. The proposed identifications of 2CG135+01 are also discussed

The results of a second observation in the Virgo region by the ESA COS-B satellite are presented. The presence of a high-energy (50-800 MeV) Gamma-radiation source is confirmed, and its position is consistent with 3C273. The error box at the 90% confidence level containing the quasar is ~2.5 square degrees and the probability of a change coincidence of the gamma-ray source with 3C273 is approximately 10^-3. No variation of the gamma-ray fluxes between the two observations in July 1976 and June 1978 is observed within the ~50% uncertainty. Spectral y-ray data for the total 3C273 data set are presented and compared with low and high-energy X-ray contemporary measurements. Recent Einstein Observatory X-ray data on the short term variability of the central part of the QSO are used to show that, within reasonable assumptions, the photon-photon interaction excludes the gamma-ray source from coinciding with the variable X-ray source, thus ruling out comptonisation models for the production of energetic photons.

The recently proposed nearby (distance approximately 0.03 pc) cloud of Vidal-Madjar et al. (1978) has an angular extent encompassing the Rho Oph region and the COS-B CG 353+16 source. In an attempt to analyze a possible association between the gamma-ray source and the cloud, usage is made of SAS-3 low energy X-ray data to put an upper limit on the columnar density, and thus such an association can easily be excluded. Furthermore, the possibility is analyzed that the COS-B source is associated with the Rho Oph dark cloud complex, and that the responsible process is the interaction of cosmic rays with the cloud mass. It is seen that a standard Black and Fazio (1973) mechanism can hardly be at work so the quantitative requirements are given for an improved gamma-ray production rate, obtainable, for instance, within the model of Forman et al. (1979).

Gamma-ray observations from the COS-B satellite and radio pulse-timing measurements from the Tidbinbilla Deep Space Station are used to determine accurately the relative phases of the radio (2295 MHz) and gamma-ray (above 50 MeV) pulses from the Vela pulsar. Two independent analysis procedures are employed, and both show that the peak of the radio pulse leads the peak of the first gamma-ray pulse by 11.2 plus or minus 0.5 ms. This result, together with the data of Manchester et al. (1978), indicates that the phase delay relative to the radio pulse peak of the pulse components at optical and gamma-ray frequencies are about 11 ms (first gamma-ray), 21 ms (first optical), 42 ms (second optical), and 49 ms (second gamma-ray).