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Thompson, D. J.

Publications and source records attributed to Thompson, D. J..

At least 127 records · Page 7
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NASA NTRS1976

The relationship between the galactic matter distribution, cosmic ray dynamics, and gamma ray production

Theoretical considerations and analysis of the results of gamma ray astronomy suggest that the galactic cosmic rays are dynamically coupled to the interstellar matter through the magnetic fields, and hence the cosmic ray density should be enhanced where the matter density is greatest on the scale of galactic arms. This concept has been explored in a galactic model using recent 21 cm radio observations of the neutral hydrogen and 2.6 mm observations of carbon monoxide, which is considered to be a tracer of molecular hydrogen. The model assumes: (1) cosmic rays are galactic and not universal; (2) on the scale of galactic arms, the cosmic ray column (surface) density is proportional to the total interstellar gas column density; (3) the cosmic ray scale height is significantly larger than the scale height of the matter; and (4) ours is a spiral galaxy characterized by an arm to interarm density ratio of about 3:1.

Kniffen, D. A.↗
NASA NTRS1976

A search of the SAS-2 data for pulsed gamma-ray emission from radio pulsars

Data from the SAS-2 high-energy (above 35 MeV) gamma-ray experiment have been examined for pulsed emission from each of 75 radio pulsars which were viewed by the instrument and which have sufficiently well-defined period and period-derivative information from radio observations to allow for gamma-ray periodicity searches. When gamma-ray arrival times were converted to pulsar phase using the radio reference timing information, two pulsars, PSR 1747-46 and PSR 1818-04, showed positive effects, each with a probability of less than 1 part in 10,000 of being a random fluctuation in the data for that pulsar. These are in addition to PSR 0531+21 and PSR 0833-45, previously reported. The results of this study suggest that gamma-ray astronomy has reached the detection threshold for gamma-ray pulsars and that work in the near future should give important new information on the nature of pulsars.

Ogelman, H.↗
NASA NTRS1976

Observation of gamma rays with a 4.8 hour periodicity from CYG X-3

Energetic (E35 MeV) Gamma rays were observed from Cyg X-3 with the SAS-2 Gamma ray telescope. They are modulated at the 4.8 sup h period observed in the X-ray and infrared regions, and within the statistical error are in phase with this emission. The flux above 100 MeV has an average value of (4.4 + or - 1.1)x 10 to the -6 power/sq cm/sec. If the distance to Cyg X-3 is 10 kpcs, this flux implies a luminosity of more than 10 to the 37th power ergs/s if the radiation is isotropic and about 10 to the 36th power ergs/s if the radiation is restricted to a cone of one steradian, as it might be in a pulsar.

Lamb, R. C.↗
NASA NTRS1976

Significance of medium-energy gamma-ray astronomy in the study of cosmic rays

The paper examines the medium-energy (about 10-30 MeV) galactic gamma-ray radiation from primary and secondary electrons and calculates the expected gamma-ray distribution for the specific model of Bignami et al. (1975) on the assumption that the cosmic rays are correlated with the matter on the scale of galactic arms. The energy spectrum typical of regions near the galactic center indicates a dramatic shift from a predominantly cosmic-ray nucleonic mechanism at higher energies to a cosmic-ray electron mechanism at the lower energies. This provides a most important and direct means of probing the cosmic-ray electrons as a function of galactic position by making gamma-ray observations in the few to 40 MeV energy range. Medium-energy gamma-ray astronomy is shown to be a valuable tool in galactic research.

Fichtel, C. E.↗
NASA NTRS1976

Final SAS-2 gamma ray results on sources in the galactic anticenter region

Analysis of SAS-2 high energy Gamma ray data from the direction of the galactic anticenter shows that this region is characterized by: a diffuse emission from the galactic plane which has a maximum along b=0 deg and an enhancement toward negative latitudes associated with Gould's Belt, a strong point source in the direction of the Crab nebula, and a second intense localized source near galactic coordinates 195 deg, +5 deg. Gamma ray emission from the Crab source is dominated by a pulsed flux from PSR 0531+21. The total flux above 100MeV is 3.7 + or - 0.8 million/sq cm s. The source near 195 deg, + 5 deg has a flux above 100 MeV of 4.3 + or - 0.9 million/sq cm s. Its spectrum appears flatter than that of the Crab. The diffuse galactic plane emission at negative lattitudes shows a general correlation with the local matter distribution associated with Gould's Belt. The calculated Gamma ray intensity agrees well with the SAS-2 observations.

Thompson, D. J.↗
NASA NTRS1976

SAS-2 galactic gamma ray results, 1

Continuing analysis of the data from the SAS-2 high energy gamma-ray experiment has produced an improved picture of the sky at photon energies above 35 MeV. On a large scale, the diffuse emission from the galactic plane is the dominant feature observed by SAS-2. This galactic plane emission is most intense between galactic longitude 310 and 45 deg, corresponding to a region within 7kpc of the galactic center. Within the high-intensity region, SAS-2 observes peaks around galactic longitudes 315 deg, 330 deg, 345 deg, 0 deg, and 35 deg. These peaks appear to be correlated with such galactic features and components as molecular hydrogen, atomic hydrogen, magnetic fields, cosmic ray concentrations, and photon fields.

Thompson, D. J.↗
NASA NTRS1976

SAS-2 galactic gamma ray results. 2. Localized sources

Gamma-ray emission was detected from the radio pulsars PSR1818-04 and PSR1747-46, in addition to the previously reported gamma-ray emission from the Crab and Vela pulsars. Since the Crab pulsar is the only one observed in the optical and X-ray bands, these gamma-ray observations suggest a uniquely gamma-ray phenomenon occurring in a fraction of the radio pulsars. Using distance estimates it is found that PSR1818-04 has a gamma-ray luminosity comparable to that of the Crab pulsar, while the luminosities of PSR1747-46 and the Vela pulsar are approximately an order of magnitude lower. This survey of SAS-2 data for pulsar correlations has also yielded upper limits to gamma-ray luminosity for 71 other radio pulsars.

Hartman, R. C.↗
NASA NTRS1976

Gamma rays and large scale galactic structure

Many theoretical models were developed in an attempt to explain the spatial structure in the observed emission which results from interactions of energetic cosmic rays with the interstellar gas. The peaks in the observed distribution are remarkably well correlated with longitudes corresponding to tangential directions to known spiral arm features. Based on theoretical and experimental arguments, it is assumed that on the scale of galactic arms the cosmic rays are more intense where the mass of the gas to which they are coupled is greatest. Refining this model with the results of recent surveys of the interstellar gas, a good fit to the observations is obtained whether the cosmic rays are confined to the spiral arms in the disk or are more evenly confined as in a flat halo model.

Kniffen, D. A.↗
NASA NTRS1976

SAS-2 gamma-ray observations of PSR 1747-46

Observations with the SAS-2 high energy ( 35 MeV) gamma-ray telescope show evidence of gamma-ray emission from the radio pulsar PSR 1747-46. When the arrival times of gamma-rays from the region of the pulsar were converted to pulsar phases using the radio period and period derivative, a single peak was found in the phase plot, with a Poisson probability of occurring by chance of .00008. Independently, the time-averaged data for the PSR 1747-46 region show an enhancement over the surrounding region of the sky at the same galactic latitude, with a Poisson probability of chance occurrence of less than .008. The probability that these results are chance is the product of these two probabilities times the number of radio pulsars examined (73). This overall probability is sufficiently small (.00005) to suggest an identification of a new gamma-ray pulsar. In the gamma-ray pulsar plot, the peak falls 0.16 + or - 0.03 period after the radio pulsar peak. This phase shift is, within uncertainties, the same as that observed between the single radio peak and the first of the two gamma-ray peaks seen in the phase plot for PSR-0833-45 (the Vela pulsar).

Thompson, D. J.↗
NASA NTRS1976

SAS-2 gamma-ray observations of PSR 1747-46

Evidence is reported for the observation of gamma-ray emission from the radio pulsar PSR 1747-46 by the gamma-ray telescope aboard SAS 2. The evidence is based on the presence of both an approximately 3-sigma enhancement of gamma rays at the pulsar's location and an approximately 4-sigma peak in the phase plot of 79 gamma-ray events whose phase was calculated from the pulsar's known period. The gamma-ray pulsation is found to appear at a phase lag of about 0.16 from that predicted by the radio observations. The pulsed gamma-ray fluxes above 35 MeV and 100 MeV are estimated, and it is shown that the gamma-ray pulse width is similar to the radio pulse width. It is concluded that PSR 1747-46 is a most likely candidate for pulsed gamma-ray emission.

Thompson, D. J.↗
NASA NTRS1975

Interpretation of the pulsed gamma-ray emission from Vela

A model is proposed for the Vela pulsar in which the radio emission originates near the surface of the neutron star while the pulsed gamma-ray emission is produced by synchrotron radiation near the speed-of-light cylinder. This model can explain the energy flux, double pulse structure, and phase shift (with respect to the radio) of the gamma-ray emission, and offers approximate quantitative predictions for other X- and gamma-ray fluxes.

Thompson, D. J.↗
NASA NTRS1975

Significance of medium energy gamma ray astronomy in the study of cosmic rays

Medium energy (about 10 to 30 MeV) gamma ray astronomy provides information on the product of the galactic electron cosmic ray intensity and the galactic matter to which the electrons are dynamically coupled by the magnetic field. Because high energy (greater than 100 MeV) gamma ray astronomy provides analogous information for the nucleonic cosmic rays and the relevant matter, a comparison between high energy and medium energy gamma ray intensities provides a direct ratio of the cosmic ray electrons and nucleons throughout the galaxy. A calculation of gamma ray production by electron bremsstrahlung shows that: bremsstrahlung energy loss is probably not negligible over the lifetime of the electrons in the galaxy; and the approximate bremsstrahlung calculation often used previously overestimates the gamma ray intensity by about a factor of two. As a specific example, expected medium energy gamma ray intensities are calculated for the speral arm model.

Fichtel, C. E.↗
NASA NTRS1975

SAS-2 high-energy gamma-ray observations of the Vela pulsar

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.

Thompson, D. J.↗
NASA NTRS1975

High-energy galactic gamma radiation from cosmic rays concentrated in spiral arms

A model for the emission of high-energy (exceeding 100 Mev) gamma-rays from the galactic disk has been developed and compared with recent SAS-2 observations. In the calculation, it is assumed that (1) the high energy galactic gamma-rays result primarily from the interaction of cosmic rays with galactic matter, (2) the cosmic-ray density is proportional to the matter density on the scale of galactic arms, and (3) the matter in the Galaxy is distributed in a spiral pattern consistent with density-wave theory and experimental data on the matter distribution that is available, including the 21-cm H I line emission, continuum emission from H II regions, and data currently being used to estimate the H2 density. The calculated galactic-longitude distribution of gamma rays is in good agreement with the SAS-2 observations in relative shape and absolute flux. As a corollary, the nonuniform cosmic-ray distribution of this model tends to support the galactic origin of the fraction of cosmic rays which is important in the production of high-energy photons. Modifications of the basic model show that the gamma-ray flux is relatively sensitive to large variations of the assumed distribution of molecular hydrogen in the Galaxy.

Bignami, G. F.↗
NASA NTRS1975

Interpretation of the pulsed gamma ray emission from Vela

A model is proposed for the Vela pulsar in which the radio emission originates near the surface of the neutron star while the pulsed gamma ray emission is produced by synchrotron radiation near the speed of light cylinder. This model can explain the energy flux, double pulse structure, and phase shift with respect to the radio of the gamma ray emission and offers approximate quantitative predictions for other X-ray and gamma-ray fluxes.

Thompson, D. J.↗
NASA NTRS1975

High-energy gamma-ray results from the second small astronomy satellite

Data are reported which were obtained with a high-energy (exceeding 35 Mev) gamma-ray telescope flown on the second Small Astronomy Satellite (SAS-2). The high-energy galactic gamma radiation is observed to dominate over the general diffuse radiation along the entire galactic plane, and its longitudinal and latitudinal distributions appear to be generally correlated with galactic structural features, particularly with arm segments. Principally on the basis of its angular distribution and magnitude, it is suggested that this radiation results primarily from cosmic-ray interactions with interstellar matter. A uniform celestial gamma radiation appears to have been detected; the form of its differential spectrum over the energy range from about 35 to 170 MeV is obtained, and a cosmological origin is suggested for this radiation. In addition to the general galactic emission, high-energy gamma radiation was detected from the Crab Nebula, Vela X, a general region toward the galactic center, and a region located a few degrees north of the galactic plane. Upper limits to the high-energy gamma ray fluxes are set for a number of localized sources.

Fichtel, C. E.↗
NASA NTRS1975

SAS-2 High energy gamma-ray observations of the Vela pulsar

The second Small Astronomy Satellite high-energy (35 MeV) gamma-ray telescope 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, one following the radio peak by about 13 msec, and the other 0.4 period after the first. The luminosity of the pulsed emission above 100 MeV from Vela is about 0.1 that of the pulsar NP0532 in the Crab nebula, whereas the pulsed emission from Vela at optical wavelengths is less than 0.0004 that from the Crab. The relatively high intensity of the pulsed gamma-ray emission and the double peak structure, compared to the single pulse in the radio emission, suggests that the high energy gamma-ray pulsar emission may be produced under different conditions from those found at lower energies.

Thompson, D. J.↗